关于导致MySQL做全表扫描的三种情况的原因分析，这两天看到了两种可能会导致全表扫描的sql，这里给大家看一下，希望可以避免踩坑：
原因一：强制类型转换的情况下，不会使用索引，会走全表扫描。
举例如下：
首先我们创建一个表
- CREATE TABLE `test` (
- `id` int(11) NOT NULL AUTO_INCREMENT,
- `age` int(11) DEFAULT NULL,
- `score` varchar(20) NOT NULL DEFAULT '',
- PRIMARY KEY (`id`),
- KEY `idx_score` (`score`)
- ) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8
我们可以看到，这个表有三个字段，其中两个int类型，一个varchar类型。varchar类型的字段score是一个索引，而id是主键。
然后我们给这个表里面插入一些数据，插入数据之后的表如下：
- mysql：yeyztest 21:43:12>>select * from test;
- +----+------+-------+
- | id | age | score |
- +----+------+-------+
- | 1 | 1 | 5 |
- | 2 | 2 | 10 |
- | 5 | 5 | 25 |
- | 8 | 8 | 40 |
- | 9 | 2 | 45 |
- | 10 | 5 | 50 |
- | 11 | 8 | 55 |
- +----+------+-------+
- 7 rows in set (0.00 sec)
这个时候，我们使用explain语句来查看两条sql的执行情况，分别是：
- explain select * from test where score ='10';
- explain select * from test where score =10;
结果如下：
- mysql:yeyztest 21:42:29>>explain select * from test where score ='10';
- +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
- | 1 | SIMPLE | test | NULL | ref | idx_score | idx_score | 62 | const | 1 | 100.00 | NULL |
- +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
- 1 row in set, 1 warning (0.00 sec)
- mysql:yeyztest 21:43:06>>explain select * from test where score =10;
- +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
- | 1 | SIMPLE | test | NULL | ALL | idx_score | NULL | NULL | NULL | 7 | 14.29 | Using where |
- +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
- 1 row in set, 3 warnings (0.00 sec)
可以看到，如果我们使用的是varchar类型的值，那么结果中扫描的行数rows就是1，而当我们使用的是整数值10的时候，扫描行数变为了7，证明，如果出现了强制类型转换，则会导致索引失效。
原因二：反向查询不能使用索引，会导致全表扫描。
创建一个表test1，它的主键是score，然后插入6条数据：
- CREATE TABLE `test1` (
- `score` varchar(20) not null default '' ,
- PRIMARY KEY (`score`)
- ) ENGINE=InnoDB DEFAULT CHARSET=utf8
- mysql:yeyztest 22:09:37>>select * from test1;
- +-------+
- | score |
- +-------+
- | 111 |
- | 222 |
- | 333 |
- | 444 |
- | 555 |
- | 666 |
- +-------+
- 6 rows in set (0.00 sec)
当我们使用反向查找的时候，不会使用到索引，来看下面两条sql：
- explain select * from test1 where score='111';
- explain select * from test1 where score!='111';
- mysql:yeyztest 22:13:01>>explain select * from test1 where score='111';
- +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
- | 1 | SIMPLE | test1 | NULL | const | PRIMARY | PRIMARY | 62 | const | 1 | 100.00 | Using index |
- +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
- 1 row in set, 1 warning (0.00 sec)
- mysql:yeyztest 22:13:08>>explain select * from test1 where score!='111';
- +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
- | 1 | SIMPLE | test1 | NULL | index | PRIMARY | PRIMARY | 62 | NULL | 6 | 100.00 | Using where; Using index |
- +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
- 1 row in set, 1 warning (0.00 sec)
可以看到，使用！=作为条件的时候，扫描的行数是表的总记录行数。因此如果想要使用索引，我们就不能使用反向匹配规则。
原因三、某些or值条件可能导致全表扫描。
首先我们创建一个表，并插入几条数据：
- CREATE TABLE `test4` (
- `id` int(11) DEFAULT NULL,
- `name` varchar(20) DEFAULT NULL,
- KEY `idx_id` (`id`)
- ) ENGINE=InnoDB DEFAULT CHARSET=utf8
- 1 row in set (0.00 sec)
- mysql--dba_admin@127.0.0.1:yeyztest 22:23:44>>select * from test4;
- +------+------+
- | id | name |
- +------+------+
- | 1 | aaa |
- | 2 | bbb |
- | 3 | ccc |
- | 4 | yeyz |
- | NULL | yeyz |
- +------+------+
- 5 rows in set (0.00 sec)
其中表test4包含两个字段，id字段是一个索引，而name字段是varchar类型，我们来看下面三个语句的扫描行数：
- mysql:yeyztest 22:24:12>>explain select * from test4 where id is null;
- +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-----------------------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-----------------------+
- | 1 | SIMPLE | test4 | NULL | ref | idx_id | idx_id | 5 | const | 1 | 100.00 | Using index condition |
- +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-----------------------+
- 1 row in set, 1 warning (0.00 sec)
- mysql:yeyztest 22:24:17>>explain select * from test4 where id=1;
- +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
- | 1 | SIMPLE | test4 | NULL | ref | idx_id | idx_id | 5 | const | 1 | 100.00 | NULL |
- +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
- 1 row in set, 1 warning (0.00 sec)
- mysql:yeyztest 22:24:28>>explain select * from test4 where id=1 or id is null;
- +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
- | 1 | SIMPLE | test4 | NULL | ALL | idx_id | NULL | NULL | NULL | 5 | 40.00 | Using where |
- +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
- 1 row in set, 1 warning (0.00 sec)
可以看到单独使用id=1和id is null，都只会扫描一行记录，而使用or将二者连接起来就会导致扫描全表而不使用索引。
简单总结一下：
1.强制类型转换的情况下，不会使用索引，会走全表扫描
2.反向查询不能使用索引，会导致全表扫描。
3.某些or值条件可能导致全表扫描。
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