[Build] TABLE statement
create table test03(
id int primary key not null auto_increment,
c1 char(10),
c2 char(10),
c3 char(10),
c4 char(10),
c5 char(10)
);
insert into test03(c1,c2,c3,c4,c5) values('a1','a2','a3','a4','a5');
insert into test03(c1,c2,c3,c4,c5) values('b1','b2','b3','b4','b5');
insert into test03(c1,c2,c3,c4,c5) values('c1','c2','c3','c4','c5');
insert into test03(c1,c2,c3,c4,c5) values('d1','d2','d3','d4','d5');
insert into test03(c1,c2,c3,c4,c5) values('e1','e2','e3','e4','e5');
select * from test03;
[Indexing]
create index idx_test03_c1234 on test03(c1,c2,c3,c4);
show index from test03;
Question: We have created a composite index idx_test03_c1234, Analysis indexes use the following SQL?
1 explain select * from test03 where c1='a1'; 2 explain select * from test03 where c1='a1' and c2='a2'; 3 explain select * from test03 where c1='a1' and c2='a2' and c3='a3'; 4 explain select * from test03 where c1='a1' and c2='a2' and c3='a3' and c4='a4';
1）
explain select * from test03 where c1='a1' and c2='a2' and c3='a3' and c4='a4';
2）
explain select * from test03 where c1='a1' and c2='a2' and c4='a4' and c3='a3';
explain select * from test03 where c4='a4' and c3='a3' and c2='a2' and c1='a1';
3）
explain select * from test03 where c1='a1' and c2='a2' and c3>'a3' and c4='a4';
4）
explain select * from test03 where c1='a1' and c2='a2' and c4>'a4' and c3='a3';
Description: All four indices used, although in the last c3, but mysql automatic tuning.
5）
explain select * from test03 where c1='a1' and c2='a2' and c4='a4' order by c3;
c3 role in sorting instead of looking
[Index of two functions: Find and Sort]
6)
EXPLAIN the SELECT * from TEST03 the WHERE c1 = 'a1' and c2 = 'a2' c3 by the Order;
7）
explain select * from test03 where c1='a1' and c2='a2' order by c4;
出现了filesort
8）
8.1
explain select * from test03 where c1='a1' and c5='a5' order by c2,c3;
只用c1一个字段索引，但是c2、c3用于排序,无filesort
8.2
explain select * from test03 where c1='a1' and c5='a5' order by c3,c2;
出现了filesort，我们建的索引是1234，它没有按照顺序来，3 2 颠倒了
9）
explain select * from test03 where c1='a1' and c2='a2' order by c2,c3;
10）
explain select * from test03 where c1='a1' and c2='a2' and c5='a5' order by c2,c3;
用c1、c2两个字段索引，但是c2、c3用于排序,无filesort
explain select * from test03 where c1='a1' and c2='a2' and c5='a5' order by c3,c2;
本例有常量c2的情况，和8.2对比（c2='c2'已经有具体值，为常量时，无需排序）
explain select * from test03 where c1='a1' and c5='a5' order by c3,c2;
filesort出现
11）
explain select * from test03 where c1='a1' and c4='a4' group by c2,c3;
12）
explain select * from test03 where c1='a1' and c4='a4' group by c3,c2;
Using where; Using temporary; Using filesort
【group by表面理解为分组，但是要注意的是，分组之前必排序】
【结论】
【一般性建议】
1、对于单键索引，尽量选择针对当前query过滤性更好的索引
2、在选择组合索引的时候，当前Query中过滤性最好的字段在索引字段顺序中，位置越靠前（左）越好。(避免索引过滤性好的索引失效)
3、在选择组合索引的时候，尽量选择可以能够包含当前query中的where字句中更多字段的索引
4、尽可能通过分析统计信息和调整query的写法来达到选择合适索引的目的