Mysql联合索引优化

创建表，以及联合索引 b_c_d
CREATE TABLE tab_test (
id int(11) NOT NULL,
a int(11) NOT NULL,
b int(11) NOT NULL,
c varchar(255) NOT NULL,
d int(11) NOT NULL,
PRIMARY KEY (id),
KEY idx_b_c_d (b,c,d) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=latin1

能命中的索引情况。
//【左匹配】，b_c_d 索引
1 explain select * from tab_test where b = 1;

2 explain select * from tab_test where b = 1 and c= ‘e’; //交换 b,c 的位置，也能命中索引，应
为查询优化器会交换他们的位置。
3 explain select * from tab_test where c= ‘e’ and b = 1 and d= 1;
//【左匹配+其它字段】也能命中 b_c_d 索引。
1 explain select * from tab_test where b = 1 and a=6;
2 explain select * from tab_test where b = 1 and c= ‘e’ and a=6;
3 explain select * from tab_test where c= ‘e’ and b = 1 and d= 1 and a = 1;

//【左匹配+排序】
总结：
1 左匹配+左匹配排序字段 （排序字段也必须是左匹配的）
1 explain select * from tab_test where b > 8 and c = ‘e’ and d >5 order by b asc, c asc, d asc;
3 explain select * from tab_test where b > 8 order by b asc;
4 explain select * from tab_test where b > 8 order by b desc;
5 explain select * from tab_test where b > 8 and c = ‘e’ and d < 5 order by b asc;
2 左匹配+排序字段对应的都是常量
explain select * from tab_test where b > 8 and c = ‘e’ and d > 9 order by c asc; //排序字段只有 c
但 是 c 字 段 只 查 出 常 量 。

以下不能命中索引
排序字段，不是左匹配，则会使用文件排序。