《收获，不止Oracle》表的连接学以致用
6.2 三大类型
6.2.1 连接类型
嵌套循环
排序合并及散列连接
第一种方式就是数据库表连接中的嵌套循环连接（Nested Loops Join），而第二种方式就是表连接中的排序合并连接（Merge Sort Join）或者散列连接（Hash Join），请大家记住。
我在这里要告诉大家，在电信、金融等领域的数据库相关应用中，表连接总体的比例情况大致为，嵌套循环连接占70%左右，而散列连接占20%，剩下大致10%是排序合并连接。”
各类连接被访问次数的差异
研究嵌套循环连接的表的被访问次数前的准备
[oracle@oracle-db-19c ~]$ sqlplus / as sysdba
SQL*Plus: Release 19.0.0.0.0 - Production on Sun Jan 29 12:47:17 2023
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
SQL>
SQL> conn maxwellpan/maxwellpan@PDB1
Connected.
SQL>
SQL>
SQL> show user;
USER is "MAXWELLPAN"
SQL>
SQL>
SQL> drop table t1 cascade constraints purge;
Table dropped.
SQL> drop table t2 cascade constraints purge;
Table dropped.
SQL> create table t1 (
2 id number not null,
3 n number,
4 contents varchar2(4000)
5 );
Table created.
SQL> create table t2 (
2 id number not null,
3 t1_id number not null,
4 n number,
5 contents varchar2(4000)
6 );
Table created.
SQL> execute dbms_random.seed(0);
PL/SQL procedure successfully completed.
SQL> INSERT INTO T1
2 SELECT rownum,rownum,dbms_random.string('a',50)
3 from dual
4 connect by level <= 100
5 order by dbms_random.random;
100 rows created.
SQL> INSERT INTO T2
2 SELECT rownum,rownum,rownum,dbms_random.string('a',50)
3 from dual
4 connect by level <= 100000
5 order by dbms_random.random;
100000 rows created.
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
100
SQL> select count(*) from t2;
COUNT(*)
----------
100000
SQL> 测试表连接的写法如下：
select /*+leading(t1)use_nl(t2)*/
*
from t1, t2
where t1.id = t2.t1_id;研究嵌套循环连接，t2表被访问100次
SQL> set linesize 1000
SQL> alter session set statistics_level=all;
Session altered.
SQL> select /*+leading(t1)use_nl(t2)*/
2 *
3 from t1, t2
4 where t1.id = t2.t1_id;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
12 12
LIoEWaAdhvdNAhpdbdpIawQFQDmzebzEqwwChvFHDhoRfrlVrc
12 12 12
OtnpOjHihZjuQvFTrbUnzdudicUrhVdHalzMpNSmzDmxpXjChe
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
22 22
oXzAinzjnUtJrhwPzWHTktpdZPTPHTkYAyjsmYMawihqyolJuT
22 22 22
hsGLxPQWFPExbxjfezItjqsfjSYObxxfmcDjjjhxiUIWORrFhQ
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
10 10
InblvsVdzHVVPFElOsSWtRLkqAJwytraTSsWMoEaoYNAuAoIrI
10 10 10
uLeVTecJSNhjCPcXeiyONxMIFQvaqLuFyuvWDvBuyiHLeWzlbh
100 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID fx7936hqx6118, child number 0
-------------------------------------
select /*+leading(t1)use_nl(t2)*/ * from t1, t2 where t1.id =
t2.t1_id
Plan hash value: 1967407726
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 100 |00:00:00.30 | 100K|
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | NESTED LOOPS | | 1 | 100 | 100 |00:00:00.30 | 100K|
| 2 | TABLE ACCESS FULL| T1 | 1 | 100 | 100 |00:00:00.01 | 14 |
|* 3 | TABLE ACCESS FULL| T2 | 100 | 1 | 100 |00:00:00.29 | 100K|
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("T1"."ID"="T2"."T1_ID")
Note
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----
- dynamic statistics used: dynamic sampling (level=2)
25 rows selected.
SQL> 100次怎么变成了2 次
在刚才的基础上，我们继续跟踪观察如下语句，差别在于增加了t1的条件：
select /*+leading(t1)use_nl(t2)*/
*
from t1, t2
where t1.id = t2.t1_id
and t1.n in (17, 19);换个语句，这次t2表被访问2次
SQL>
SQL> select /*+leading(t1)use_nl(t2)*/
2 *
3 from t1, t2
4 where t1.id = t2.t1_id
5 and t1.n in (17, 19);
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
17 17
mcbXTSZhvccAomKZZJdfSanBDAlVYefHAurNoryMikSJjeobGT
17 17 17
RXWlAksGJyRTYvRYzcAApNTJZhZSOXvBPBpdwujRLERFshXvCm
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID gfdsv34x0r9gx, child number 0
-------------------------------------
select /*+leading(t1)use_nl(t2)*/ * from t1, t2 where t1.id =
t2.t1_id and t1.n in (17, 19)
Plan hash value: 1967407726
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 2 |00:00:00.01 | 2019 |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | NESTED LOOPS | | 1 | 2 | 2 |00:00:00.01 | 2019 |
|* 2 | TABLE ACCESS FULL| T1 | 1 | 2 | 2 |00:00:00.01 | 8 |
|* 3 | TABLE ACCESS FULL| T2 | 2 | 1 | 2 |00:00:00.01 | 2011 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(("T1"."N"=17 OR "T1"."N"=19))
3 - filter("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL> 2次怎么变成1 次
继续换个语句，这次t2表被访问1次
SQL>
SQL> select /*+leading(t1)use_nl(t2)*/
2 *
3 from t1, t2
4 where t1.id = t2.t1_id
5 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID b08tygwk2nmbh, child number 0
-------------------------------------
select /*+leading(t1)use_nl(t2)*/ * from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 1967407726
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1014 |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.01 | 1014 |
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 8 |
|* 3 | TABLE ACCESS FULL| T2 | 1 | 1 | 1 |00:00:00.01 | 1006 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("T1"."N"=19)
3 - filter("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL> 1次又变成 0次
改写到最后，t2表居然被访问0次
SQL>
SQL> select /*+leading(t1)use_nl(t2)*/
2 *
3 from t1, t2
4 where t1.id = t2.t1_id
5 and t1.n =999999999;
no rows selected
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID b30dn2nuuz6zt, child number 0
-------------------------------------
select /*+leading(t1)use_nl(t2)*/ * from t1, t2 where t1.id =
t2.t1_id and t1.n =999999999
Plan hash value: 1967407726
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.01 | 7 |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | NESTED LOOPS | | 1 | 1 | 0 |00:00:00.01 | 7 |
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 0 |00:00:00.01 | 7 |
|* 3 | TABLE ACCESS FULL| T2 | 0 | 1 | 0 |00:00:00.01 | 0 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("T1"."N"=999999999)
3 - filter("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL>
嵌套循环连接访问次数的最终结论
t1表的查询返回多少条记录，t2表就被访问多少次。第一次t1表返回100条记录，是因为t1表全表就是100条记录，无条件查询当然就是返回100条；而第二次AND t1.n in（17,19）的条件让t1表返回2条记录，所以t2表被访问2次；第三次AND t1.n=19的条件让t1表的查询只返回1条记录，所以t2表被访问1次；最后一次AND t1.n=999999999这个条件显然是从t1表中查不出任何记录的，所以t2表被访问0次，干脆就不被访问了！
分析t2表被访问次数不同的原因
SQL> ---解释t2表为什么被访问100次
SQL> select count(*) from t1;
COUNT(*)
----------
100
SQL>
SQL> ---解释t2表为什么被访问2次
SQL> select count(*) from t1 where t1.n in (17, 19);
COUNT(*)
----------
2
SQL>
SQL> ---解释t2表为什么被访问1次
SQL> select count(*) from t1 where t1.n = 19;
COUNT(*)
----------
1
SQL>
SQL>
SQL> ---解释t2表为什么被访问0次
SQL> select count(*) from t1 where t1.n = 999999999;
COUNT(*)
----------
0
SQL> 散列连接的表被访问的次数
测试t2表仅被访问1次
select /*+leading(t1)use_hash(t2)*/*
from t1, t2
where t1.id = t2.t1_id;散列连接中t2表只会被访问1次或0次
SQL>
SQL> select /*+leading(t1)use_hash(t2)*/*
2 from t1, t2
3 where t1.id = t2.t1_id;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
29 29
UdyZOgyQOYrowfGAmMLgKAwNXFpmsbAhvkwzESohhOaHvLSHfD
29 29 29
NBgeTJqQwABblhuRRZbsaTdpulkvFzlpGKYwNGnuTiEHxbBQqp
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
96 96
PDjlvYcPtkTrFCITsiRPtGAdqEImtZYckVgLyJzYQpSYKwnbDy
96 96 96
vxBFUdYBjpaXikSdvkYxaTZulJFcZrHAkDmeBrDYtghvZakvex
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
51 51
OSvQYvjxmyzzGMnFGccgQPuuiLtDgJNADGvxNkSLCWEeCFXWpP
51 51 51
xwcqdBlQwpumvozClyiAdXkfsfzoQfhfuSyKctENvbfITolNtj
100 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 8m9zqja907n7j, child number 0
-------------------------------------
select /*+leading(t1)use_hash(t2)*/* from t1, t2 where t1.id =
t2.t1_id
Plan hash value: 1838229974
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 100 |00:00:00.01 | 1083 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 1 | HASH JOIN | | 1 | 100 | 100 |00:00:00.01 | 1083 | 1000K| 1000K| 1346K (0)|
| 2 | TABLE ACCESS FULL| T1 | 1 | 100 | 100 |00:00:00.01 | 7 | | | |
| 3 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1012 | | | |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."ID"="T2"."T1_ID")
Note
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----
- dynamic statistics used: dynamic sampling (level=2)
25 rows selected.
SQL> 散列连接准确结论
不过更准确的说法应该是：在散列连接中，驱动表和被驱动表都只会被访问0次或者1次。
Hash Join中t2表被访问0次的情况
SQL>
SQL> select /*+leading(t1)use_hash(t2)*/*
2 from t1, t2
3 where t1.id = t2.t1_id
4 and t1.n = 999999999;
no rows selected
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 9p23u2b7c4tjw, child number 0
-------------------------------------
select /*+leading(t1)use_hash(t2)*/* from t1, t2 where t1.id =
t2.t1_id and t1.n = 999999999
Plan hash value: 1838229974
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.01 | 7 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 1 | HASH JOIN | | 1 | 1 | 0 |00:00:00.01 | 7 | 685K| 685K| 179K (0)|
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 0 |00:00:00.01 | 7 | | | |
| 3 | TABLE ACCESS FULL| T2 | 0 | 98810 | 0 |00:00:00.01 | 0 | | | |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."ID"="T2"."T1_ID")
2 - filter("T1"."N"=999999999)
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL>
散列连接中t1表和t2表都被访问0次的情况
SQL>
SQL> select /*+leading(t1)use_hash(t2)*/*
2 from t1, t2
3 where t1.id = t2.t1_id
4 and 1=2;
no rows selected
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 4jcf0cucqftuu, child number 0
-------------------------------------
select /*+leading(t1)use_hash(t2)*/* from t1, t2 where t1.id =
t2.t1_id and 1=2
Plan hash value: 487071653
----------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.01 |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 1 | FILTER | | 1 | | 0 |00:00:00.01 |
|* 2 | HASH JOIN | | 0 | 100 | 0 |00:00:00.01 |
| 3 | TABLE ACCESS FULL| T1 | 0 | 100 | 0 |00:00:00.01 |
| 4 | TABLE ACCESS FULL| T2 | 0 | 98810 | 0 |00:00:00.01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(NULL IS NOT NULL)
2 - access("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
27 rows selected.
SQL>
排序合并连接的表被访问的次数
排序合并连接的访问情况和散列连接一样
SQL>
SQL> select /*+ordered use_merge(t2)*/*
2 from t1, t2
3 where t1.id = t2.t1_id;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 1
rMLTDXxxqXOZnqYRJwInlGfGBTxNkAszBGEUGELqTSRnFjRGbi
1 1 1
oEDjYpmoOoPPCgRApZewlJvzMTsosHAFerbsFWoxLAExnGcaPH
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2 2
UEDJsfIgoYqwreSuuvjIcPZarpxMdCthpDCsgPlJfvIiylLiBS
2 2 2
RcGqBYvXQxnttfqIICFyBaiuYPXacaNFUOrGCmzJDKhrrrMGqL
100 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID b3dw96yzr1k2k, child number 0
-------------------------------------
select /*+ordered use_merge(t2)*/* from t1, t2 where t1.id = t2.t1_id
Plan hash value: 412793182
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 100 |00:00:00.03 | 1012 | | | |
| 1 | MERGE JOIN | | 1 | 100 | 100 |00:00:00.03 | 1012 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 2 | SORT JOIN | | 1 | 100 | 100 |00:00:00.01 | 7 | 13312 | 13312 |12288 (0)|
| 3 | TABLE ACCESS FULL| T1 | 1 | 100 | 100 |00:00:00.01 | 7 | | | |
|* 4 | SORT JOIN | | 100 | 98810 | 100 |00:00:00.03 | 1005 | 9762K| 1209K| 8677K (0)|
| 5 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1005 | | | |
-----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."ID"="T2"."T1_ID")
filter("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
27 rows selected.
SQL>显然可以看出来，在访问次数上，排序合并连接和散列连接是一样的，t1表和t2表都只会被访问0次或者1次。
排序合并连接根本就没有驱动和被驱动的概念，而嵌套循环连接和散列连接要考虑驱动和被驱动情况。
各类连接驱动顺序的区别
嵌套循环连接的表驱动顺序
嵌套循环连接中t1表先被访问的情况
SQL>
SQL> alter session set statistics_level=all;
Session altered.
SQL> select /*+leading(t1)use_nl(t2)*/
2 *
3 from t1, t2
4 where t1.id = t2.t1_id
5 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID b08tygwk2nmbh, child number 0
-------------------------------------
select /*+leading(t1)use_nl(t2)*/ * from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 1967407726
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1014 |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.01 | 1014 |
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 8 |
|* 3 | TABLE ACCESS FULL| T2 | 1 | 1 | 1 |00:00:00.01 | 1006 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("T1"."N"=19)
3 - filter("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL> 嵌套循环连接中t2表先被访问的情况
SQL>
SQL> select /*+leading(t2)use_nl(t1)*/
2 *
3 from t1, t2
4 where t1.id = t2.t1_id
5 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID dyt6y9v61aprm, child number 0
-------------------------------------
select /*+leading(t2)use_nl(t1)*/ * from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 4016936828
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.58 | 701K|
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.58 | 701K|
| 2 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1006 |
|* 3 | TABLE ACCESS FULL| T1 | 100K| 1 | 1 |00:00:00.55 | 700K|
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(("T1"."ID"="T2"."T1_ID" AND "T1"."N"=19))
Note
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----
- dynamic statistics used: dynamic sampling (level=2)
25 rows selected.
SQL> 散列连接的表驱动顺序
散列连接中t1表先被访问的情况
SQL>
SQL> ---观察t1先被访问的情况
SQL> alter session set statistics_level=all;
Session altered.
SQL> select /*+leading(t1)use_hash(t2)*/*
2 from t1, t2
3 where t1.id = t2.t1_id
4 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 5fthzm536j4c3, child number 0
-------------------------------------
select /*+leading(t1)use_hash(t2)*/* from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 1838229974
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1077 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 1 | HASH JOIN | | 1 | 1 | 1 |00:00:00.01 | 1077 | 1000K| 1000K| 423K (0)|
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 7 | | | |
| 3 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1006 | | | |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."ID"="T2"."T1_ID")
2 - filter("T1"."N"=19)
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL> 散列连接中t2表先被访问的情况
SQL> ---观察t2先被访问的情况
SQL> alter session set statistics_level=all;
Session altered.
SQL> select /*+leading(t2)use_hash(t1)*/*
2 from t1, t2
3 where t1.id = t2.t1_id
4 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 9mgbxa9k7ttms, child number 0
-------------------------------------
select /*+leading(t2)use_hash(t1)*/* from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 2959412835
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.03 | 1013 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 1 | HASH JOIN | | 1 | 1 | 1 |00:00:00.03 | 1013 | 12M| 2564K| 15M (0)|
| 2 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1005 | | | |
|* 3 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 8 | | | |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."ID"="T2"."T1_ID")
3 - filter("T1"."N"=19)
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
26 rows selected.
SQL> t1表先被访问的情况下Buffers是1013，而t2表先被访问的情况下，Buffers也是1013，但是Used-Mem却差异明显，前者是286KB，后者是11MB，说明排序尺寸差异明显。再结合时间来看，前者是0.04秒，后者是0.1秒，差别也不小。“说明散列连接中驱动表的顺序非常重要，性能差别也很明显！
排序合并的表驱动顺序
排序合并连接中t1表先被访问的情况
SQL> ---观察t1先被访问的情况
SQL> alter session set statistics_level=all;
Session altered.
SQL> select /*+leading(t1)use_merge(t2)*/*
2 from t1, t2
3 where t1.id = t2.t1_id
4 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID dxvtjqg2dgjfy, child number 0
-------------------------------------
select /*+leading(t1)use_merge(t2)*/* from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 412793182
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.03 | 1012 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | MERGE JOIN | | 1 | 1 | 1 |00:00:00.03 | 1012 | | | |
| 2 | SORT JOIN | | 1 | 1 | 1 |00:00:00.01 | 7 | 2048 | 2048 | 2048 (0)|
|* 3 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 7 | | | |
|* 4 | SORT JOIN | | 1 | 98810 | 1 |00:00:00.03 | 1005 | 9762K| 1209K| 8677K (0)|
| 5 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1005 | | | |
-----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("T1"."N"=19)
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
4 - access("T1"."ID"="T2"."T1_ID")
filter("T1"."ID"="T2"."T1_ID")
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
29 rows selected.
SQL> 排序合并连接中t2表先被访问的情况
SQL> ---观察t2先被访问的情况
SQL> alter session set statistics_level=all;
Session altered.
SQL> select /*+leading(t2)use_merge(t1)*/*
2 from t1, t2
3 where t1.id = t2.t1_id
4 and t1.n =19;
ID N
---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID T1_ID N
---------- ---------- ----------
CONTENTS
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
19 19
jAOYAJHBzwhNWHImZeFUOaxvMycbQsCLKhsUnSFwZpyctEahjK
19 19 19
GjEZVArATZoLjgeBswAqZZshWrhgQeEKLYScUQqMtlFclNtjHK
SQL> SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID cythx275yxzr7, child number 0
-------------------------------------
select /*+leading(t2)use_merge(t1)*/* from t1, t2 where t1.id =
t2.t1_id and t1.n =19
Plan hash value: 1792967693
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.04 | 1012 | | | |
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | MERGE JOIN | | 1 | 1 | 1 |00:00:00.04 | 1012 | | | |
| 2 | SORT JOIN | | 1 | 98810 | 20 |00:00:00.04 | 1005 | 9762K| 1209K| 8677K (0)|
| 3 | TABLE ACCESS FULL| T2 | 1 | 98810 | 100K|00:00:00.01 | 1005 | | | |
|* 4 | SORT JOIN | | 20 | 1 | 1 |00:00:00.01 | 7 | 2048 | 2048 | 2048 (0)|
|* 5 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 7 | | | |
-----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."ID"="T2"."T1_ID")
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
filter("T1"."ID"="T2"."T1_ID")
5 - filter("T1"."N"=19)
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
29 rows selected.
SQL> 嵌套循环连接和散列连接有驱动顺序，驱动表的顺序不同将影响表连接的性能，而排序合并连接没有驱动的概念，无论哪张表在前都无妨。