MySQL 查询一个分区表，当查询条件存在数据时执行效率OK,当不存在数据时执行不完，一直在sending data,当去掉desc就没问题。换个版本貌似也没问题。

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mysql> select version();

+------------+

| version()  |

+------------+

| 5.7.11-log |

+------------+

1 row in set (0.00 sec)

mysql> use zabbix

Database changed

mysql> SELECT * FROM history h WHERE h.itemid='106107' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

+--------+------------+-----------+-----------+

| itemid | clock      | value     | ns        |

+--------+------------+-----------+-----------+

| 106107 | 1533828123 | 1792.0000 | 151803000 |

| 106107 | 1533828003 | 1792.0000 |  44536142 |

+--------+------------+-----------+-----------+

2 rows in set (0.00 sec)

mysql> explain SELECT * FROM history h WHERE h.itemid='106107' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

| id | select_type | table | partitions                                                                                                                              | type  | possible_keys               | key       | key_len | ref  | rows | filtered | Extra                 |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

|  1 | SIMPLE      | h     | p201808,p201809,p201810,p201811,p201812,p201901,p201902,p201903,p201904,p201905,p201906,p201907,p201908,p201909,p201910,p201911,p201912 | range | history_1,idx_history_clock | history_1 | 12      | NULL |  172 |   100.00 | Using index condition |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.05 sec)

ysql> SELECT * FROM history h WHERE h.itemid='1061055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

^C^C -- query aborted

ERROR 1317 (70100): Query execution was interrupted

mysql> explain SELECT * FROM history h WHERE h.itemid='1061055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

| id | select_type | table | partitions                                                                                                                              | type  | possible_keys               | key       | key_len | ref  | rows | filtered | Extra                 |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

|  1 | SIMPLE      | h     | p201808,p201809,p201810,p201811,p201812,p201901,p201902,p201903,p201904,p201905,p201906,p201907,p201908,p201909,p201910,p201911,p201912 | range | history_1,idx_history_clock | history_1 | 12      | NULL |    1 |   100.00 | Using index condition |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.00 sec)

mysql> SELECT * FROM history h WHERE h.itemid='1061055' AND h.clock>1533723653 ORDER BY h.clock  LIMIT 2 OFFSET 0;

Empty set (0.00 sec)

mysql> explain SELECT * FROM history h WHERE h.itemid='1061055' AND h.clock>1533723653 ORDER BY h.clock  LIMIT 2 OFFSET 0;

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

| id | select_type | table | partitions                                                                                                                              | type  | possible_keys               | key       | key_len | ref  | rows | filtered | Extra                 |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

|  1 | SIMPLE      | h     | p201808,p201809,p201810,p201811,p201812,p201901,p201902,p201903,p201904,p201905,p201906,p201907,p201908,p201909,p201910,p201911,p201912 | range | history_1,idx_history_clock | history_1 | 12      | NULL |    1 |   100.00 | Using index condition |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.00 sec)

创建了降序索引，效率提升

mysql> create index idx_history_2 on  history (itemid desc);

Query OK, 0 rows affected (36 min 50.11 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> 

mysql>  SELECT * FROM history h WHERE h.itemid='1060001055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

Empty set (0.00 sec)

mysql> explain SELECT * FROM history h WHERE h.itemid='1060001055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+------+-------------------------------------------+-----------+---------+-------+------+----------+-------------+

| id | select_type | table | partitions                                                                                                                              | type | possible_keys                             | key       | key_len | ref   | rows | filtered | Extra       |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+------+-------------------------------------------+-----------+---------+-------+------+----------+-------------+

|  1 | SIMPLE      | h     | p201808,p201809,p201810,p201811,p201812,p201901,p201902,p201903,p201904,p201905,p201906,p201907,p201908,p201909,p201910,p201911,p201912 | ref  | history_1,idx_history_clock,idx_history_2 | history_1 | 8       | const |    1 |    31.59 | Using where |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+------+-------------------------------------------+-----------+---------+-------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

升级了版本，效率提升

mysql> 

mysql> select version();

+---------------+

| version()     |

+---------------+

| 5.7.22-22-log |

+---------------+

1 row in set (0.00 sec)

mysql> SELECT * FROM history h WHERE h.itemid='1060001055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

ERROR 1046 (3D000): No database selected

mysql> show databses;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'databses' at line 1

mysql> show databases;

+--------------------+

| Database           |

+--------------------+

| information_schema |

| mysql              |

| performance_schema |

| sys                |

| zabbix             |

+--------------------+

5 rows in set (0.00 sec)

mysql> use zabbix

Database changed

mysql> SELECT * FROM history h WHERE h.itemid='1060001055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

Empty set (0.01 sec)

mysql> explain  SELECT * FROM history h WHERE h.itemid='1060001055' AND h.clock>1533723653 ORDER BY h.clock DESC LIMIT 2 OFFSET 0;

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

| id | select_type | table | partitions                                                                                                                              | type  | possible_keys               | key       | key_len | ref  | rows | filtered | Extra                 |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

|  1 | SIMPLE      | h     | p201808,p201809,p201810,p201811,p201812,p201901,p201902,p201903,p201904,p201905,p201906,p201907,p201908,p201909,p201910,p201911,p201912 | range | history_1,idx_history_clock | history_1 | 12      | NULL |    1 |   100.00 | Using index condition |

+----+-------------+-------+-----------------------------------------------------------------------------------------------------------------------------------------+-------+-----------------------------+-----------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.00 sec)

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