高效使用索引的首要条件是知道什么样的查询会使用到索引,这个问题和B+Tree中的“最左前缀原理”有关,下面通过例子说明最左前缀原理。 这里先说一下联合索引的概念。在上文中,我们都是假设索引只引用了单个的列,实际上,MySQL中的索引可以以一定顺序引用多个列,这种索引叫做联合索引,一般的,一个联合索引是一个有序元组,其中各个元素均为数据表的一列,实际上要严格定义索引需要用到关系代数,但是这里我不想讨论太多关系代数的话题,因为那样会显得很枯燥,所以这里就不再做严格定义。另外,单列索引可以看成联合索引元素数为1的特例。 以employees.titles表为例,下面先查看其上都有哪些索引: ~~~ SHOW INDEX FROM employees.titles; +--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Null | Index_type | +--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+ | titles | 0 | PRIMARY | 1 | emp_no | A | NULL | | BTREE | | titles | 0 | PRIMARY | 2 | title | A | NULL | | BTREE | | titles | 0 | PRIMARY | 3 | from_date | A | 443308 | | BTREE | | titles | 1 | emp_no | 1 | emp_no | A | 443308 | | BTREE | +--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+ ~~~ 从结果中可以到titles表的主索引为,还有一个辅助索引。为了避免多个索引使事情变复杂(MySQL的SQL优化器在多索引时行为比较复杂),这里我们将辅助索引drop掉: ~~~ ALTER TABLE employees.titles DROP INDEX emp_no; ~~~ 这样就可以专心分析索引PRIMARY的行为了。 ## 情况一:全列匹配。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND title='Senior Engineer' AND from_date='1986-06-26'; +----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+ | 1 | SIMPLE | titles | const | PRIMARY | PRIMARY | 59 | const,const,const | 1 | | +----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+ ~~~ 很明显,当按照索引中所有列进行精确匹配(这里精确匹配指“=”或“IN”匹配)时,索引可以被用到。这里有一点需要注意,理论上索引对顺序是敏感的,但是由于MySQL的查询优化器会自动调整where子句的条件顺序以使用适合的索引,例如我们将where中的条件顺序颠倒: ~~~ EXPLAIN SELECT * FROM employees.titles WHERE from_date='1986-06-26' AND emp_no='10001' AND title='Senior Engineer'; +----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+ | 1 | SIMPLE | titles | const | PRIMARY | PRIMARY | 59 | const,const,const | 1 | | +----+-------------+--------+-------+---------------+---------+---------+-------------------+------+-------+ ~~~ 效果是一样的。 ## 情况二:最左前缀匹配。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001'; +----+-------------+--------+------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | titles | ref | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+--------+------+---------------+---------+---------+-------+------+-------+ ~~~ 当查询条件精确匹配索引的左边连续一个或几个列时,如或,所以可以被用到,但是只能用到一部分,即条件所组成的最左前缀。上面的查询从分析结果看用到了PRIMARY索引,但是key_len为4,说明只用到了索引的第一列前缀。 ## 情况三:查询条件用到了索引中列的精确匹配,但是中间某个条件未提供。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND from_date='1986-06-26'; +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | titles | ref | PRIMARY | PRIMARY | 4 | const | 1 | Using where | +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+ ~~~ 此时索引使用情况和情况二相同,因为title未提供,所以查询只用到了索引的第一列,而后面的from_date虽然也在索引中,但是由于title不存在而无法和左前缀连接,因此需要对结果进行扫描过滤from_date(这里由于emp_no唯一,所以不存在扫描)。如果想让from_date也使用索引而不是where过滤,可以增加一个辅助索引,此时上面的查询会使用这个索引。除此之外,还可以使用一种称之为“隔离列”的优化方法,将emp_no与from_date之间的“坑”填上。 首先我们看下title一共有几种不同的值: ~~~ SELECT DISTINCT(title) FROM employees.titles; +--------------------+ | title | +--------------------+ | Senior Engineer | | Staff | | Engineer | | Senior Staff | | Assistant Engineer | | Technique Leader | | Manager | +--------------------+ ~~~ 只有7种。在这种成为“坑”的列值比较少的情况下,可以考虑用“IN”来填补这个“坑”从而形成最左前缀: ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND title IN ('Senior Engineer', 'Staff', 'Engineer', 'Senior Staff', 'Assistant Engineer', 'Technique Leader', 'Manager') AND from_date='1986-06-26'; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 59 | NULL | 7 | Using where | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ ~~~ 这次key_len为59,说明索引被用全了,但是从type和rows看出IN实际上执行了一个range查询,这里检查了7个key。看下两种查询的性能比较: ~~~ SHOW PROFILES; +----------+------------+-------------------------------------------------------------------------------+ | Query_ID | Duration | Query | +----------+------------+-------------------------------------------------------------------------------+ | 10 | 0.00058000 | SELECT * FROM employees.titles WHERE emp_no='10001' AND from_date='1986-06-26'| | 11 | 0.00052500 | SELECT * FROM employees.titles WHERE emp_no='10001' AND title IN ... | +----------+------------+-------------------------------------------------------------------------------+ ~~~ “填坑”后性能提升了一点。如果经过emp_no筛选后余下很多数据,则后者性能优势会更加明显。当然,如果title的值很多,用填坑就不合适了,必须建立辅助索引。 ## 情况四:查询条件没有指定索引第一列。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE from_date='1986-06-26'; +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+ | 1 | SIMPLE | titles | ALL | NULL | NULL | NULL | NULL | 443308 | Using where | +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+ ~~~ 由于不是最左前缀,索引这样的查询显然用不到索引。 ## 情况五:匹配某列的前缀字符串。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND title LIKE 'Senior%'; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 56 | NULL | 1 | Using where | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ ~~~ 此时可以用到索引,~~但是如果通配符不是只出现在末尾,则无法使用索引。~~(原文表述有误,如果通配符%不出现在开头,则可以用到索引,但根据具体情况不同可能只会用其中一个前缀) ## 情况六:范围查询。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no < '10010' and title='Senior Engineer'; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 4 | NULL | 16 | Using where | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ ~~~ 范围列可以用到索引(必须是最左前缀),但是范围列后面的列无法用到索引。同时,索引最多用于一个范围列,因此如果查询条件中有两个范围列则无法全用到索引。 ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no < '10010' AND title='Senior Engineer' AND from_date BETWEEN '1986-01-01' AND '1986-12-31'; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 4 | NULL | 16 | Using where | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ ~~~ 可以看到索引对第二个范围索引无能为力。这里特别要说明MySQL一个有意思的地方,那就是仅用explain可能无法区分范围索引和多值匹配,因为在type中这两者都显示为range。同时,用了“between”并不意味着就是范围查询,例如下面的查询: ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no BETWEEN '10001' AND '10010' AND title='Senior Engineer' AND from_date BETWEEN '1986-01-01' AND '1986-12-31'; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | titles | range | PRIMARY | PRIMARY | 59 | NULL | 16 | Using where | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+ ~~~ 看起来是用了两个范围查询,但作用于emp_no上的“BETWEEN”实际上相当于“IN”,也就是说emp_no实际是多值精确匹配。可以看到这个查询用到了索引全部三个列。因此在MySQL中要谨慎地区分多值匹配和范围匹配,否则会对MySQL的行为产生困惑。 ## 情况七:查询条件中含有函数或表达式。 很不幸,如果查询条件中含有函数或表达式,则MySQL不会为这列使用索引(虽然某些在数学意义上可以使用)。例如: ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no='10001' AND left(title, 6)='Senior'; +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | titles | ref | PRIMARY | PRIMARY | 4 | const | 1 | Using where | +----+-------------+--------+------+---------------+---------+---------+-------+------+-------------+ ~~~ 虽然这个查询和情况五中功能相同,但是由于使用了函数left,则无法为title列应用索引,而情况五中用LIKE则可以。再如: ~~~ EXPLAIN SELECT * FROM employees.titles WHERE emp_no - 1='10000'; +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+ | 1 | SIMPLE | titles | ALL | NULL | NULL | NULL | NULL | 443308 | Using where | +----+-------------+--------+------+---------------+------+---------+------+--------+-------------+ ~~~ 显然这个查询等价于查询emp_no为10001的函数,但是由于查询条件是一个表达式,MySQL无法为其使用索引。看来MySQL还没有智能到自动优化常量表达式的程度,因此在写查询语句时尽量避免表达式出现在查询中,而是先手工私下代数运算,转换为无表达式的查询语句。