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。