Question

查询

SELECT SQL_NO_CACHE contacts.id,
       contacts.date_modified contacts__date_modified
FROM contacts
INNER JOIN
  (SELECT tst.team_set_id
   FROM team_sets_teams tst
   INNER JOIN team_memberships team_membershipscontacts ON (team_membershipscontacts.team_id = tst.team_id)
   AND (team_membershipscontacts.user_id = '5daa2e92-c347-11e9-afc5-525400a80916')
   AND (team_membershipscontacts.deleted = 0)
   GROUP BY tst.team_set_id) contacts_tf ON contacts_tf.team_set_id = contacts.team_set_id
LEFT JOIN contacts_cstm contacts_cstm ON contacts_cstm.id_c = contacts.id
WHERE contacts.deleted = 0
ORDER BY contacts.date_modified DESC,
         contacts.id DESC
LIMIT 21;

耗时极长（2M记录2分钟）。 我无法更改此查询，因为它是系统生成的

这是解释：

+----+-------------+--------------------------+------------+--------+-------------------------------------------------------------------------------------------------------+----------------------------+---------+-------------------------------------------+---------+----------+---------------------------------------------------------------------+
| id | select_type | table                    | partitions | type   | possible_keys                                                                                         | key                        | key_len | ref                                       | rows    | filtered | Extra                                                               |
+----+-------------+--------------------------+------------+--------+-------------------------------------------------------------------------------------------------------+----------------------------+---------+-------------------------------------------+---------+----------+---------------------------------------------------------------------+
|  1 | PRIMARY     | contacts                 | NULL       | ref    | idx_contacts_tmst_id,idx_del_date_modified,idx_contacts_del_last,idx_cont_del_reports,idx_del_id_user | idx_del_date_modified      | 2       | const                                     | 1113718 |   100.00 | Using temporary; Using filesort                                     |
|  1 | PRIMARY     | <derived3>               | NULL       | ALL    | NULL                                                                                                  | NULL                       | NULL    | NULL                                      |       2 |    50.00 | Using where; Using join buffer (Block Nested Loop)                  |
|  1 | PRIMARY     | contacts_cstm            | NULL       | eq_ref | PRIMARY                                                                                               | PRIMARY                    | 144     | sugarcrm.contacts.id                      |       1 |   100.00 | Using index                                                         |
|  3 | DERIVED     | team_membershipscontacts | NULL       | ref    | idx_team_membership,idx_teammemb_team_user,idx_del_team_user                                          | idx_team_membership        | 145     | const                                     |       2 |    99.36 | Using index condition; Using where; Using temporary; Using filesort |
|  3 | DERIVED     | tst                      | NULL       | ref    | idx_ud_set_id,idx_ud_team_id,idx_ud_team_set_id,idx_ud_team_id_team_set_id                            | idx_ud_team_id_team_set_id | 144     | sugarcrm.team_membershipscontacts.team_id |       1 |   100.00 | Using index                                                         |
+----+-------------+--------------------------+------------+--------+-------------------------------------------------------------------------------------------------------+----------------------------+---------+-------------------------------------------+---------+----------+---------------------------------------------------------------------+

但是当我使用force index(idx_del_date_modified)（与explain中使用的索引相同）时，查询仅需0.01s，而我得到的解释也略有不同。

+----+-------------+--------------------------+------------+--------+----------------------------------------------------------------------------+----------------------------+---------+-------------------------------------------+---------+----------+---------------------------------------------------------------------+
| id | select_type | table                    | partitions | type   | possible_keys                                                              | key                        | key_len | ref                                       | rows    | filtered | Extra                                                               |
+----+-------------+--------------------------+------------+--------+----------------------------------------------------------------------------+----------------------------+---------+-------------------------------------------+---------+----------+---------------------------------------------------------------------+
|  1 | PRIMARY     | contacts                 | NULL       | ref    | idx_del_date_modified                                                      | idx_del_date_modified      | 2       | const                                     | 1113718 |   100.00 | Using where                                                         |
|  1 | PRIMARY     | <derived2>               | NULL       | ALL    | NULL                                                                       | NULL                       | NULL    | NULL                                      |       2 |    50.00 | Using where                                                         |
|  1 | PRIMARY     | contacts_cstm            | NULL       | eq_ref | PRIMARY                                                                    | PRIMARY                    | 144     | sugarcrm.contacts.id                      |       1 |   100.00 | Using index                                                         |
|  2 | DERIVED     | team_membershipscontacts | NULL       | ref    | idx_team_membership,idx_teammemb_team_user,idx_del_team_user               | idx_team_membership        | 145     | const                                     |       2 |    99.36 | Using index condition; Using where; Using temporary; Using filesort |
|  2 | DERIVED     | tst                      | NULL       | ref    | idx_ud_set_id,idx_ud_team_id,idx_ud_team_set_id,idx_ud_team_id_team_set_id | idx_ud_team_id_team_set_id | 144     | sugarcrm.team_membershipscontacts.team_id |       1 |   100.00 | Using index                                                         |
+----+-------------+--------------------------+------------+--------+----------------------------------------------------------------------------+----------------------------+---------+-------------------------------------------+---------+----------+---------------------------------------------------------------------+

第一个查询使用临时表和文件排序，但是带有force index的查询仅使用where。查询不应该相同吗？为什么带有强制索引的查询要快得多-使用的索引仍然相同。

Answer 1

根据MySQL手册：

可以在以下条件下创建临时表：

如果有一个ORDER BY子句和另一个GROUP BY子句，或者   ORDER BY或GROUP BY包含表中除   连接队列中的第一个表，将创建一个临时表。

DISTINCT与ORDER BY结合使用可能需要一个临时表。

如果使用SQL_SMALL_RESULT选项，MySQL将使用内存   临时表，除非查询还包含元素（描述   稍后）需要磁盘存储。

同样，由于MySQL中有查询优化器组件，因此性能更好。

如果创建索引，则即使索引存在，查询优化器也无法使用索引列。使用force index(..)迫使MySql使用索引。

请考虑一个详细的示例here。

查询使用没有强制索引的临时表

1 个答案: