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本篇內容主要講解“ACS與PL/SQL的工作情況分析”，感興趣的朋友不妨來看看。本文介紹的方法操作簡單快捷，實用性強。下面就讓小編來帶大家學習“ACS與PL/SQL的工作情況分析”吧!
我們來看下ACS在PL/SQL里的工作情況，結果可能會令你非常失望。
首先構造一個PL/SQL，里面使用到了我們在本章中創建的表T：
SQL>var sql_id varchar2(255) SQL>alter system flush shared_pool; System altered. 2 x integer; 3 n number; 4 begin 5 for i in 1..10 loop 6 if i = 1 then 7 x := 500000; 8 else 9 x := 1; 10 end if; 11 select count(object_id) into n from t where id > x; 12 end loop; 13 end; 14 / PL/SQL procedure successfully completed. |
這段PL/SQL首先會執行‘select count(object_id) into n from t where id > 100000 ’1次，然后會執行‘select count(object_id) into n from t where id >1’ 9次，執行完成后，我們來看看是否會使用到ACS。
SQL>select 2 sql_id 3 , child_number 4 , executions 5 , parse_calls 6 , buffer_gets 7 , is_bind_sensitive 8 , is_bind_aware 9 from 10 v$sql 11 where 12 sql_id =' gp03v5aw085v3'; SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS IS IS --------------- ------------ ---------- ----------- ----------- -- -- gp03v5aw085v3 0 10 1 646875 Y N |
非常可惜，這個SQL并沒有產生多個子游標，雖然已經識別到這個SQL為綁定敏感is_bind_sensitive='Y'，但是is_bind_aware='N'。
SQL> SELECT hash_value, sql_id, child_number, bucket_id, COUNT 2 FROM v$sql_cs_histogram 3 WHERE sql_id='gp03v5aw085v3' 4 ORDER BY sql_id, child_number; HASH_VALUE SQL_ID CHILD_NUMBER BUCKET_ID COUNT ---------- --------------- ------------ ---------- ---------- 3087275875 gp03v5aw085v3 0 0 1 3087275875 gp03v5aw085v3 0 2 9 3087275875 gp03v5aw085v3 0 1 0 |
雖然v$sql_cs_histogram已經監控到了處理行數的巨大改變，但是卻沒有生成新的游標。
PLAN_TABLE_OUTPUT ---------------------------------------------------------------------------------------- SQL_ID gp03v5aw085v3, child number 0 ------------------------------------- SELECT COUNT(OBJECT_ID) FROM T WHERE ID > :B1 Plan hash value: 3694077449 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 4 (100)| | | 1 | SORT AGGREGATE | | 1 | 10 | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 1 | 10 | 4 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | I | 1 | | 3 (0)| 00:00:01 | ------------------------------------------------------------------------------------- Peeked Binds (identified by position): -------------------------------------- 1 - :B1 (NUMBER): 5000000 Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("ID">:B1) |
執行計劃一直沿用的是第一次產生的執行計劃，根據Peeked Binds (identified by position):的值為5000000可以推斷出來。我們嘗試在SQL里增加bind_aware hint看看，這個hint的作用在本章的最佳實踐小節中介紹，這里不再贅述。
SQL>declare 2 x integer; 3 n number; 4 begin 5 for i in 1..10 loop 6 if i = 1 then 7 x := 5000000; 8 else 9 x := 1; 10 end if; 11 select /*+ bind_aware */count(object_id) into n from t where id > x; 12 end loop; 13 end; 14 / PL/SQL procedure successfully completed. SQL>select 2 sql_id 3 , child_number 4 , executions 5 , parse_calls 6 , buffer_gets 7 , is_bind_sensitive 8 , is_bind_aware 9 from 10 v$sql 11 where 12 sql_id = '5542a2rzny69v'; SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS IS IS --------------- ------------ ---------- ----------- ----------- -- -- 5542a2rzny69v 0 10 1 687396 Y Y |
雖然優化器已經標記這個游標is_bind_aware='Y'了，但是依然沒有新的游標產生出來。在MOS上查找類似的問題，會發現一個BUG：
Bug 8357294 : ADAPTIVE cursor SHARING DOESN'T WORK FOR STATIC SQL cursorS FROM PL/SQL
標題的意思是由于BUG，ACS不能工作在PL/SQL的靜態游標里。但是根據測試動態游標也不能工作。
SQL>declare 2 x integer; 3 n number; 4 begin 5 for i in 1..10 loop 6 if i = 1 then 7 x := 5000000; 8 else 9 x := 1; 10 end if; 11 execute immediate 12 'select count(object_id) from t where id > :x' into n using x; 13 end loop; 14 end; 15 / PL/SQL procedure successfully completed. SQL>select 2 sql_id 3 , child_number 4 , executions 5 , parse_calls 6 , buffer_gets 7 , is_bind_sensitive 8 , is_bind_aware 9 from 10 v$sql 11 where 12 sql_id = '6qwg6gauwbpm8'; SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS IS IS --------------- ------------ ---------- ----------- ----------- -- -- 6qwg6gauwbpm8 0 10 1 687580 Y N |
文中提到了Session_Cached_Cursors在設置為0后，ACS就可以正常工作了，經過試驗也如它所說。
SQL>alter session set Session_Cached_Cursors=0; Session altered. SQL>alter system flush shared_pool; System altered. SQL>declare 2 x integer; 3 n number; 4 begin 5 for i in 1..10 loop 6 if i = 1 then 7 x := 5000000; 8 else 9 x := 1; 10 end if; 11 select count(object_id) into n from t where id > x; 12 end loop; 13 end; 14 / PL/SQL procedure successfully completed. SQL>select 2 sql_id 3 , child_number 4 , executions 5 , parse_calls 6 , buffer_gets 7 , is_bind_sensitive 8 , is_bind_aware 9 from 10 v$sql 11 where 12 sql_id = ' gp03v5aw085v3'; SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS IS IS --------------- ------------ ---------- ----------- ----------- -- -- gp03v5aw085v3 0 2 3 76405 Y N gp03v5aw085v3 1 8 7 517480 Y Y SQL>select * from table(dbms_xplan.display_cursor('gp03v5aw085v3',null)); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------- SQL_ID gp03v5aw085v3, child number 0 ------------------------------------- SELECT COUNT(OBJECT_ID) FROM T WHERE ID > :B1 Plan hash value: 3694077449 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 4 (100)| | | 1 | SORT AGGREGATE | | 1 | 10 | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 1 | 10 | 4 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | I | 1 | | 3 (0)| 00:00:01 | ------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("ID">:B1) SQL_ID gp03v5aw085v3, child number 1 ------------------------------------- SELECT COUNT(OBJECT_ID) FROM T WHERE ID > :B1 Plan hash value: 2966233522 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 14373 (100)| | | 1 | SORT AGGREGATE | | 1 | 10 | | | |* 2 | TABLE ACCESS FULL| T | 4999K| 47M| 14373 (2)| 00:02:53 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("ID">:B1) |
可以看到ACS已經工作了，在v$sql_cs_histogram里也為新游標產生了新的行。
SQL> SELECT hash_value, sql_id, child_number, bucket_id, COUNT 2 FROM v$sql_cs_histogram 3 WHERE sql_id='gp03v5aw085v3' 4 ORDER BY sql_id, child_number; HASH_VALUE SQL_ID CHILD_NUMBER BUCKET_ID COUNT ---------- --------------- ------------ ---------- ---------- 3087275875 gp03v5aw085v3 0 1 0 3087275875 gp03v5aw085v3 0 0 1 3087275875 gp03v5aw085v3 0 2 1 3087275875 gp03v5aw085v3 1 1 0 3087275875 gp03v5aw085v3 1 0 0 3087275875 gp03v5aw085v3 1 2 8 |
到此，相信大家對“ACS與PL/SQL的工作情況分析”有了更深的了解，不妨來實際操作一番吧！這里是億速云網站，更多相關內容可以進入相關頻道進行查詢，關注我們，繼續學習！
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