MySQL查询explain extra疑惑
CREATE TABLE `course` ( `id` int(10) NOT NULL AUTO_INCREMENT, `name` varchar(50) NOT NULL, PRIMARY KEY (`id`), KEY `my_index` (`name`)) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=latin1;
?使用查询：
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explain select name from course where name='java';
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?结果：
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+----+-------------+--------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table ?| type | possible_keys | key ? ? ?| key_len | ref ? | rows | Extra ? ? ? ? ? ? ? ? ? ?|
+----+-------------+--------+------+---------------+----------+---------+-------+------+--------------------------+
| ?1 | SIMPLE ? ? ?| course | ref ?| my_index ? ? ?| my_index | 52 ? ? ?| const | ? ?1 | Using where; Using index |
+----+-------------+--------+------+---------------+----------+---------+-------+------+--------------------------+
如果name列没有索引的话，explain结果的extra列将是Using where，没有Using index。
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接着使用id作条件查询：
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explain select name from course where id=1;explain select * from course where id=1;?
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结果是：
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+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table ?| type ?| possible_keys | key ? ? | key_len | ref ? | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
| ?1 | SIMPLE ? ? ?| course | const | PRIMARY ? ? ? | PRIMARY | 4 ? ? ? | const | ? ?1 | ? ? ? |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
如果这样查询：
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explain select id from course where id=1;
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?结果是：
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+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table ?| type ?| possible_keys | key ? ? | key_len | ref ? | rows | Extra ? ? ? |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| ?1 | SIMPLE ? ? ?| course | const | PRIMARY ? ? ? | PRIMARY | 4 ? ? ? | const | ? ?1 | Using index |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
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问题：
?? ? ?同样是针对有索引的id（主键）和name列查询，extra列的结果不一样，前者查询id是extra 为空，如果查询整列或name列则是using index，即覆盖查询。后者不管查询什么，都是using where和using index.不清楚为什么。
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