爲了更好的理解連接類型（type），將根據查詢條件的不同對連接類型進行簡單歸納。
表定義如下：
1.id爲主鍵
mysql> show create table key_id; +--------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +--------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | key_id | CREATE TABLE `key_id` ( `id` char(128) NOT NULL DEFAULT '', `data` char(128) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +--------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
2.id爲unique索引
mysql> show create table key_id; +--------+---------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +--------+---------------------------------------------------------------------------------------------------------------------------------------------------------+ | key_id | CREATE TABLE `key_id` ( `id` int(11) NOT NULL DEFAULT '0', `data` char(128) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +--------+---------------------------------------------------------------------------------------------------------------------------------------------------------+
3.id爲普通索引
mysql> show create table index_id; +----------+------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +----------+------------------------------------------------------------------------------------------------------------------------------------------------+ | index_id | CREATE TABLE `index_id` ( `id` int(11) DEFAULT NULL, `data` char(128) DEFAULT NULL, KEY`id` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +----------+------------------------------------------------------------------------------------------------------------------------------------------------+
一、等值查詢：
1.const
mysql> explain extended select * fromkey_id where id='8'; +----+-------------+--------+-------+---------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | type | possible_keys | key |key_len | ref | rows | filtered | Extra| +----+-------------+--------+-------+---------------+---------+---------+-------+------+----------+-------+ | 1| SIMPLE | key_id | const |PRIMARY | PRIMARY | 384 | const | 1 | 100.00 | NULL | +----+-------------+--------+-------+---------------+---------+---------+-------+------+----------+-------+
mysql> explain select id from key_id where id=4; +----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key |key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+ | 1| SIMPLE | key_id | const |PRIMARY | PRIMARY | 4 | const | 1 | Using index | +----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
可以看到當unique和primary key索引被作爲等值條件時觸發。const非常快，在執行優化解析時值已經被算出來了。
2. ref
mysql> explain select * fromindex_id where id=2; +----+-------------+----------+------+---------------+------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+-------+------+-------+ | 1 |SIMPLE | index_id | ref | id | id | 5 | const | 1 | NULL | +----+-------------+----------+------+---------------+------+---------+-------+------+-------+
普通索引被作爲等值條件時觸發。
3.all
mysql> explain select * from index_id where data='a'; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id| select_type | table | type |possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | index_id | ALL | NULL | NULL | NULL | NULL | 10 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
當條件列沒有任何索引的情況下觸發。此時爲全表搜索
二、範圍查詢
1.range
mysql> explain extended select * from key_id whereid>8; +----+-------------+--------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | type | possible_keys | key |key_len | ref | rows | filtered | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1| SIMPLE | key_id | range |PRIMARY | PRIMARY | 4 | NULL | 2 | 100.00 | Using where | +----+-------------+--------+-------+---------------+---------+---------+------+------+----------+-------------+
可以看到當unique和primary key索引被作爲範圍查詢條件時觸發。而普通索引（根據經驗）讀取的數據不能超過表中數據的20%才觸發，不然將會是全表搜索。
mysql> explain select * from index_id where id >9; +----+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len |ref | rows | Extra | +----+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+ | 1| SIMPLE | index_id | range |id | id | 5 | NULL | 1 | Using indexcondition | +----+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+
因爲innodb通過索引來尋找數據時，首先會遍歷輔助索引，然後根據輔助索引葉節點的指針獲得主鍵索引的主鍵，最後再根據主鍵索引得到完整的行數據。並且輔助索引與主鍵索引不在同一個數據段，所以普通索引的訪問是隨機讀的過程，當需要的數據量大時，通過普通索引搜索數據沒有全表搜索這種順序讀來得快。
2.all
mysql> explain select * from index_id where data>'f'; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1| SIMPLE | index_id | ALL | NULL | NULL | NULL | NULL | 10 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
mysql> explain select * fromindex_id where id>2; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1| SIMPLE | index_id | ALL | id | NULL | NULL |NULL | 6 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
可以看出當條件列沒有任何索引以及普通索引列獲取數據的行數大於表中數據20%的情況下觸發全表搜索。
三、不帶條件
1.index
mysql> explain extended select id fromkey_id; +----+-------------+--------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | type | possible_keys | key |key_len | ref | rows | filtered |Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1| SIMPLE | key_id | index |NULL | PRIMARY | 384 | NULL | 4 | 100.00 | Using index | +----+-------------+--------+-------+---------------+---------+---------+------+------+----------+-------------+
可以看到當查詢的列爲索引列將觸發index。此時只需要對索引頁進行遍歷。
2.all
mysql> explain extended select * fromkey_id; +----+-------------+--------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+----------+-------+ | 1| SIMPLE | key_id | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | NULL | +----+-------------+--------+------+---------------+------+---------+------+------+----------+-------+
當查詢字段有未加非索引的情況則需要遍歷整張表。