2013-04-22不适当的索引导致的性能问题--影响CBO的判断
现场反馈系统慢，做了一个AWR报告，平时上班时间一个小时正常DB time为100min，现在已经远远超过这个数。数据库服务器CPU有4个。
| Snap Id | Snap Time | Sessions | Cursors/Session | |
|---|---|---|---|---|
| Begin Snap: | 31355 | 22-Apr-13 11:00:48 | 91 | 21.4 |
| End Snap: | 31356 | 22-Apr-13 12:01:01 | 105 | 17.5 |
| Elapsed: | 60.21 (mins) | |||
| DB Time: | 1,777.30 (mins) |
| Event | Waits | Time(s) | Avg Wait(ms) | % Total Call Time | Wait Class |
|---|---|---|---|---|---|
| latch: cache buffers chains | 34,983 | 11,720 | 335 | 11.0 | Concurrency |
| CPU time | 11,527 | 10.8 | |||
| latch free | 442 | 88 | 200 | .1 | Other |
| db file sequential read | 61,933 | 71 | 1 | .1 | User I/O |
| db file scattered read | 13,744 | 15 | 1 | .0 | User I/O |
可以看到SQL的执行时间远远大于SQL消耗CPU的时间。老问题，单条SQL逻辑读过高，在并发的情况下，产生大量资源争用，平均每次逻辑读1,109,335。
| Elapsed Time (s) | CPU Time (s) | Executions | Elap per Exec (s) | % Total DB Time | SQL Id | SQL Module | SQL Text |
|---|---|---|---|---|---|---|---|
| 44,559 | 4,145 | 113 | 394.32 | 41.79 | 435xza3c04ac9 | SELECT GG_JOB_ITEM.*, PROD_... | |
| 44,551 | 4,392 | 113 | 394.25 | 41.78 | 8jjqdn6bw4f2p | SELECT COUNT(1) C FROM GG_JOB... |
问题SQL语句：
SELECT COUNT(1) C
FROM GG_JOB_ITEM, GG_JOB_PATROL_DETAIL
WHERE GG_JOB_ITEM.TASK_ID = GG_JOB_PATROL_DETAIL.TASK_ID(+)
AND GG_JOB_ITEM.TASK_KIND = :1
AND GG_JOB_ITEM.TASK_SOURCE = :2
AND GG_JOB_ITEM.IS_COMPLETE = 5
AND NOT EXISTS (SELECT 1
FROM GG_JOB_ITEM A
WHERE A.TASK_KIND = :3
AND A.TASK_SOURCE != :4
AND A.TASK_SOURCE_ID = GG_JOB_ITEM.TASK_ID)
AND EXISTS
(SELECT 1
FROM GG_JOB_ITEM B
WHERE B.TASK_KIND = :5
AND B.IS_COMPLETE != 5
AND B.TASK_ID = GG_JOB_ITEM.TASK_SOURCE_ID)
AND ((GG_JOB_ITEM.TASK_BEGIN_DATE >= :6 AND
GG_JOB_ITEM.TASK_BEGIN_DATE <= :7) OR
(GG_JOB_ITEM.TASK_END_DATE >= :8 AND
GG_JOB_ITEM.TASK_END_DATE <= :9) OR
(GG_JOB_ITEM.TASK_BEGIN_DATE <= :10 AND
GG_JOB_ITEM.TASK_END_DATE >= :11))
AND GG_JOB_ITEM.TASK_KIND = :12
AND GG_JOB_ITEM.IS_COMPLETE = :13
AND GG_JOB_ITEM.TASK_TYPE IN
(1, 2, 6, 4, 21, 3, 7, 8, 9, 22, 23, 26, 29, 11, 12, 14, 19, 24);
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1353 (100)| |
| 1 | SORT AGGREGATE | | 1 | 63 | | |
|* 2 | FILTER | | | | | |
| 3 | NESTED LOOPS ANTI | | 1 | 63 | 1353 (1)| 00:00:17 |
| 4 | NESTED LOOPS SEMI | | 1 | 52 | 1352 (1)| 00:00:17 |
| 5 | NESTED LOOPS OUTER | | 1 | 42 | 1351 (1)| 00:00:17 |
|* 6 | TABLE ACCESS BY INDEX ROWID| GG_JOB_ITEM | 1 | 37 | 1350 (1)| 00:00:17 |
|* 7 | INDEX RANGE SCAN | INDEX_TASK_KIND | 107K| | 63 (0)| 00:00:01 |
|* 8 | INDEX UNIQUE SCAN | PK_GG_JOB_PATROL_DETAILA | 1 | 5 | 1 (0)| 00:00:01 |
|* 9 | TABLE ACCESS BY INDEX ROWID | GG_JOB_ITEM | 86225 | 842K| 1 (0)| 00:00:01 |
|* 10 | INDEX UNIQUE SCAN | PK_GG_JOB_ITEM | 1 | | 1 (0)| 00:00:01 |
|* 11 | TABLE ACCESS BY INDEX ROWID | GG_JOB_ITEM | 107K| 1151K| 1 (0)| 00:00:01 |
|* 12 | INDEX RANGE SCAN | INDEX_TASK_KIND | 3 | | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------------
对表GG_JOB_ITEM进行分析，记录有三十多万，有8个索引，其中在TASK_KIND和TASK_TYPE上建了索引，TASK_KIND只有3个值，TASK_TYPE只有20个值，选择性如此差的字段不能建索引，处理方法是全部drop。
只是删除了两个索引，更新表的统计信息后再次执行SQL，执行计划为：
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 966 | 4980 (2)|
| 1 | NESTED LOOPS OUTER | | 3 | 966 | 4980 (2)|
| 2 | NESTED LOOPS SEMI | | 3 | 891 | 4979 (2)|
| 3 | HASH JOIN ANTI | | 233 | 66871 | 4932 (2)|
| 4 | TABLE ACCESS FULL | PROD_TASK_ITEM | 234 | 64584 | 2466 (2)|
| 5 | TABLE ACCESS FULL | PROD_TASK_ITEM | 105K| 1129K| 2465 (2)|
| 6 | TABLE ACCESS BY INDEX ROWID| PROD_TASK_ITEM | 1347 | 13470 | 1 (0)|
| 7 | INDEX UNIQUE SCAN | PK_PROD_TASK_ITEM | 1 | | 1 (0)|
| 8 | TABLE ACCESS BY INDEX ROWID | PROD_TASK_PATROL_DETAIL | 1 | 25 | 1 (0)|
| 9 | INDEX UNIQUE SCAN | PK_PROD_TASK_PATROL_DETAILA | 1 | | 1 (0)|
--------------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
22722 consistent gets
27 physical reads
0 redo size
17193 bytes sent via SQL*Net to client
281 bytes received via SQL*Net from client
8 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
93 rows processed