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一.语法
explain < table_name >
例如: explain select * from t3 where id=3952602;
二.explain输出解释
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
1.id |
1.我
ぐぢóғҊ理解是SQL执行〮ÑѱѨ顺利ぱゃĔҴҫ标识,SQL从大到小こづöҖҍ执行.
例如:
mysql> explain select * from (select * from ( select * from t3 where id=3952602) a) b;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | | system | NULL | NULL | NULL | NULL | 1 | |
| 3 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+ |
很显然这条SQL是从里向外ぜのÿҟҖ执行,就是从id=3 向上执行.
2. select_type就是select类型,可以有以下几种
(1) SIMPLE
简单SELECT(不使用UNION或子查询等) 例如:
mysql> explain select * from t3 where id=3952602;
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | const | 1 | |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+ |
(2). PRIMARY
我いざç҇Ѿ理解是最外层〜〮¿џіselect.例如:
mysql> explain select * from (select * from t3 where id=3952602) a ;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+ |
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