MySQL优化之distinct

distinct实际上和group by的操作非常相似，只不过是在group by之后的每组中取出一条记录而已。

但是distinct分组的时候是不使用排序来做分组的。

同样distinct也分为松散索引扫描和紧凑索引扫描

松散索引

测试过程中我发现如果distinct中的字段本身是唯一的或者没有数据则Extra中不是Using index for group-by，而是显示using Index，说明MySQL在这块还是比较智能的。

如果插入了数据需要更新表信息，analyze table group_message

mysql> explain select distinct gid from group_message \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: group_message
         type: range
possible_keys: IDX_GID_UID_GMT
          key: IDX_GID_UID_GMT
      key_len: 5
          ref: NULL
         rows: 3
        Extra: Using index for group-by
1 row in set (0.03 sec)

mysql> explain select distinct id from group_message \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: group_message
         type: index
possible_keys: PRIMARY,IDX_GID_UID_GMT
          key: IDX_GID_UID_GMT
      key_len: 16
          ref: NULL
         rows: 5
        Extra: Using index
1 row in set (0.03 sec)

紧凑索引扫描：

mysql> explain select distinct uid from group_message where gid = 1\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: group_message
         type: ref
possible_keys: IDX_GID_UID_GMT
          key: IDX_GID_UID_GMT
      key_len: 5
          ref: const
         rows: 5
        Extra: Using where; Using index
1 row in set (0.32 sec)

distinct不能使用索引的情况：我们发现只使用了临时表并没有使用到排序。

mysql> explain select distinct uid from group_message \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: group_message
         type: index
possible_keys: IDX_GID_UID_GMT
          key: IDX_GID_UID_GMT
      key_len: 16
          ref: NULL
         rows: 5
        Extra: Using index; Using temporary
1 row in set (0.02 sec)

表结构：

mysql> show create table group_message \G
*************************** 1. row ***************************
       Table: group_message
Create Table: CREATE TABLE `group_message` (
  `id` int(11) NOT NULL,
  `message` varchar(1000) DEFAULT NULL,
  `gid` int(11) DEFAULT NULL,
  `uid` int(11) DEFAULT NULL,
  `gmt_ctreate` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `IDX_GID_UID_GMT` (`gid`,`uid`,`gmt_ctreate`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.03 sec)

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