一、示例自适应游标共享 1、创建演示环境
SQL> select * from v$version where rownum<2; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production SQL> create table t(id,owner,object_id) as 2 select rownum,owner,object_id from all_objects where rownum<=1000; SQL> alter table t add constraint t_pk primary key(id); SQL> exec dbms_stats.gather_table_stats('SCOTT','T',cascade=>true); SQL> select count(id),count(distinct id),min(id),max(id) from t; COUNT(ID) COUNT(DISTINCTID) MIN(ID) MAX(ID) ---------- ----------------- ---------- ---------- 1000 1000 1 1000 2、使用绑定变量执行SQL语句并获得首次执行情况
SQL> var v_id number; SQL> exec :v_id:=9; SQL> set linesize 180 SQL> select sum(object_id) from t where id<:v_id; SUM(OBJECT_ID) -------------- 2078 SQL> select * from table(dbms_xplan.display_cursor(null,null,'typical -predicate')); SQL_ID 7qcp6urqh7d2j, child number 0 ------------------------------------- select sum(object_id) from t where id<:v_id -->变量值为9时，使用了正确的执行计划，且预估的行数也准确 Plan hash value: 4270555908 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 8 | 64 | 3 (0)| 00:00:01 | | 3 | INDEX RANGE SCAN | T_PK | 8 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------- SQL> col SQL_TEXT format a45 -->下面的语句获得自适应游标共享的3个字段值 SQL> select sql_id,sql_text,child_number,executions,is_bind_sensitive,is_bind_aware,is_shareable 2 from v$sql where sql_text like '%select sum(object_id) from t where%' and sql_text not like '%from v$sql%'; SQL_ID SQL_TEXT CHILD_NUMBER EXECUTIONS I I I ------------- --------------------------------------------- ------------ ---------- - - - 7qcp6urqh7d2j select sum(object_id) from t where id<:v_id 0 1 Y N Y
3、自适应游标共享的外在体现
自适应游标共享主要通过三个字段来得以体现，即is_bind_sensitive,is_bind_aware,is_shareable。(注:此三个字段仅在Oracle 11g 中存在)。通过上面从v$sql(v$sqlarea中不存在is_shareable)的查询可知，三个字段分别被赋予了不同的值，代表了不同的含义。 is_bind_sensitive(绑定是否敏感) 表示该子游标中是否使用了绑定变量要素，且采用bind peeking方法进行执行计划生成。如果执行计划依赖于窥探到的值，此处为Y， 否则为N。 is_bind_aware(绑定是否可知) 表示该子游标是否使用了extended cursor sharing技术，是则为Y，否则为N，如为N，则该游标将废弃，不再可用。 is_shareable(是否可共享) 表示该子游标可否被下次软解析是否可共享使用。可共享则为Y，否则为N，表示该子游标失去了共享价值，按LRU算法淘汰。 由于该SQL语句为首次执行，因此从v$sql查询的结果中得知 is_bind_sensitive 为Y值(首次运行，执行了bind peeking) is_bind_aware 为N值(首次运行，不被extended cursor sharing支持) is_shareable 为Y值(执行计划可共享)4、重新赋值后观察游标共享情况
SQL> exec :v_id:=900; SQL> select sum(object_id) from t where id<:v_id; SUM(OBJECT_ID) -------------- 1826561 SQL> select * from table(dbms_xplan.display_cursor(null,null,'typical -predicate')); SQL_ID 7qcp6urqh7d2j, child number 0 ------------------------------------- select sum(object_id) from t where id<:v_id -->此次执行的变量值为900，执行计划位上次变量为9的执行计划 -->此时为非正确的执行计划，等同于Oracle 9i,10g中的情形 Plan hash value: 4270555908 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 8 | 64 | 3 (0)| 00:00:01 | | 3 | INDEX RANGE SCAN | T_PK | 8 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------- -->自适应游标共享的3个字段值并未发生任何变化 SQL> select sql_id,sql_text,child_number,executions,is_bind_sensitive,is_bind_aware,is_shareable 2 from v$sql where sql_text like '%select sum(object_id) from t where%' and sql_text not like '%from v$sql%'; SQL_ID SQL_TEXT CHILD_NUMBER EXECUTIONS I I I ------------- --------------------------------------------- ------------ ---------- - - - 7qcp6urqh7d2j select sum(object_id) from t where id<:v_id 0 2 Y N Y SQL> select sum(object_id) from t where id<:v_id; -->再次执行变量为900值的SQL语句 SUM(OBJECT_ID) -------------- 1826561 SQL> select * from table(dbms_xplan.display_cursor(null,null,'typical -predicate')); SQL_ID 7qcp6urqh7d2j, child number 1 ------------------------------------- select sum(object_id) from t where id<:v_id -->此时执行计划较上一次发生了变化，使用了全表扫描,Rows接近于实际值 -->自适应游标共享特性得以体现 Plan hash value: 2966233522 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS FULL| T | 900 | 7200 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- -->自适应游标共享特性的几个值发生了变化，生成了新的子游标，其子游标号为1 SQL> select sql_id,sql_text,child_number,executions,is_bind_sensitive,is_bind_aware,is_shareable 2 from v$sql where sql_text like '%select sum(object_id) from t where%' and sql_text not like '%from v$sql%'; SQL_ID SQL_TEXT CHILD_NUMBER EXECUTIONS I I I ------------- --------------------------------------------- ------------ ---------- - - - 7qcp6urqh7d2j select sum(object_id) from t where id<:v_id 0 2 Y N Y 7qcp6urqh7d2j select sum(object_id) from t where id<:v_id 1 1 Y Y Y SQL> exec :v_id:=800 -->为变量赋于不同的值 SQL> select sum(object_id) from t where id<:v_id; -->利用新的变量值执行SQL语句 SUM(OBJECT_ID) -------------- 1548431 SQL> select sql_id,child_number,executions,is_bind_sensitive,is_bind_aware,is_shareable 2 from v$sql where sql_text like '%select sum(object_id) from t where%' and sql_text not like '%from v$sql%'; SQL_ID CHILD_NUMBER EXECUTIONS I I I ------------- ------------ ---------- - - - 7qcp6urqh7d2j 0 2 Y N Y 7qcp6urqh7d2j 1 1 Y Y N 7qcp6urqh7d2j 2 1 Y Y Y -->生成了新的子游标号为2 SQL> exec :v_id:=500; -->为变量赋于新值 SQL> select sum(object_id) from t where id<:v_id; -->利用新的变量值执行SQL语句 SUM(OBJECT_ID) -------------- 826694 /**************************************************/ /* Author: Robinson Cheng */ /* Blog: http://blog.csdn.net/robinson_0612 */ /* MSN: robinson_0612@hotmail.com */ /* QQ: 645746311 */ /**************************************************/ SQL> select sql_id,child_number,executions,is_bind_sensitive,is_bind_aware,is_shareable 2 from v$sql where sql_text like '%select sum(object_id) from t where%' and sql_text not like '%from v$sql%'; SQL_ID CHILD_NUMBER EXECUTIONS I I I ------------- ------------ ---------- - - - 7qcp6urqh7d2j 0 2 Y N Y 7qcp6urqh7d2j 1 1 Y Y N 7qcp6urqh7d2j 2 1 Y Y N -->注意看子游标1,2的is_shareable值为N，表示不可共享 7qcp6urqh7d2j 3 1 Y Y Y -->生成了新的子游标号为3， -->查看最终该SQL语句的不同子游标的所有执行计划 SQL> select * from table(dbms_xplan.display_cursor('7qcp6urqh7d2j',null,'typical -predicate')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------ SQL_ID 7qcp6urqh7d2j, child number 0 ------------------------------------- select sum(object_id) from t where id<:v_id -->0号子游标为索引范围扫描 Plan hash value: 4270555908 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 8 | 64 | 3 (0)| 00:00:01 | | 3 | INDEX RANGE SCAN | T_PK | 8 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------- SQL_ID 7qcp6urqh7d2j, child number 1 ------------------------------------- select sum(object_id) from t where id<:v_id -->1号子游标为全表扫描，其预估的行数接近实际影响行数的值为900 Plan hash value: 2966233522 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS FULL| T | 900 | 7200 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- SQL_ID 7qcp6urqh7d2j, child number 2 ------------------------------------- select sum(object_id) from t where id<:v_id -->2号子游标为全表扫描，但其预估的行数接近实际影响行数的值为800 Plan hash value: 2966233522 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS FULL| T | 800 | 6400 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- SQL_ID 7qcp6urqh7d2j, child number 3 ------------------------------------- select sum(object_id) from t where id<:v_id -->3号子游标为全表扫描，但其预估的行数等于实际影响行数的值499 Plan hash value: 2966233522 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 3 (100)| | | 1 | SORT AGGREGATE | | 1 | 8 | | | | 2 | TABLE ACCESS FULL| T | 499 | 3992 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- 二、自适应游标共享的几个相关视图
1、v$sql_cs_statistics 用于监控自适应游标共享的相关统计信息.下面的查询中列出了每个子游标的peeking情况,以及执行次数,预处理行数,BUFFER_GETS等
SQL> select child_number,bind_set_hash_value,peeked,executions,rows_processed,buffer_gets 2 from v$sql_cs_statistics where sql_id='7qcp6urqh7d2j' 3 order by 1; CHILD_NUMBER BIND_SET_HASH_VALUE P EXECUTIONS ROWS_PROCESSED BUFFER_GETS ------------ ------------------- - ---------- -------------- ----------- 0 1706589901 Y 1 17 69 -->17行，索引范围扫描 1 3116944019 Y 1 900 5 -->900行，全表扫描 2 1328865654 Y 1 800 5 -->800行，全表扫描 3 1624350242 Y 1 500 5 -->500行，全表扫描
2、v$sql_cs_selectivity
显示每个子游标的游标的选择性范围。下面的查询中列出了谓词，选择性范围，列上的选择性的值SQL> select child_number,predicate,range_id,low,high from v$sql_cs_selectivity 2 where sql_id='7qcp6urqh7d2j' order by 1; CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH ------------ ------------------ ---------- ---------- ---------- 1
3、v$sql_cs_histogram
用于决定一个查询是否允许自适应游标共享，以直方图形式存储SQL> select CHILD_NUMBER,BUCKET_ID,COUNT from v$sql_cs_histogram where SQL_ID='7qcp6urqh7d2j' 2 order by 1; CHILD_NUMBER BUCKET_ID COUNT ------------ ---------- ---------- 0 1 1 0 0 1 0 2 0 1 1 0 1 0 1 1 2 0 2 1 0 2 0 1 2 2 0 3 1 0 3 0 1 3 2 0
三、总结 1、自适应游标共享在SQL语句首次执行时(使用绑定变量)，进行窥探，并记录窥探结果，如果后续有相同的的SQL语句执行，则对窥探结果 进行比较以判断是否需要生成新的执行计划。此即为绑定变量是否敏感。 2、绑定变量的可知性用于判断当前的游标是否为可扩展性游标共享，当不可知时，则游标被废弃。 3、自适应游标共享的实质是在Oracle 10g以前的基础上实现了多次绑定变量窥探，增加了获取最佳执行计划选择的机率。 4、尽管使用自适应游标共享特性，但并不能保证每次执行SQL语句一定按最佳计划执行，本次演示中SQL语句的第二次执行并非最佳执行计划。 5、自适应游标共享也不能保证两次执行相同SQL语句一定按相同的执行计划执行，因为自适应游标共享会不断尝试peeking。 四、延伸参考
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