Oracle技术之在OLTP OR OLAP上绑定变量
我在这里不会介绍OLTP和OLAP的概念，这些入行的DBA都是必懂的。今天的话题是绑定变量为什么适合在OLTP系统而不宜于OLAP系统。从某种角度上我们可以这样看待OLTP和OLAP系统，OLTP系统小查询多而OLAP系统基本都是大查询，而它们致命的区别就是执行时间上(结果集有很大的区别)，所以我们也可以说OLTP系统适合短查询、OLAP系统则适合长查询。不管怎样，在OLTP系统，我们需要注重小查询的快速执行，为此而能够做的优化操作之一就是提高SQL命中率、使用索引(索引访问小结果集)，而绑定变量特性恰恰能够提高SQL的重复使用频率，减少反复解析次数，进而极大节省系统资源的消耗。在OLAP系统，报表类查询里经常有聚合排序操作，一般都是庞大的结果集，在这里解析所耗的系统资源基本可以忽略，为达到加快查询的优化目的，我们要注重执行计划的正确选择，而绑定变量的使用很可能让CBO选择错误的执行计划，故不适合使用。
下面通过示例查看绑定变量影响OLTP和OLAP系统的效果。
下面简要介绍OLAP系统下的影响：
-- 我有一张大表
luocs@MAA> select count(*) from tt1;
COUNT(*)
----------
338752
luocs@MAA> select index_name, index_type, status from user_indexes where table_name='TT1';
INDEX_NAME INDEX_TYPE STATUS
--------------- --------------- ----------------
INX_TT1_OWNER BITMAP VALID
luocs@MAA> select COLUMN_NAME, NUM_DISTINCT from user_tab_cols where table_name='TT1';
COLUMN_NAME NUM_DISTINCT
------------------------------------------------------------ ------------
OWNER 15
OBJECT_NAME 16272
SUBOBJECT_NAME 333
OBJECT_ID 21204
DATA_OBJECT_ID 6221
OBJECT_TYPE 36
CREATED 855
LAST_DDL_TIME 907
TIMESTAMP 935
STATUS 1
TEMPORARY 2
GENERATED 2
SECONDARY 2
NAMESPACE 15
EDITION_NAME 0
-- 查看OWNER为SYS和SYSTEM的记录数
luocs@MAA> select count(*) from tt1 where owner='SYS';
COUNT(*)
----------
152960
luocs@MAA> select count(*) from tt1 where owner='SYSTEM';
COUNT(*)
----------
8208
-- 查看执行计划
luocs@MAA> explain plan for select count(*) from tt1 where owner='SYS';
Explained.
luocs@MAA> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 945689350
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 7 | 7 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 7 | | |
| 2 | BITMAP CONVERSION COUNT | | 153K| 1047K| 7 (0)| 00:00:01 |
|* 3 | BITMAP INDEX SINGLE VALUE| INX_TT1_OWNER | | | | |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("OWNER"='SYS')
15 rows selected.
luocs@MAA> explain plan for select count(*) from tt1 where owner='SYSTEM';
Explained.
luocs@MAA> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 945689350
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 7 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 7 | | |
| 2 | BITMAP CONVERSION COUNT | | 8390 | 58730 | 1 (0)| 00:00:01 |
|* 3 | BITMAP INDEX SINGLE VALUE| INX_TT1_OWNER | | | | |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("OWNER"='SYSTEM')
15 rows selected.
-- 通过10046事件查看使用绑定变量后的结果
luocs@MAA> alter session set events '10046 trace name context forever, level 4';
Session altered.
luocs@MAA> variable l varchar2(10)
luocs@MAA> exec :l:='SYS'
PL/SQL procedure successfully completed.
luocs@MAA> select count(*) from tt1 where owner=:l;
COUNT(*)
----------
152960
luocs@MAA> exec :l:='SYSTEM'
PL/SQL procedure successfully completed.
luocs@MAA> select count(*) from tt1 where owner=:l;
COUNT(*)
----------
8208
luocs@MAA> alter session set events '10046 trace name context off';
Session altered.
luocs@MAA> select value from v$diag_info where name = 'Default Trace File';
VALUE
----------------------------------------------------------------------------------------------------
/u01/app/oracle/diag/rdbms/maa/maa/trace/maa_ora_9081.trc
-- 格式化下
[oracle@maa3 ~]$ tkprof /u01/app/oracle/diag/rdbms/maa/maa/trace/maa_ora_9081.trc luocs.txt aggregate=no
-- trace部分内容
SQL ID: 6yw9t0j4w7hvh Plan Hash: 0
BEGIN :l:='SYS'; END;
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 1
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 0.00 0.00 0 0 0 1
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 51
********************************************************************************
SQL ID: dbuap8tqp73th Plan Hash: 945689350
select count(*)
from
tt1 where owner=:l
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 7 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.00 0.00 0 7 0 1
Misses in library cache during parse: 1
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 51
Number of plan statistics captured: 1
Rows (1st) Rows (avg) Rows (max) Row Source Operation
---------- ---------- ---------- ---------------------------------------------------
1 1 1 SORT AGGREGATE (cr=7 pr=0 pw=0 time=276 us)
11 11 11 BITMAP CONVERSION COUNT (cr=7 pr=0 pw=0 time=225 us cost=7 size=1072232 card=153176)
11 11 11 BITMAP INDEX SINGLE VALUE INX_TT1_OWNER (cr=7 pr=0 pw=0 time=67 us)(object id 25884)
-- 注意，这里第一次绑定变量值为SYS的时候card=153176，执行计划很准确
********************************************************************************
SQL ID: fvp79xru6ftzd Plan Hash: 0
BEGIN :l:='SYSTEM'; END;
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 1
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 0.00 0.00 0 0 0 1
Misses in library cache during parse: 1
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 51
********************************************************************************
SQL ID: dbuap8tqp73th Plan Hash: 945689350
select count(*)
from
tt1 where owner=:l
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 2 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.00 0.00 0 2 0 1
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 51
Number of plan statistics captured: 1
Rows (1st) Rows (avg) Rows (max) Row Source Operation
---------- ---------- ---------- ---------------------------------------------------
1 1 1 SORT AGGREGATE (cr=2 pr=0 pw=0 time=81 us)
1 1 1 BITMAP CONVERSION COUNT (cr=2 pr=0 pw=0 time=68 us cost=7 size=1072232 card=153176)
1 1 1 BITMAP INDEX SINGLE VALUE INX_TT1_OWNER (cr=2 pr=0 pw=0 time=41 us)(object id 25884)
-- 但紧接给变量传SYSTEM值，CBO认为card依然为153176，这是不正确的，也可能会影响执行计划。
********************************************************************************
以上简单的示例可以证明，绑定变量适合于OLTP系统而不适合于OLAP系统。
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