创建索引前后执行计划的变更和软硬解析的比较
创建索引前后执行计划的变更和软硬解析的比较
环境:OEL+Oracle 10.2.0.5 Rac
今天在itpub上回答一个网友的提问,RT:我第一次执行了一条sql之后,这条sql通过了硬解析,得到了执行计划,当再次执行这条sql时,会进行软解析是吧,不会通过优化器得到新的执行计划。如果我增加了一条索引,通过索引执行这条sql更好,在执行这条sql是进行软解析吗?(统计信息的改变,会导致sql进行硬解析吗?)
我当时的回答是:有索引了,统计信息变了。走索引了,执行计划变了。 但是软硬解析是对于SQL语句而言的吧?只要共享池中存在此SQL副本,将直接执行软解析;个人认为未经易做图yze表前,会被软解析
答案应该是硬解析;
中午午休的时候,趴那儿回顾了一下这个案例,但是思前想后总感觉有点不对,一切以事实说话,决定起来测试一下;测试过程和结果如下:
[sql] SQL> show user USER is "SYS" SQL> drop index tt_idx; drop index tt_idx * ERROR at line 1: ORA-01418: specified index does not exist SQL> drop table tt purge; drop table tt purge * ERROR at line 1: ORA-00942: table or view does not exist
创建一张新表TT:
[sql] SQL> create table tt as select * from dba_objects;
Table created.
打开autotrace并开始第一次根据条件检索新表tt:
[sql] SQL> set autotrace on; SQL> select object_id,object_name from tt where object_id=10; OBJECT_ID OBJECT_NAME</span> ---------- -------------------------------------- 10 C_USER# Execution Plan ---------------------------------------------------------- Plan hash value: 264906180 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 8 | 632 | 156 (1)| 00:00:02 | |* 1 | TABLE ACCESS FULL| TT | 8 | 632 | 156 (1)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("OBJECT_ID"=10) Note ----- - dynamic sampling used for this statement Statistics ---------------------------------------------------------- 68 recursive calls 0 db block gets 785 consistent gets 701 physical reads 0 redo size 481 bytes sent via SQL*Net to client 400 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL>
可以很清晰的从执行计划中看出,优化器走了全表扫描,并执行了硬解析;
再次执行上述查询:
[sql] SQL> select object_id,object_name from tt where object_id=10; OBJECT_ID OBJECT_NAME ---------- --------------------------------------- 10 C_USER# Execution Plan ---------------------------------------------------------- Plan hash value: 264906180 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 8 | 632 | 156 (1)| 00:00:02 | |* 1 | TABLE ACCESS FULL| TT | 8 | 632 | 156 (1)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("OBJECT_ID"=10) Note ----- - dynamic sampling used for this statement Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 707 consistent gets 0 physical reads 0 redo size 481 bytes sent via SQL*Net to client 400 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL>
执行计划虽依然走全表扫描,但是执行了软解析,这里的主要原因是在共享池中存在同样的SQL语句的一个副本(两条语句一模一样)
[sql] SQL> select object_id,OBJECT_NAME from tt where object_id=10; OBJECT_ID OBJECT_NAME ---------- ------------------------------------------------- 10 C_USER# Execution Plan ---------------------------------------------------------- Plan hash value: 264906180 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 8 | 632 | 156 (1)| 00:00:02 | |* 1 | TABLE ACCESS FULL| TT | 8 | 632 | 156 (1)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("OBJECT_ID"=10) Note ----- - dynamic sampling used for this statement Statistics ---------------------------------------------------------- 4 recursive calls 0 db block gets 779 consistent gets 0 physical reads 0 redo size 481 bytes sent via SQL*Net to client 400 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> select OBJECT_ID,OBJECT_NAME from tt where object_id=10; OBJECT_ID OBJECT_NAME ---------- ---------------------------------------------- 10 C_USER# Execution Plan ---------------------------------------------------------- Plan hash value: 264906180 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 8 | 632 | 156 (1)| 00:00:02 | |* 1 | TABLE ACCESS FULL| TT | 8 | 632 | 156 (1)| 00:00:02 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("OBJECT_ID"=10) Note ----- - dynamic sampling used for this statement Statistics ---------------------------------------------------------- 4 recursive calls 0 db block gets 779 consistent gets 0 physical reads 0 redo size 481 bytes sent via SQL*Net to client 400 bytes received via SQL*Net from client
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