【读书笔记】【收获，不止Oracle】嵌套循环表(Nested Loops Join)

本次试验用于探究嵌套循环表的访问次数。

1.构造t1和t2表，用于嵌套查询。

SYS@ orcl>DROP TABLE t1 CASCADE CONSTRAINTS PURGE;

Table dropped.

SYS@ orcl>DROP TABLE t2 CASCADE CONSTRAINTS PURGE;

Table dropped.

SYS@ orcl>CREATE TABLE t1(
  2  id number not null,
  3  n number,
  4  contents varchar2(4000)
  5  )
  6  ;

Table created.

SYS@ orcl>CREATE TABLE t2(
  2  id number not null,
  3  t1_id number not null,
  4  n number,
  5  contents varchar2(4000)
  6  )
  7  ;

Table created.

2.t1表投入100条数据，t2表投入100000条数据。、

SYS@ orcl>execute dbms_random.seed(0);

PL/SQL procedure successfully completed.

SYS@ orcl>INSERT INTO t1
  2  SELECT 
  3  rownum,rownum,dbms_random.string('a',50)
  4  FROM dual 
  5  CONNECT BY level<=100
  6  ORDER BY dbms_random.random;

100 rows created.

SYS@ orcl>INSERT INTO t2
  2  SELECT rownum,rownum,rownum,dbms_random.string('b',50)
  3  FROM dual
  4  CONNECT BY level<=100000
  5  ORDER BY dbms_random.random; 

100000 rows created.


SYS@ orcl>commit;

Commit complete.


SYS@ orcl>select count(*) from t1;

  COUNT(*)
----------
       100

SYS@ orcl>select count(*) from t2;

  COUNT(*)
----------
    100000

3.只是【t1.id=t2.t1_id】的情况下，执行如下语句。

SELECT /*+leading(t1) use_nl(t2)*/*
FROM t1,t2
WHERE t1.id=t2.t1_id;

观察执行计划

SYS@ orcl>select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  6cy5taz2tutgz, 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 |
-------------------------------------------------------------------------------------
|   1 |  NESTED LOOPS      |      |      1 |    100 |    100 |00:00:00.83 |   98517 |
|   2 |   TABLE ACCESS FULL| T1   |      1 |    100 |    100 |00:00:00.01 |      10 |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|*  3 |   TABLE ACCESS FULL| T2   |    100 |      1 |    100 |00:00:00.83 |   98507 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("T1"."ID"="T2"."T1_ID")

Note
-----
   - dynamic sampling used for this statement

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


23 rows selected.

4.多了【t1.n in(17,19)】条件

SELECT /*+leading(t1) use_nl(t2)*/*
FROM t1,t2
WHERE t1.id=t2.t1_id
AND t1.n in(17,19);

观察执行计划

SYS@ orcl>select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  bh8yn920472bt, 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 |
-------------------------------------------------------------------------------------
|   1 |  NESTED LOOPS      |      |      1 |      2 |      2 |00:00:00.01 |    1975 |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|*  2 |   TABLE ACCESS FULL| T1   |      1 |      2 |      2 |00:00:00.01 |       4 |
|*  3 |   TABLE ACCESS FULL| T2   |      2 |      1 |      2 |00:00:00.01 |    1971 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter(("T1"."N"=17 OR "T1"."N"=19))
   3 - filter("T1"."ID"="T2"."T1_ID")

Note

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----
   - dynamic sampling used for this statement


25 rows selected.

5.改变检索条件【t1.n = 19】

SELECT /*+leading(t1) use_nl(t2)*/*
FROM t1,t2
WHERE t1.id=t2.t1_id
AND t1.n = 19;

观察执行计划

SYS@ orcl>select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  7pzkjc7sd65a6, 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 |
-------------------------------------------------------------------------------------
|   1 |  NESTED LOOPS      |      |      1 |      1 |      1 |00:00:00.01 |     990 |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|*  2 |   TABLE ACCESS FULL| T1   |      1 |      1 |      1 |00:00:00.01 |       4 |
|*  3 |   TABLE ACCESS FULL| T2   |      1 |      1 |      1 |00:00:00.01 |     986 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("T1"."N"=19)
   3 - filter("T1"."ID"="T2"."T1_ID")

Note

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----
   - dynamic sampling used for this statement


25 rows selected.

6.改变检索条件【AND t1.n = 9999999999】

SYS@ orcl>select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  5kpy1wbydnghk, child number 0
-------------------------------------
SELECT /*+leading(t1) use_nl(t2)*/* FROM t1,t2 WHERE t1.id=t2.t1_id AND t1.n
= 9999999999

Plan hash value: 1967407726

-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   1 |  NESTED LOOPS      |      |      1 |      1 |      0 |00:00:00.01 |       3 |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|*  2 |   TABLE ACCESS FULL| T1   |      1 |      1 |      0 |00:00:00.01 |       3 |
|*  3 |   TABLE ACCESS FULL| T2   |      0 |      1 |      0 |00:00:00.01 |       0 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("T1"."N"=9999999999)
   3 - filter("T1"."ID"="T2"."T1_ID")

Note

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----
   - dynamic sampling used for this statement


25 rows selected.

总结：

循环嵌套表中驱动表(t1)返回多少条记录，被驱动表就访问多少次。