CBO一个错误的索引选择会带来的后果
在对20亿记录大表的查询中，发现Oracle的执行计划选择并不稳定，当然这是CBO的正常行为，然而当选择不同时，结果是巨大的。
在以下查询中，使用指定的索引，查询快速得出结果，但是这依赖于Hints的强制指定：
SQL> select /*+ index(smsmg IDX_smsmg_DEST_MDN) */ count(*)
2 from smsmg where msg_to_dest_mdn='861318888888' and service_id='54';
COUNT(*)
----------
1
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 1659057974
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 18 | 98 (0)| 00:00:02 | | |
| 1 | SORT AGGREGATE | | 1 | 18 | | | | |
|* 2 | TABLE ACCESS BY GLOBAL INDEX ROWID| smsmg | 1 | 18 | 98 (0)| 00:00:02 | ROWID | ROWID |
|* 3 | INDEX RANGE SCAN | IDX_smsmg_msg_to_des_mdn | 106 | | 4 (0)| 00:00:01 | | |
--------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("SERVICE_ID"='54')
3 - access("msg_to_dest_mdn"='861318888888')
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
82 consistent gets
0 physical reads
0 redo size
515 bytes sent via SQL*Net to client
469 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
而如果不加Hints，查询是一时无法得出结果的：
SQL> select count(*) from smsmg where msg_to_dest_mdn='861318888888' and service_id='54';
select count(*) from smsmg where msg_to_dest_mdn='861318888888' and service_id='54'
*
ERROR at line 1:
ORA-01013: user requested cancel of current operation
Elapsed: 00:04:27.88
其执行计划显示，这一缺省的执行方式导致了错误的索引选择：
SQL> set autotrace trace explain
SQL> select count(*) from smsmg where msg_to_dest_mdn='861318888888' and service_id='54';
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 1152948967
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 18 | 5 (0)| 00:00:01 | | |
| 1 | SORT AGGREGATE | | 1 | 18 | | | | |
|* 2 | TABLE ACCESS BY GLOBAL INDEX ROWID| smsmg | 1 | 18 | 5 (0)| 00:00:01 | ROWID | ROWID |
|* 3 | INDEX RANGE SCAN | IDX_smsmg_SERVICE_ID | 1 | | 4 (0)| 00:00:01 | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("msg_to_dest_mdn"='861318888888')
3 - access("SERVICE_ID"='54')
说CBO是Oracle最为博大精深的技术一点也不为过，只是这技术越复杂越深奥出错的机会就越多了。
这个表的数据量大约是500G：
SQL> select segment_name,partition_name,bytes/1024/1024/1024 GB,blocks from dba_segments
2 where owner='SMSMSG' and segment_name='SMSSENDMSG';
SEGMENT_NAME PARTITION_NAME GB BLOCKS
------------------------------ --------------- ---------- ----------
SMSSENDMSG M01 30.625 4014080
SMSSENDMSG M02 29.875 3915776
SMSSENDMSG M03 43.25 5668864
SMSSENDMSG M04 38 4980736
SMSSENDMSG M05 43.1875 5660672
SMSSENDMSG M06 50.6875 6643712
SMSSENDMSG M08 55.4375 7266304
SMSSENDMSG M09 32.125 4210688
SMSSENDMSG M10 23.9375 3137536
SMSSENDMSG M11 25.6875 3366912
SMSSENDMSG M12 31.9375 4186112
SEGMENT_NAME PARTITION_NAME GB BLOCKS
------------------------------ --------------- ---------- ----------
SMSSENDMSG M13 .000061035 8
SMSSENDMSG M07 58.5 7667712
13 rows selected.
SQL> select sum(bytes)/1024/1024/1024 GB from dba_segments where owner='SMSMSG' and segment_name='SMSSENDMSG';
GB
----------
463.250061
每个分区的记录数量大约是1~2亿条：
SQL> select table_owner,table_name,partition_name,num_rows from dba_tab_partitions
2 where table_owner='SMS9885' and table_name='SMS_TO_ISMG';
TABLE_OWNER TABLE_NAME PARTITION_NAME NUM_ROWS
------------------------------ ------------------------------ --------------- ----------
SMSMSG SMSSENDMSG M01 135605804
SMSMSG SMSSENDMSG M02 134599287
SMSMSG SMSSENDMSG M03 187959758
SMSMSG SMSSENDMSG M04 169663942
SMSMSG SMSSENDMSG M05 187435468
SMSMSG SMSSENDMSG M06 222079762
SMSMSG SMSSENDMSG M07 256482704
SMSMSG SMSSENDMSG M08 229089535
SMSMSG SMSSENDMSG M09 122453724
SMSMSG SMSSENDMSG M10 104093080
SMSMSG SMSSENDMSG M11 116095184
TABLE_OWNER TABLE_NAME PARTITION_NAME NUM_ROWS
------------------------------ ------------------------------ --------------- ----------
SMSMSG SMSSENDMSG M12 143216009
SMSMSG SMSSENDMSG M13 0
13 rows selected.
前面使用的都是单键值索引，其索引选择性参考前几天的记录。
-The End-
oracle视频教程请关注:http://u.youku.com/user_video/id_UMzAzMjkxMjE2.html