简单谈谈MySQL的loose index scan_MySQL
select count(distinct a) from table1;
列a上有一个索引，那么按照简单的想法来讲，如何扫描呢？很简单，一条一条的扫描，这样一来，其实做了一次索引全扫描，效率很差。这种扫描方式会扫描到很多很多的重复的索引，这样说的话优化的办法也是很容易想到的：跳过重复的索引就可以了。于是网上能搜到这样的一个优化的办法：
select count(*) from (select distinct a from table1) t;
从已经搜索到的资料看，这样的执行计划中的extra就从using index变成了using index for group-by。
但是，但是，但是，好在我们现在已经没有使用5.1的版本了，大家基本上都是5.5以上了，这些现代版本，已经实现了loose index scan：
很好很好，就不需要再用这种奇技淫巧去优化SQL了。
文档里关于group by这里写的有点意思，说是最大众化的办法就是进行全表扫描并且创建一个临时表，这样执行计划就会难看的要命了，肯定有ALL和using temporary table了。
5.0之后group by在特定条件下可能使用到loose index scan，
CREATE TABLE log_table ( id INT NOT NULL PRIMARY KEY, log_machine VARCHAR(20) NOT NULL, log_time DATETIME NOT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE INDEX ix_log_machine_time ON log_table (log_machine, log_time);
1
SELECT MAX(log_time) FROM log_table；
SELECT MAX(log_time) FROM log_table WHERE log_machine IN ('Machine 1')；
这两条sql都只需一次index seek便可返回，源于索引的有序排序，优化器意识到min/max位于最左/右块，从而避免范围扫描；
extra显示Select tables optimized away ；
2
代码如下:
SELECT MAX(log_time) FROM log_table WHERE log_machine IN (‘Machine 1','Machine 2','Machine 3','Machine 4');
执行计划type 为range(extra显示using where; using index)，即执行索引范围扫描，先读取所有满足log_machine约束的记录，然后对其遍历找出max value；
改进
代码如下:
SELECT MAX(log_time) FROM log_table WHERE log_machine IN (‘Machine 1','Machine 2','Machine 3','Machine 4') group by log_machine order by 1 desc limit 1;
这满足group by选择loose index scan的要求，执行计划的extra显示using index for group-by，执行效果等值于
SELECT MAX(log_time) FROM log_table WHERE log_machine IN (‘Machine 1') Union SELECT MAX(log_time) FROM log_table WHERE log_machine IN (‘Machine 2') …..
即对每个log_machine执行loose index scan，rows从原来的82636下降为16(该表总共1,000,000条记录)。
Group by何时使用loose index scan?
适用条件：
1 针对单表操作
2 Group by使用索引的最左前缀列
3 只支持聚集函数min()/max()
4 Where条件出现的列必须为=constant操作 ， 没出现在group by中的索引列必须使用constant
5 不支持前缀索引，即部分列索引 ，如index(c1(10))
执行计划的extra应该显示using index for group-by
假定表t1有个索引idx(c1,c2,c3)
SELECT c1, c2 FROM t1 GROUP BY c1, c2; SELECT DISTINCT c1, c2 FROM t1; SELECT c1, MIN(c2) FROM t1 GROUP BY c1; SELECT c1, c2 FROM t1 WHERE c1 < const GROUP BY c1, c2; SELECT MAX(c3), MIN(c3), c1, c2 FROM t1 WHERE c2 > const GROUP BY c1, c2; SELECT c2 FROM t1 WHERE c1 < const GROUP BY c1, c2; SELECT c1, c2 FROM t1 WHERE c3 = const GROUP BY c1, c2 SELECT c1, c3 FROM t1 GROUP BY c1, c2;--无法使用松散索引
而SELECT c1, c3 FROM t1 where c3= const GROUP BY c1, c2;则可以
紧凑索引扫描tight index scan
Group by在无法使用loose index scan，还可以选择tight，若两者都不可选，则只能借助临时表；
扫描索引时，须读取所有满足条件的索引键，要么是全索引扫描，要么是范围索引扫描；
Group by的索引列不连续；或者不是从最左前缀开始，但是where条件里出现最左列；
SELECT c1, c2, c3 FROM t1 WHERE c2 = 'a' GROUP BY c1, c3; SELECT c1, c2, c3 FROM t1 WHERE c1 = 'a' GROUP BY c2, c3;
5.6的改进
事实上，5.6的index condition push down可以弥补loose index scan缺失带来的性能损失。
KEY(age,zip)
mysql> explain SELECT name FROM people WHERE age BETWEEN 18 AND 20 AND zip IN (12345,12346, 12347); +----+-------------+--------+-------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE | people | range | age | age | 4 | NULL | 90556 | Using where | +----+-------------+--------+-------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.01 sec)
根据key_len=4可以推测出sql只用到索引的第一列，即先通过索引查出满足age (18,20)的行记录，然后从server层筛选出满足zip约束的行；
pre-5.6，对于复合索引，只有当引导列使用"="时才有机会在索引扫描时使用到后面的索引列。
mysql> explain SELECT name FROM people WHERE age=18 AND zip IN (12345,12346, 12347); +----+-------------+--------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | people | range | age | age | 8 | NULL | 3 | Using where | +----+-------------+--------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
对比一下查询效率
mysql> SELECT sql_no_cache name FROM people WHERE age=19 AND zip IN (12345,12346, 12347); +----------------------------------+ | name | +----------------------------------+ | 888ba838661aff00bbbce114a2a22423 | +----------------------------------+ 1 row in set (0.06 sec) mysql> SELECT SQL_NO_CACHE name FROM people WHERE age BETWEEN 18 AND 22 AND zip IN (12345,12346, 12347); +----------------------------------+ | name | +----------------------------------+ | ed4481336eb9adca222fd404fa15658e | | 888ba838661aff00bbbce114a2a22423 | +----------------------------------+ 2 rows in set (1 min 56.09 sec)
对于第二条sql，可以使用union改写，
mysql> SELECT name FROM people WHERE age=18 AND zip IN (12345,12346, 12347) -> UNION ALL -> SELECT name FROM people WHERE age=19 AND zip IN (12345,12346, 12347) -> UNION ALL -> SELECT name FROM people WHERE age=20 AND zip IN (12345,12346, 12347) -> UNION ALL -> SELECT name FROM people WHERE age=21 AND zip IN (12345,12346, 12347) -> UNION ALL -> SELECT name FROM people WHERE age=22 AND zip IN (12345,12346, 12347);
而mysql5.6引入了index condition pushdown，从优化器层面解决了此类问题。
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