对Group By 语句的一次优化过程(hellodba)

来源：互联网发布：淘宝宝贝上架时间技巧编辑：程序博客网时间：2024/06/06 14:14

生产环境中发现一条语句很慢，拿回来一看，其实是一个简单的Group By语句:

表CCMMT的数据量比较大，5M多条记录。

1、SQL> select CDE, CID  2  from CCMMT  3  GROUP BY CDE, CID  4  having max(ADT) < sysdate - 180; 707924 rows selected. Elapsed: 00:06:17.49 Execution Plan----------------------------------------------------------   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=414 Card=238583 Bytes=4771660)   1    0   FILTER   2    1     SORT (GROUP BY NOSORT) (Cost=414 Card=238583 Bytes=4771660)   3    2       TABLE ACCESS (BY INDEX ROWID) OF 'CCMMT' (Cost=414 Card=57969096 Bytes=1159381920)   4    3         INDEX (FULL SCAN) OF 'CCMMT_TEMP_IDX' (NON-UNIQUE) (Cost=26 Card=57969096) Statistics----------------------------------------------------------          0  recursive calls          0  db block gets    2769177  consistent gets    1089991  physical reads          0  redo size   23926954  bytes sent via SQL*Net to client     519785  bytes received via SQL*Net from client      47196  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)     707924  rows processed

要6min多返回。尝试调整语句写法，用minus代替Group By:

2、

SQL> select DISTINCT CDE, CID  2  from CCMMT  3  where ADT < sysdate - 180  4  minus  5  select DISTINCT CDE, CID  6  from CCMMT  7  where ADT >= sysdate - 180; 707924 rows selected. Elapsed: 00:00:21.53 Execution Plan----------------------------------------------------------   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=190624 Card=2794940          Bytes=111797600)   1    0   MINUS   2    1     SORT* (UNIQUE) (Cost=95312 Card=2794940 Bytes=55898800)                                               :Q13049001   3    2       INDEX* (FAST FULL SCAN) OF 'CCMMT_UQ1' (UNIQUE) (Cost=77305 Card=2898455 Bytes=57969100)    :Q13049000   4    1     SORT* (UNIQUE) (Cost=95312 Card=2794940 Bytes=55898800)                                               :Q13050001   5    4       INDEX* (FAST FULL SCAN) OF 'CCMMT_UQ1' (UNIQUE) (Cost=77305 Card=2898455 Bytes=57969100)    :Q13050000    2 PARALLEL_TO_SERIAL            SELECT DISTINCT C0 C0,C1 C1 FROM :Q13049000 ORDER BY C0,C1   3 PARALLEL_TO_PARALLEL          SELECT /*+ INDEX_RRS(A1 "CCMMT_UQ1")*/ A1."CDE" C0,A1."CA   4 PARALLEL_TO_SERIAL            SELECT DISTINCT C0 C0,C1 C1 FROM :Q13050000 ORDER BY C0,C1   5 PARALLEL_TO_PARALLEL          SELECT /*+ INDEX_RRS(A1 "CCMMT_UQ1")*/ A1."CDE" C0,A1."CA Statistics----------------------------------------------------------          0  recursive calls         33  db block gets     126566  consistent gets     129243  physical reads          0  redo size   18461368  bytes sent via SQL*Net to client     519785  bytes received via SQL*Net from client      47196  SQL*Net roundtrips to/from client          4  sorts (memory)          2  sorts (disk)     707924  rows processed

效果不错，Consistent gets 和 Physical Reads都下降了，同时只需要21s就返回了。但从查询计划看，用到了并行查询，因此会消耗更多的CPU。

在(ADT, CDE, CID )上创建索引，再次执行：

3、

SQL> select DISTINCT CDE, CID  2  from CCMMT  3  where ADT < sysdate - 180  4  minus  5  select DISTINCT CDE, CID  6  from CCMMT  7  where ADT >= sysdate - 180; 707924 rows selected. Elapsed: 00:00:26.94 Execution Plan----------------------------------------------------------   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=36018 Card=2794940 Bytes=111797600)   1    0   MINUS   2    1     SORT (UNIQUE) (Cost=18009 Card=2794940 Bytes=55898800)   3    2       INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE) (Cost=2 Card=2898455 Bytes=57969100)   4    1     SORT (UNIQUE) (Cost=18009 Card=2794940 Bytes=55898800)   5    4       INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE) (Cost=2 Card=2898455 Bytes=57969100) Statistics----------------------------------------------------------          0  recursive calls        118  db block gets      22565  consistent gets      31604  physical reads          0  redo size   18461368  bytes sent via SQL*Net to client     519785  bytes received via SQL*Net from client      47196  SQL*Net roundtrips to/from client          1  sorts (memory)          1  sorts (disk)     707924  rows processed

效果也比较理想，consistent gets和physical reads再次大大下降，返回时间和上面差不多，在一个数量级上，但是不再使用并行查询了。

用NOT Exists代替minus:

4、

SQL> select DISTINCT CDE, CID  2  from CCMMT a  3  where ADT < sysdate - 180  4  AND NOT EXISTS  5  (SELECT CDE, CID FROM  6  (select DISTINCT CDE, CID  7  from CCMMT  8  where ADT >= sysdate - 180) b  9  WHERE a.CDE = b.CDE 10  AND a.CID = b.CID); 707924 rows selected. Elapsed: 00:10:35.70 Execution Plan----------------------------------------------------------   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=600 Card=144923 Bytes=2898460)   1    0   SORT (UNIQUE) (Cost=600 Card=144923 Bytes=2898460)   2    1     INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE)(Cost=2 Card=144923 Bytes=2898460)   3    2       TABLE ACCESS (BY INDEX ROWID) OF 'CCMMT' (Cost=2 Card=1 Bytes=20)   4    3         INDEX (RANGE SCAN) OF 'CCMMT_TEMP_IDX' (NON-UNIQUE) (Cost=1 Card=9) Statistics----------------------------------------------------------          5  recursive calls        118  db block gets   40535587  consistent gets    3157604  physical reads          0  redo size   18461368  bytes sent via SQL*Net to client     519785  bytes received via SQL*Net from client      47196  SQL*Net roundtrips to/from client          2  sorts (memory)          1  sorts (disk)     707924  rows processed FT! consistent gets和physical reads爆涨，10min才返回结果！用Not In换掉Not Exists:

5、

SQL> select DISTINCT CDE, CID  2  from CCMMT a  3  where ADT < sysdate - 180  4  AND (CDE, CID) NOT IN  5  (select DISTINCT CDE, CID  6  from CCMMT  7  where ADT >= sysdate - 180); 707924 rows selected. Elapsed: 00:01:00.70 Execution Plan----------------------------------------------------------   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=36425 Card=1 Bytes=40)   1    0   SORT (UNIQUE NOSORT) (Cost=36425 Card=1 Bytes=40)   2    1     MERGE JOIN (ANTI) (Cost=36423 Card=1 Bytes=40)   3    2       SORT (JOIN) (Cost=18212 Card=2898455 Bytes=57969100)   4    3         INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE) (Cost=2 Card=2898455 Bytes=57969100)   5    2       SORT (UNIQUE) (Cost=18212 Card=2898455 Bytes=57969100)   6    5         INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE) (Cost=2 Card=2898455 Bytes=57969100) Statistics----------------------------------------------------------          0  recursive calls        419  db block gets      22565  consistent gets      98692  physical reads          0  redo size   18461368  bytes sent via SQL*Net to client     519785  bytes received via SQL*Net from client      47196  SQL*Net roundtrips to/from client          1  sorts (memory)          1  sorts (disk)     707924  rows processed

恩，consistent gets和建了索引时的minus方式一样，但是physical reads太大，返回时间太长---1min。同时用到了刚才建的索引。（呵呵，所以说，NOT EXISTS并不是什么情况下都比NOT IN更优啊）

在尝试用left join + is null代替not in:

6、

SQL> SELECT a.CDE, a.CID  2  FROM  3  (select DISTINCT CDE, CID  4  from CCMMT  5  where ADT < sysdate - 180) a,  6  (select DISTINCT CDE, CID  7  from CCMMT  8  where ADT >= sysdate - 180) b  9  WHERE a.CDE = b.CDE(+) 10  AND a.CID = b.CID(+) 11  AND b.CDE IS NULL; 707924 rows selected. Elapsed: 00:00:25.46 Execution Plan----------------------------------------------------------   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=54675 Card=2794940 Bytes=117387480)   1    0   FILTER   2    1     MERGE JOIN (OUTER)   3    2       VIEW (Cost=18009 Card=2794940 Bytes=58693740)   4    3         SORT (UNIQUE) (Cost=18009 Card=2794940 Bytes=55898800)   5    4           INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE) (Cost=2 Card=2898455 Bytes=57969100)   6    2       SORT (JOIN) (Cost=36667 Card=2794940 Bytes=58693740)   7    6         VIEW (Cost=18009 Card=2794940 Bytes=58693740)   8    7           SORT (UNIQUE) (Cost=18009 Card=2794940 Bytes=55898800)   9    8             INDEX (RANGE SCAN) OF 'CCMMT_IDX3' (NON-UNIQUE) (Cost=2 Card=2898455 Bytes=57969100) Statistics----------------------------------------------------------         10  recursive calls        118  db block gets      22569  consistent gets      31300  physical reads          0  redo size   18461368  bytes sent via SQL*Net to client     519785  bytes received via SQL*Net from client      47196  SQL*Net roundtrips to/from client          6  sorts (memory)          1  sorts (disk)     707924  rows processed

效果不错，和有索引时使用minus在同一数量级上。

总结，以上几种方式中，效果最好的应该是第3种和第6种，buffer gets、磁盘IO和CPU消耗都比较少，返回时间大大减少，但是需要新建一个索引，消耗更多磁盘空间，并存在影响其它语句的正常查询计划的风险。而第2种方式应该是次好的。在返回时间上，和上面两种差不多，不需要新的索引，但是会消耗更多的内存、磁盘和CPU资源。

出于综合考虑，采用了第2种方式对生产库进行了优化。