一个超超超烂SQL的优化

最近遇到一个超超超烂的SQL，由于SQL涉及某某税务局的数据，表名字是经过处理的。

原始SQL如下：

select * from ( select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='0') 无评价, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='1') 满意, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='2') 较满意, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='3') 一般, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='4') 较不满意, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='5') 不满意 from xxx t WHERE t.yxbz='Y' )where slzl<>0 ;

不用看执行计划就知道，表test_v会被扫描多次，所以这个SQL必须改写。如何改写这个SQL呢？请看下面分析：

首先，这个SQL语句是拼出来的，我们可以把复杂的问题简单化，所以我先看这个SQL：

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl from CC_ZXJBXX t WHERE t.yxbz='Y';

执行计划如下：

SQL> select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm, 2 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl 3 from CC_ZXJBXX t 4 WHERE t.yxbz='Y'; 已选择1956行。 已用时间: 00: 00: 00.37 执行计划 ---------------------------------------------------------- Plan hash value: 4286326665 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1956 | 105K| 11 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 22 | | | |* 2 | TABLE ACCESS FULL| TEST_V | 4 | 88 | 5 (0)| 00:00:01 | |* 3 | TABLE ACCESS FULL | CC_ZXJBXX | 1956 | 105K| 11 (0)| 00:00:01 | -------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("A"."ZCRYZH"=TO_CHAR(:B1)) 3 - filter("T"."YXBZ"='Y')

上面的SQL语句会返回1956行，根据执行计划可以看到它进行的是filter操作，讲上面的SQL改写，初步改写如下：

SQL> select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,count(*) from 2 CC_ZXJBXX t,test_v a where to_char(t.zxid) = a.ZCRYZH 3 and t.yxbz='Y' group by t.zxid,t.gh,t.xm,t.bm,t.fzjgdm; 已选择20行。 已用时间: 00: 00: 00.03 执行计划 ---------------------------------------------------------- Plan hash value: 2833546679 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 32157 | 2418K| 22 (28)| 00:00:01 | | 1 | HASH GROUP BY | | 32157 | 2418K| 22 (28)| 00:00:01 | |* 2 | HASH JOIN | | 32157 | 2418K| 17 (6)| 00:00:01 | | 3 | TABLE ACCESS FULL| TEST_V | 411 | 9042 | 5 (0)| 00:00:01 | |* 4 | TABLE ACCESS FULL| CC_ZXJBXX | 1956 | 105K| 11 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("A"."ZCRYZH"=TO_CHAR("T"."ZXID")) 4 - filter("T"."YXBZ"='Y')

改写之后，SQL只返回了20条数据，但是原始SQL要返回1956条数据，问题出在哪里呢？ 问题在于，第一个SQL是filter，它是过滤，并且是count过滤

而第二个SQL是等值jion，第一个SQL里面的count 不管你过滤成功与否，都会返回数据，过滤失败，count=0 即可，所以对第二个SQL应该用 left join 于是SQL 更改如下：

SQL> select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,count(*) from 2 CC_ZXJBXX t,test_v a where to_char(t.zxid) = a.ZCRYZH(+) 3 and t.yxbz='Y' group by t.zxid,t.gh,t.xm,t.bm,t.fzjgdm; 已选择1956行。 已用时间: 00: 00: 00.12 执行计划 ---------------------------------------------------------- Plan hash value: 3281235561 ------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 32157 | 2418K| 22 (28)| 00:00:01 | | 1 | HASH GROUP BY | | 32157 | 2418K| 22 (28)| 00:00:01 | |* 2 | HASH JOIN RIGHT OUTER| | 32157 | 2418K| 17 (6)| 00:00:01 | | 3 | TABLE ACCESS FULL | TEST_V | 411 | 9042 | 5 (0)| 00:00:01 | |* 4 | TABLE ACCESS FULL | CC_ZXJBXX | 1956 | 105K| 11 (0)| 00:00:01 | ------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("A"."ZCRYZH"(+)=TO_CHAR("T"."ZXID")) 4 - filter("T"."YXBZ"='Y')

这样改写之后，返回记录条数是对了，但是SQL逻辑有点点改变，第一个SQL是过滤的，count()如果不匹配会返回0，但是第二个SQL是 join的，如果不匹配，会返回1

所以SQL需要改写如下：

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,decode(count(*),1,0,count(*)) from CC_ZXJBXX t,test_v a where to_char(t.zxid) = a.ZCRYZH(+) and t.yxbz='Y' group by t.zxid,t.gh,t.xm,t.bm,t.fzjgdm order by 1;

改写完了一个，后面的就简单了，所以整体的SQL可以改写如下：

select t.zxid, t.gh, t.xm, t.bm, t.fzjgdm, decode(count(*), 1, 0, count(*)), SUM(DECODE(A.MYD, '0', 1, 0)) 无评价, SUM(DECODE(A.MYD, '1', 1, 0)) 满意, SUM(DECODE(A.MYD, '2', 1, 0)) 较满意, SUM(DECODE(A.MYD, '3', 1, 0)) 一般, SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意, SUM(DECODE(A.MYD, '5', 1, 0)) 不满意 from CC_ZXJBXX t, test_v a where to_char(t.zxid) = a.ZCRYZH(+) and t.yxbz = 'Y' group by t.zxid, t.gh, t.xm, t.bm, t.fzjgdm ;

上面的SQL等价于

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='0') 无评价, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='1') 满意, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='2') 较满意, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='3') 一般, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='4') 较不满意, (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='5') 不满意 from xxx t WHERE t.yxbz='Y'；

由于原始SQL要去掉 slzl <>0 所以再包装一下

select * from (select t.zxid, t.gh, t.xm, t.bm, t.fzjgdm, decode(count(*), 1, 0, count(*))slzl, SUM(DECODE(A.MYD, '0', 1, 0)) 无评价, SUM(DECODE(A.MYD, '1', 1, 0)) 满意, SUM(DECODE(A.MYD, '2', 1, 0)) 较满意, SUM(DECODE(A.MYD, '3', 1, 0)) 一般, SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意, SUM(DECODE(A.MYD, '5', 1, 0)) 不满意 from CC_ZXJBXX t, test_v a where to_char(t.zxid) = a.ZCRYZH(+) and t.yxbz = 'Y' group by t.zxid, t.gh, t.xm, t.bm, t.fzjgdm ) where slzl<>0;

原始SQL的执行计划 以及 更改后的执行计划对比：

SQL> select * from ( 2 select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm, 3 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl, 4 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='0') 无评价, 5 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='1') 满意, 6 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='2') 较满意, 7 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='3') 一般, 8 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='4') 较不满意, 9 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='5') 不满意 10 from xxx t 11 WHERE t.yxbz='Y' 12 ) where slzl<>0; 已选择20行。 已用时间: 00: 00: 00.28 执行计划 ---------------------------------------------------------- Plan hash value: 2669170809 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1956 | 76284 | 17 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 4 | | | |* 2 | TABLE ACCESS FULL | TEST_V | 16 | 64 | 5 (0)| 00:00:01 | | 3 | SORT AGGREGATE | | 1 | 6 | | | |* 4 | TABLE ACCESS FULL | TEST_V | 9 | 54 | 5 (0)| 00:00:01 | | 5 | SORT AGGREGATE | | 1 | 6 | | | |* 6 | TABLE ACCESS FULL | TEST_V | 5 | 30 | 5 (0)| 00:00:01 | | 7 | SORT AGGREGATE | | 1 | 6 | | | |* 8 | TABLE ACCESS FULL | TEST_V | 1 | 6 | 5 (0)| 00:00:01 | | 9 | SORT AGGREGATE | | 1 | 6 | | | |* 10 | TABLE ACCESS FULL | TEST_V | 2 | 12 | 5 (0)| 00:00:01 | | 11 | SORT AGGREGATE | | 1 | 6 | | | |* 12 | TABLE ACCESS FULL | TEST_V | 1 | 6 | 5 (0)| 00:00:01 | | 13 | SORT AGGREGATE | | 1 | 6 | | | |* 14 | TABLE ACCESS FULL | TEST_V | 1 | 6 | 5 (0)| 00:00:01 | |* 15 | FILTER | | | | | | |* 16 | TABLE ACCESS FULL | XXX | 1956 | 76284 | 12 (0)| 00:00:01 | | 17 | SORT AGGREGATE | | 1 | 4 | | | |* 18 | TABLE ACCESS FULL| TEST_V | 16 | 64 | 5 (0)| 00:00:01 | ------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("A"."ZCRYZH"=TO_CHAR(:B1)) 4 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='0') 6 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='1') 8 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='2') 10 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='3') 12 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='4') 14 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='5') 15 - filter( (SELECT COUNT(*) FROM "TEST_V" "A" WHERE "A"."ZCRYZH"=TO_CHAR(:B1))<>0) 16 - filter("T"."YXBZ"='Y') 18 - filter("A"."ZCRYZH"=TO_CHAR(:B1)) 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 33261 consistent gets 0 physical reads 0 redo size 2011 bytes sent via SQL*Net to client 411 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 20 rows processed

原始SQL需要消耗33261个逻辑读，扫描TEST_V表8次，更改之后的SQL

SQL> select * from 2 (select t.zxid, 3 t.gh, 4 t.xm, 5 t.bm, 6 t.fzjgdm, 7 decode(count(*), 1, 0, count(*))slzl, 8 SUM(DECODE(A.MYD, '0', 1, 0)) 无评价, 9 SUM(DECODE(A.MYD, '1', 1, 0)) 满意, 10 SUM(DECODE(A.MYD, '2', 1, 0)) 较满意, 11 SUM(DECODE(A.MYD, '3', 1, 0)) 一般, 12 SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意, 13 SUM(DECODE(A.MYD, '5', 1, 0)) 不满意 14 from CC_ZXJBXX t, test_v a 15 where to_char(t.zxid) = a.ZCRYZH(+) 16 and t.yxbz = 'Y' 17 group by t.zxid, t.gh, t.xm, t.bm, t.fzjgdm 18 ) where slzl<>0; 已选择20行。 已用时间: 00: 00: 00.01 执行计划 ---------------------------------------------------------- Plan hash value: 1348291491 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 98 | 4410 | 18 (12)| 00:00:01 | |* 1 | FILTER | | | | | | | 2 | HASH GROUP BY | | 98 | 4410 | 18 (12)| 00:00:01 | |* 3 | HASH JOIN RIGHT OUTER| | 1956 | 88020 | 17 (6)| 00:00:01 | | 4 | TABLE ACCESS FULL | TEST_V | 411 | 2466 | 5 (0)| 00:00:01 | |* 5 | TABLE ACCESS FULL | CC_ZXJBXX | 1956 | 76284 | 11 (0)| 00:00:01 | ------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(DECODE(COUNT(*),1,0,COUNT(*))<>0) 3 - access("A"."ZCRYZH"(+)=TO_CHAR("T"."ZXID")) 5 - filter("T"."YXBZ"='Y') 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 62 consistent gets 0 physical reads 0 redo size 1958 bytes sent via SQL*Net to client 411 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 20 rows processed

逻辑读降低到62，表TEST_V只扫描1次，其实这个SQL只是一个汇总的SQL，稍微有点开发常识的人都知道，最开始写SQL的那个人绝对是在乱写SQL。SQL的优化到这里还没完，

因为我们已经知道这个SQL的目的其实就是汇总，所以直接抛弃刚才的优化思路，直接重写SQL如下：

SQL> SELECT * 2 FROM (SELECT T.ZXID, 3 T.GH, 4 T.XM, 5 T.BM, 6 T.FZJGDM, 7 SUM(1) SLZL, 8 SUM(DECODE(A.MYD, '0', 1, 0)) 无评价, 9 SUM(DECODE(A.MYD, '1', 1, 0)) 满意, 10 SUM(DECODE(A.MYD, '2', 1, 0)) 较满意, 11 SUM(DECODE(A.MYD, '3', 1, 0)) 一般, 12 SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意, 13 SUM(DECODE(A.MYD, '5', 1, 0)) 不满意 14 FROM CC_ZXJBXX T ,test_v A 15 where A.ZCRYZH=T.ZXID 16 and T.YXBZ = 'Y' 17 GROUP BY T.ZXID, T.GH, T.XM, T.BM, T.FZJGDM) order by 1; 已选择20行。 已用时间: 00: 00: 00.01 执行计划 ---------------------------------------------------------- Plan hash value: 1317439664 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 411 | 18495 | 18 (12)| 00:00:01 | | 1 | SORT GROUP BY | | 411 | 18495 | 18 (12)| 00:00:01 | |* 2 | HASH JOIN | | 411 | 18495 | 17 (6)| 00:00:01 | | 3 | TABLE ACCESS FULL| TEST_V | 411 | 2466 | 5 (0)| 00:00:01 | |* 4 | TABLE ACCESS FULL| CC_ZXJBXX | 1956 | 76284 | 11 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("T"."ZXID"=TO_NUMBER("A"."ZCRYZH")) 4 - filter("T"."YXBZ"='Y') 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 62 consistent gets 0 physical reads 0 redo size 1945 bytes sent via SQL*Net to client 411 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 20 rows processed

有时候开发人员，为了完成任务，他才不管你SQL写得好不好呢，能完成功能就行了，相信各位DBA深有体会。