下面我们为表t2创建一个索引：
[SQL] 纯文本查看 复制代码| 001 002 003 004 005 006 007 008 009 010 011 |
mysql> create index in_id on t2(id);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show index from t2;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| t2 | 1 | in_id | 1 | id | A | NULL | NULL | NULL | YES | BTREE | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
1 row in set (0.00 sec)
|
并插入1000万行的数据。
然后再用：
| 001 |
explain select * from t2 where id=1;
|
或者
| 001 |
desc select * from t2 where id=1;
|
查询其影响行数。决定其是否需要优化
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