后台某维度统计语句

SELECT products_id, sku, px_id, sj_id, cat_path, COUNT(*) AS pv, COUNT(DISTINCT ip) AS ip_numbers, SUM(is_bounce) AS bounce_numbers, SUM(remain_time) AS remain_time
    FROM dm_pv_records_search
    WHERE 1 AND add_date >= '2014-02-26 10:00:00' AND products_id > 0
GROUP BY products_id,sku, px_id, sj_id, cat_path
order by ip_numbers desc limit   0,20;

每次有同事在后台查询这类数据时，都反应数据非常慢,偶尔不能出来数据。

explain

+----+-------------+----------------------+-------+----------------------+----------+---------+------+---------+----------------------------------------------+
| id | select_type | table                | type  | possible_keys        | key      | key_len | ref  | rows    | Extra                                        |
+----+-------------+----------------------+-------+----------------------+----------+---------+------+---------+----------------------------------------------+
|  1 | SIMPLE      | dm_pv_records_search | range | products_id,add_date | add_date | 8       | NULL | 12831019 | Using where; Using temporary; Using filesort |
+----+-------------+----------------------+-------+----------------------+----------+---------+------+---------+----------------------------------------------+

这条语句索引使用是正确的，不过结果集及其大，而且需要对结果集group by操作。

观察该sql语句发现，sku,px_id,sj_id,cat_path四个字段为冗余字段，都可以从其他表连表获得。

而改sql又存在order by .. limit m,n，可知道后期结果集只有20个，更加适合后期连接相关表获取其他必要的字段sku,px_id,sj_id,cat_path。

如此一转换，可以将group by中的sku,px_id,sj_id,cat_path去掉(其中sku和cat_path为字符串)，节省后期大结果集中排序的内存。

而group by和distinct优化的一种方式是，尽量减少不必要的字段，可以参考简朝阳写的《mysql性能调优和架构设计》的第8.6小结，或者参考以下连接：http://isky000.com/database/mysql_group_by_implement 。

我们可以尝试将sql改成

SELECT products_id, COUNT(*) AS pv, COUNT(DISTINCT ip) AS ip_numbers, SUM(is_bounce) AS bounce_numbers, SUM(remain_time) AS remain_time
    FROM dm_pv_records_search
    WHERE 1 AND add_date >= '2014-02-26 10:00:00' AND products_id > 0
GROUP BY products_id
order by ip_numbers desc limit   0,20;


对比两次profiling,前者执行时间是58s,后者执行时间是5s.

前者profiling如下

+----------------------+-----------+----------+------------+--------------+---------------+
| Status               | Duration  | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+-----------+----------+------------+--------------+---------------+
| starting             |  0.000133 | 0.000000 |   0.000000 |            0 |             0 |
| checking permissions |  0.000014 | 0.000000 |   0.000000 |            0 |             0 |
| Opening tables       |  0.000031 | 0.000000 |   0.000000 |            0 |             0 |
| System lock          |  0.000018 | 0.000000 |   0.000000 |            0 |             0 |
| init                 |  0.000061 | 0.000000 |   0.000000 |            0 |             0 |
| optimizing           |  0.000019 | 0.000000 |   0.000000 |            0 |             0 |
| statistics           |  0.000073 | 0.000000 |   0.000000 |            0 |             0 |
| preparing            |  0.000051 | 0.000000 |   0.000000 |            0 |             0 |
| Creating tmp table   |  0.000051 | 0.000000 |   0.000000 |            0 |             0 |
| Sorting for group    | 47.735389 | 4.614299 |  10.773362 |         3632 |       2811456 |
| executing            |  0.000010 | 0.000000 |   0.000000 |            0 |             0 |
| Copying to tmp table | 11.566292 | 0.910861 |   0.586911 |          256 |          4408 |
| Sorting result       |  0.030459 | 0.025996 |   0.004000 |            0 |             0 |
| Sending data         |  0.000057 | 0.000000 |   0.000000 |            0 |             0 |
| end                  |  0.000005 | 0.000000 |   0.000000 |            0 |             0 |
| removing tmp table   |  0.008139 | 0.000000 |   0.008998 |            0 |             0 |
| end                  |  0.000007 | 0.000000 |   0.000000 |            0 |             0 |
| query end            |  0.000004 | 0.000000 |   0.000000 |            0 |             0 |
| closing tables       |  0.000011 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items        |  0.000071 | 0.000000 |   0.000000 |            0 |             0 |
| removing tmp table   |  0.000006 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items        |  0.000331 | 0.000000 |   0.000000 |            0 |             0 |
| logging slow query   |  0.000006 | 0.000000 |   0.000000 |            0 |             0 |
| logging slow query   |  0.000047 | 0.000000 |   0.000000 |            0 |             8 |
| cleaning up          |  0.000005 | 0.000000 |   0.000000 |            0 |             0 |
+----------------------+-----------+----------+------------+--------------+---------------+

去掉多余字段后的profiling

+--------------------------------+----------+----------+------------+--------------+---------------+
| Status                         | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+--------------------------------+----------+----------+------------+--------------+---------------+
| starting                       | 0.000027 | 0.000000 |   0.000000 |            0 |             0 |
| Waiting for query cache lock   | 0.000008 | 0.000000 |   0.000000 |            0 |             0 |
| checking query cache for query | 0.000082 | 0.000000 |   0.000000 |            0 |             0 |
| checking permissions           | 0.000011 | 0.000000 |   0.000000 |            0 |             0 |
| Opening tables                 | 0.000020 | 0.000000 |   0.000000 |            0 |             0 |
| System lock                    | 0.000014 | 0.000000 |   0.000000 |            0 |             0 |
| Waiting for query cache lock   | 0.000038 | 0.000000 |   0.000000 |            0 |             0 |
| init                           | 0.000040 | 0.000000 |   0.000000 |            0 |             0 |
| optimizing                     | 0.000019 | 0.000000 |   0.000000 |            0 |             0 |
| statistics                     | 0.000061 | 0.000000 |   0.000000 |            0 |             0 |
| preparing                      | 0.000051 | 0.000000 |   0.000000 |            0 |             0 |
| Creating tmp table             | 0.000042 | 0.000000 |   0.000000 |            0 |             0 |
| Sorting for group              | 5.275880 | 2.539614 |   6.813964 |          120 |        287368 |
| executing                      | 0.000011 | 0.000000 |   0.000000 |            0 |             0 |
| Copying to tmp table           | 0.492344 | 0.478927 |   0.021997 |           24 |           264 |
| Sorting result                 | 0.018007 | 0.016997 |   0.001000 |            0 |             0 |
| Sending data                   | 0.000039 | 0.000000 |   0.000000 |            0 |             0 |
| end                            | 0.000005 | 0.000000 |   0.000000 |            0 |             0 |
| removing tmp table             | 0.001028 | 0.000000 |   0.000999 |            0 |             0 |
| end                            | 0.000007 | 0.000000 |   0.000000 |            0 |             0 |
| query end                      | 0.000004 | 0.000000 |   0.000000 |            0 |             0 |
| closing tables                 | 0.000011 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items                  | 0.000073 | 0.000000 |   0.000000 |            0 |             0 |
| removing tmp table             | 0.000006 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items                  | 0.000008 | 0.000000 |   0.000000 |            0 |             0 |
| Waiting for query cache lock   | 0.000003 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items                  | 0.000290 | 0.000000 |   0.000000 |            0 |             0 |
| Waiting for query cache lock   | 0.000007 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items                  | 0.000003 | 0.000000 |   0.000000 |            0 |             0 |
| storing result in query cache  | 0.000005 | 0.000000 |   0.000000 |            0 |             0 |
| logging slow query             | 0.000003 | 0.000000 |   0.000000 |            0 |             0 |
| logging slow query             | 0.000068 | 0.000000 |   0.000000 |            0 |             8 |
| cleaning up                    | 0.000007 | 0.000000 |   0.000000 |            0 |             0 |
+--------------------------------+----------+----------+------------+--------------+---------------+

我们可以看到没有去掉多余字段前 Sorting for group和Copying to tmp table占用了大部分io和cpu，

而去掉后Copying to tmp table占用io和cpu时间相对来说较少，而且 Sorting for group性能提升5倍之多！


注意事项

1. 优化group by和distinct我们在sql层面能做的事情是尽量减少不必要的字段