使用/*+ no_expand */避免执行路径中的CONCATENATION
昨天的一次SQL优化过程，仅作记录。
背景是在一台11gR2的机器上，用户反映一个查询操作非常慢，基本都要20~30分钟才能有结果。经过仔细查看该SQL的执行计划，发现由于SQL中使用了or，导致CBO走出了一个非常糟糕的CONCATENATION路径。因为SQL本身是通过程序动态生成的，开发人员不愿意进行大的调整，就采用了/*+ no_expand */提升的方式避免CBO选择CONCATENATION方式，调整之后执行时间提升到1分钟之内。
no_expand提示的说明是
The NO_EXPAND hint prevents the cost-based optimizer from considering OR-expansion for queries having OR conditions or IN-lists in the WHERE clause. Usually, the optimizer considers using OR expansion and uses this method if it decides that the cost is lower than not using it.
use_concat提示的说明是
The USE_CONCAT hint forces combined OR conditions in the WHERE clause of a query to be transformed into a compound query using the UNION ALL set operator. Generally, this transformation occurs only if the cost of the query using the concatenations is cheaper than the cost without them.
问题解决了，不过留下了一个疑点：在Oracle 11gR2中，在cost明显很高的情况下，CBO是不应该选用CONCATENATION的，在 wait4friend 的测试过程中，使用SQL*Plus或者GUI的开发工具执行该SQL，都不会生成CONCATENATION的执行计划，为了重现这个问题，必须使用/*+ use_concat */来模拟。那么问题就奇怪了，为什么通过程序提交过来的SQL总是会选择很差的计划？到底是什么因素导致CBO做出错误的选择，目前还没有找到原因。一个可能的原因是，开发人员使用的是ojdbc14.jar的驱动，不过尚未进行验证。
语句如下。红色部分是后面修改的，为了避免CBO选择CONCATENATION路径，使用了/*+ no_expand */提示。
SELECT COUNT (*)
FROM (SELECT cu.customer_id, bi.bill_id
FROM (SELECT /*+ no_expand */ cu.customer_id
FROM ep_customer cu,
ep_cus_expands ex,
(SELECT id, project_id, name_alias
FROM no_card_channel) ch
WHERE 1 = 1
AND ex.create_time >= 20000101000000
AND ex.create_time <= 20121213235959
AND ex.TYPE = 'NC_CHANNEL'
AND ex.validity = 'true'
AND ( ex.ext_data = ch.project_id
OR ex.ext_data = ch.name_alias)
AND ex.customer_id = cu.customer_id) cu
LEFT JOIN
(SELECT b.bill_id,
b.customer_id,
b.amount,
b.trans_id,
c.settle_time
FROM ep_bill b, ep_cup_certificate c
WHERE b.status IN ('paid', 'void', 'refund')
AND c.pay_class = 'NC_PAY'
AND c.TYPE = 'purchase'
AND c.validity = 'true'
AND b.bill_id = c.bill_id
UNION ALL
SELECT b.bill_id,
b.customer_id,
b.amount,
b.trans_id,
'' settle_time
FROM ep_bill b
WHERE b.trans_id = '1382') bi
ON cu.customer_id = bi.customer_id)
WHERE bill_id IS NULL;
糟糕的执行计划
Plan hash value: 3646981028
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 120K(100)| |
| 1 | SORT AGGREGATE | | 1 | 83 | | | |
| 2 | CONCATENATION | | | | | | |
| 3 | HASH JOIN | | 517K| 40M| | 61936 (1)| 00:12:24 |
| 4 | TABLE ACCESS FULL | NO_CARD_CHANNEL | 130 | 3250 | | 3 (0)| 00:00:01 |
| 5 | FILTER | | | | | | |
| 6 | HASH JOIN RIGHT OUTER | | 653K| 36M| 1848K| 61929 (1)| 00:12:24 |
| 7 | VIEW | | 78526 | 920K| | 48749 (1)| 00:09:45 |
| 8 | UNION-ALL | | | | | | |
| 9 | HASH JOIN | | 74882 | 3436K| 2928K| 28390 (1)| 00:05:41 |
| 10 | TABLE ACCESS FULL | EP_CUP_CERTIFICATE | 74882 | 2047K| | 5369 (1)| 00:01:05 |
| 11 | TABLE ACCESS FULL | EP_BILL | 1702K| 30M| | 20366 (1)| 00:04:05 |
| 12 | TABLE ACCESS FULL | EP_BILL | 3644 | 58304 | | 20359 (1)| 00:04:05 |
| 13 | HASH JOIN | | 655K| 28M| 32M| 11287 (1)| 00:02:16 |
| 14 | TABLE ACCESS FULL | EP_CUS_EXPANDS | 655K| 24M| | 5595 (2)| 00:01:08 |
| 15 | INDEX FAST FULL SCAN| P_CUSTOMER_ID | 2654K| 15M| | 1791 (1)| 00:00:22 |
| 16 | HASH JOIN | | 1 | 83 | | 58518 (1)| 00:11:43 |
| 17 | MERGE JOIN CARTESIAN | | 1 | 43 | | 52919 (1)| 00:10:36 |
| 18 | FILTER | | | | | | |
| 19 | HASH JOIN RIGHT OUTER| | 1 | 18 | 1848K| 52916 (1)| 00:10:35 |
| 20 | VIEW | | 78526 | 920K| | 48749 (1)| 00:09:45 |
| 21 | UNION-ALL | | | | | | |
| 22 | HASH JOIN | | 74882 | 3436K| 2928K| 28390 (1)| 00:05:41 |
| 23 | TABLE ACCESS FULL| EP_CUP_CERTIFICATE | 74882 | 2047K| | 5369 (1)| 00:01:05 |
| 24 | TABLE ACCESS FULL| EP_BILL | 1702K| 30M| | 20366 (1)| 00:04:05 |
| 25 | TABLE ACCESS FULL | EP_BILL | 3644 | 58304 | | 20359 (1)| 00:04:05 |
| 26 | INDEX FAST FULL SCAN| P_CUSTOMER_ID | 2654K| 15M| | 1791 (1)| 00:00:22 |
| 27 | BUFFER SORT | | 130 | 3250 | | 52919 (1)| 00:10:36 |
| 28 | TABLE ACCESS FULL | NO_CARD_CHANNEL | 130 | 3250 | | 3 (0)| 00:00:01 |
| 29 | TABLE ACCESS FULL | EP_CUS_EXPANDS | 655K| 24M| | 5595 (2)| 00:01:08 |
---------------------------------------------------------------------------------------------------------
正常情况下的执行计划
Plan hash value: 3689968997
--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 60823 (100)| |
| 1 | SORT AGGREGATE | | 1 | 97 | | | |
| 2 | NESTED LOOPS | | 4979 | 471K| | 60823 (1)| 00:12:10 |
| 3 | FILTER | | | | | | |
| 4 | HASH JOIN OUTER | | 3441 | 241K| | 56150 (1)| 00:11:14 |
| 5 | HASH JOIN | | 3452 | 155K| | 7399 (2)| 00:01:29 |
| 6 | TABLE ACCESS FULL | EP_CUS_EXPANDS | 3452 | 134K| | 5595 (2)| 00:01:08 |
| 7 | INDEX FAST FULL SCAN| P_CUSTOMER_ID | 2654K| 15M| | 1791 (1)| 00:00:22 |
| 8 | VIEW | | 78526 | 1993K| | 48749 (1)| 00:09:45 |
| 9 | UNION-ALL | | | | | | |
| 10 | HASH JOIN | | 74882 | 3436K| 2928K| 28390 (1)| 00:05:41 |
| 11 | TABLE ACCESS FULL | EP_CUP_CERTIFICATE | 74882 | 2047K| | 5369 (1)| 00:01:05 |
| 12 | TABLE ACCESS FULL | EP_BILL | 1702K| 30M| | 20366 (1)| 00:04:05 |
| 13 | TABLE ACCESS FULL | EP_BILL | 3644 | 58304 | | 20359 (1)| 00:04:05 |
| 14 | TABLE ACCESS FULL | NO_CARD_CHANNEL | 1 | 25 | | 1 (0)| 00:00:01 |
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