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：
情况1：
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
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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
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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：
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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)
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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：
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explain select * from test where score ='10';
explain select * from test where score =10;
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结果如下：
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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)
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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。
情况2：
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：
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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)
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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：
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explain select * from test1 where score='111';
explain select * from test1 where score!='111';
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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)
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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。
情况3：
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：
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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)
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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：
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explain select * from test4 where id=1;
explain select * from test4 where id is null;
explain select * from test4 where id=1 or id is null;
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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)
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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。
以上就是导致mysql做全表扫描的几种情况的详细内容，更多关于mysql 全表扫描的资料请关注服务器之家其它相关文章！
原文链接：https://mp.weixin.qq.com/s/5G1xGrxb6ii_gpcWE1hC6A