我有一个带有 myisam 引擎的 mysql 数据库。与许多其他表一起，我有这个表“et”，它有大约 8137037 条记录。
我已经创建了索引（列 hname 和 pnum 的单独索引，后来创建了 hname 和 pnum 的联合索引并帮助在一秒钟内执行）这样查询如下
select st from et where hname='name' and pnum='1' limit 1
快速执行（在一秒钟内），但问题是我必须执行这个查询
select st from et where hname='name' and pnum='1' order by id limit 1
其中 id 是表的主键，此查询有时需要 145 秒 :(
我该如何解决这个问题？
表结构
mysql> describe et;
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| client_id | int(11) | YES | MUL | NULL | |
| down_id | bigint(20) | YES | MUL | NULL | |
| port_index | int(11) | YES | | NULL | |
| date1 | bigint(20) | YES | MUL | NULL | |
| username | varchar(50) | YES | | NULL | |
| hname | varchar(80) | YES | MUL | NULL | |
| pnum | varchar(10) | YES | MUL | NULL | |
| status | int(3) | YES | MUL | NULL | |
| department | int(2) | YES | MUL | NULL | |
| comments | text | YES | | NULL | |
| issue | int(1) | YES | MUL | NULL | |
+------------+-------------+------+-----+---------+----------------+
12 rows in set (0.06 sec)
索引
mysql> show indexes from et;
+-----+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
|Table| Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| et | 0 | PRIMARY | 1 | id | A | 8137037 | NULL | NULL | | BTREE | |
| et | 1 | status | 1 | status | A | 12 | NULL | NULL | YES | BTREE | |
| et | 1 | down_id | 1 | down_id | A | 4068518 | NULL | NULL | YES | BTREE | |
| et | 1 | issue_idx | 1 | issue | A | 8137037 | NULL | NULL | YES | BTREE | |
| et | 1 | hname_idx | 1 | hname | A | 283 | NULL | NULL | YES | BTREE | |
| et | 1 | pname_idx | 1 | pnum | A | 136 | NULL | NULL | YES | BTREE | |
| et | 1 | clientid_idx | 1 | client_id | A | 8137037 | NULL | NULL | YES | BTREE | |
| et | 1 | date1_idx | 1 | date1 | A | 8137037 | NULL | NULL | YES | BTREE | |
| et | 1 | department_idx | 1 | department | A | 2712345 | NULL | NULL | YES | BTREE | |
| et | 1 | hp_idx | 1 | hname | A | 283 | NULL | NULL | YES | BTREE | |
| et | 1 | hp_idx | 2 | pnum | A | 4834 | NULL | NULL | YES | BTREE | |
+-----+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
11 rows in set (0.06 sec)
解释计划
mysql> explain select status from et where hname='mmah' and port_num='1' limit 1;
+----+-------------+------------+------+----------------------------------+--------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+----------------------------------+--------------+---------+-------------+------+-------------+
| 1 | SIMPLE | error_trap | ref | hname_idx,pname_idx,hostport_idx | hostport_idx | 96 | const,const | 37 | Using where |
+----+-------------+------------+------+----------------------------------+--------------+---------+-------------+------+-------------+
1 row in set (0.02 sec)
mysql> explain select status from et where host_name='mmah' and pnum='1' order by id desc limit 1;
+----+-------------+------------+------+----------------------------------+--------------+---------+-------------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+----------------------------------+--------------+---------+-------------+------+-----------------------------+
| 1 | SIMPLE | error_trap | ref | hname_idx,pname_idx,hostport_idx | hostport_idx | 96 | const,const | 37 | Using where; Using filesort |
+----+-------------+------------+------+----------------------------------+--------------+---------+-------------+------+-----------------------------+
1 row in set (0.00 sec)