导致MySQL做全表扫描的几种情况
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2024-04-02 08:01:02
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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
?| 1 2 3 4 5 6 7 | 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：
?| 1 2 3 4 5 6 7 8 9 10 11 12 13 | 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：
?| 1 2 3 | explain select * from test where score = '10' ; explain select * from test where score =10; |
结果如下：
?| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | 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。
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：
?| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | 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：
?| 1 2 3 | explain select * from test1 where score= '111' ; explain select * from test1 where score!= '111' ; |
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | 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。
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：
?| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | 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：
?| 1 2 3 4 5 | 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 ; |
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | 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。
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
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