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)
