MySQL 索引失效是一个相当普遍的问题,尤其在处理慢查询时特别需要注意是否存在索引失效的情况。
排查索引失效问题的第一步,必须定位要分析的 SQL 语句,然后通过EXPLAIN
来查看其执行计划。主要关注type
、key
和extra
这几个字段。
我们需根据 key、type 和 extra 判断 SQL 查询是否利用了索引。若是,是否为覆盖索引、索引下推、整体索引扫描,或是索引跳跃扫描等情况。
通常,优化的索引使用情况包括以下几种:
首先,key 字段必须有值,不得为 NULL。
其次,type 应该是 ref、eq_ref、range、const 等几种类型。
此外,extra 字段如果为 NULL 或者包含"using index"、"using index condition"都是可以接受的情况。
如果执行计划显示一条 SQL 语句没有有效利用索引,例如 type = ALL,key = NULL,extra = Using where。
那么就需要进一步分析未能有效利用索引的原因。需要了解的是,是否需要使用索引以及应该使用哪个索引,这是由 MySQL 的优化器决定的,它会根据成本估算做出决策。
以下是导致未能有效利用索引的几种可能情况:
索引未正确创建:当查询语句中的 where 条件涉及的字段未创建索引,或者索引未满足最左前缀匹配条件时,就未能正确创建索引。
索引区分度不足:如果索引的区分度不高,可能导致未使用索引,因为在这种情况下,利用索引并不能有效提升查询效率。
表过小:当表中的数据量很少时,优化器可能认为全表扫描的成本不高,因此可能选择不使用索引。
查询语句中使用了函数或字段类型不匹配等情况导致索引失效。
这时候我们需要从头开始逐一分析:
如果索引未正确创建,根据 SQL 语句创建适当的索引。如果未遵守最左前缀原则,调整索引或修改 SQL 语句。
若索引区分度不高,考虑选择另一个更合适的索引字段。
对于表过小的情况,优化影响可能不大,因此是否使用索引可以不做过多优化。
排查具体的失效原因,然后有针对性地调整 SQL 语句。
可能导致索引失效的情况
假设我们有一张表(以下 SQL 实验基于 MySQL 5.7):
CREATE TABLE `mytable` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(50) NOT NULL, `age` int(11) DEFAULT NULL, `create_time` datetime DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `name` (`name`), KEY `age` (`age`), KEY `create_time` (`create_time`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4; insert into mytable(id,name,age,create_time) values (1,"paidaxing",20,now()); insert into mytable(id,name,age,create_time) values (2,"paidaxing1",21,now()); insert into mytable(id,name,age,create_time) values (3,"paidaxing2",22,now()); insert into mytable(id,name,age,create_time) values (4,"paidaxing3",20,now()); insert into mytable(id,name,age,create_time) values (5,"paidaxing4",14,now()); insert into mytable(id,name,age,create_time) values (6,"paidaxing5",43,now()); insert into mytable(id,name,age,create_time) values (7,"paidaxing6",32,now()); insert into mytable(id,name,age,create_time) values (8,"paidaxing7",12,now()); insert into mytable(id,name,age,create_time) values (9,"paidaxing8",1,now()); insert into mytable(id,name,age,create_time) values (10,"paidaxing9",43,now());
索引列参与计算
select * from mytable where age = 12; +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+ | 1 | SIMPLE | mytable | NULL | ref | age | age | 5 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
以上 SQL 语句是可以利用索引的,但如果在字段中增加计算操作,就可能导致索引失效:
select * from mytable where age +1 = 12; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
但如果计算的形式如下,仍然可以利用索引:
select * from mytable where age = 12 - 1; +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+ | 1 | SIMPLE | mytable | NULL | ref | age | age | 5 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
对索引列进行函数操作
SELECT * FROM mytable WHERE create_time = '2023-04-01 00:00:00'; +----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | mytable | NULL | ref | create_time | create_time | 6 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
以上 SQL 语句可以利用索引,但如果在字段中添加函数操作,可能会导致索引失效:
SELECT * FROM mytable WHERE YEAR(create_time) = 2022; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
使用 OR
SELECT * FROM mytable WHERE name = 'paidaxing' and age > 18; +----+-------------+---------+------------+-------+---------------+------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+------+---------+-------+------+----------+-------+ | 1 | SIMPLE | mytable | NULL | const | name,age | name | 202 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+------+---------+-------+------+----------+-------+
但如果使用 OR,并且 OR 条件中的两侧包含<或者>操作符时,可能会导致索引失效,例如:
SELECT * FROM mytable WHERE name = 'paidaxing' OR age > 18; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | name,age | NULL | NULL | NULL | 10 | 40.00 | Using where | +-
但如果 OR 条件的两侧都是等号判断,并且两个字段都有索引,仍然可以利用索引,例如:
mysql> explain SELECT * FROM mytable WHERE name = 'paidaxing' OR age = 18; +----+-------------+---------+------------+-------------+---------------+----------+---------+------+------+----------+------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------------+---------------+----------+---------+------+------+----------+------------------------------------+ | 1 | SIMPLE | mytable | NULL | index_merge | name,age | name,age | 202,5 | NULL | 2 | 100.00 | Using union(name,age); Using where | +----+-------------+---------+------------+-------------+---------------+----------+---------+------+------+----------+------------------------------------+
like 操作
SELECT * FROM mytable WHERE nick like '%paidaxing%'; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 11.11 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ SELECT * FROM mytable WHERE nick like '%paidaxing'; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 11.11 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ SELECT * FROM mytable WHERE nick like 'paidaxing%'; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | range| nick | nick | NULL | NULL | 10 | 100.00 | Using index condition | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ SELECT * FROM mytable WHERE nick like 'paida%xing'; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | range| nick | nick | NULL | NULL | 10 | 100.00 | Using index condition | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
以上四种 LIKE 模式中,"paidaxing%"和"paida%xing"这两种可以利用索引,但是如果是"%paidaxing%"和"%paidaxing"就无法利用索引。
隐式类型转换
select * from mytable where name = 1; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | name | NULL | NULL | NULL | 10 | 10.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
以上情况中,如果表中的 name 字段是 varchar 类型,但我们使用 int 类型进行查询,这会导致索引失效。
然而,有一个特例是,如果字段类型是 int 类型,而查询条件中添加了单引号或双引号,MySQL 会将参数转换为 int 类型,这种情况下也可以利用索引。
select * from mytable where age = '1'; +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+ | 1 | SIMPLE | mytable | NULL | ref | age | age | 5 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
不等于比较
SELECT * FROM mytable WHERE age != 18; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | age | NULL | NULL | NULL | 10 | 100.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
但并非所有情况都是如此,例如在以下情况下,使用 ID 进行!=比较时,可能会利用索引:
SELECT * FROM mytable WHERE id != 18; +----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 12 | 100.00 | Using where | +----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
在使用!=(不等于)操作符时,索引是否失效与多个因素有关,包括索引的选择性、数据的分布情况等,并不能简单地因为使用了!=操作符就说导致了索引失效。
is not null
以下情况是索引失效的:
SELECT * FROM mytable WHERE name is not null +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | name | NULL | NULL | NULL | 10 | 90.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
order by
SELECT * FROM mytable order by age +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+ | 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using filesort | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
当进行 ORDER BY 操作时,如果数据量非常小,数据库可能会选择在内存中进行排序,而不是使用索引。
in
使用 IN 操作时,有时会走索引,有时则不会。一般来说,当 IN 子句中的值较少时,数据库可能会选择使用索引进行优化;但如果 IN 子句中的选项较多,可能就不会使用索引。
mysql> explain select * from mytable where name in ("paidaxing"); +----+-------------+---------+------------+-------+---------------+------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+------+---------+-------+------+----------+-------+ | 1 | SIMPLE | mytable | NULL | const | name | name | 202 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+------+---------+-------+------+----------+-------+ mysql> explain select * from mytable where name in ("paidaxing","pdx"); +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | mytable | NULL | range | name | name | 202 | NULL | 2 | 100.00 | Using index condition | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+ mysql> explain select * from mytable where name in ("paidaxing","pdx","x"); +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | mytable | NULL | ALL | name | NULL | NULL | NULL | 11 | 27.27 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+