建立表：
CREATE TABLE `ygzt_test` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`a` int(11) NOT NULL,
`b` int(11) NOT NULL,
`c` int(11) NOT NULL,
`d` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `a` (`a`,`b`,`c`,`d`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci COMMENT='測試';
一、實驗一，無order by
首先加聯合索引a,b,c,d
explain select * from ygzt_test where a=1 and b=2 and c=3 and d=4
修改sql：
explain select * from ygzt_test where a>1 and b=2 and c=3 and d=4
type已經由ref降為index
修改索引b,c,d,a
explain select * from ygzt_test where a>1 and b=2 and c=3 and d=4
done
二、實驗二，order by
建立索引a，b
explain select * from ygzt_test where a>0 order by b
可以看到，a>0使用了索引，order by b 未使用
修改索引為b，a
explain select * from ygzt_test where a>0 order by b
where 與 order by 都無索引
想起此前order by＋select ＊的問題
這個問題單獨拿出來實踐下：
修改索引為b
explain select * from ygzt_test order by b
缺點：未使用索引
explain select b from ygzt_test order by b
缺點：只能返回b
explain select b,a from ygzt_test order by b
缺點：未使用索引
explain select * from ygzt_test FORCE INDEX (b) order by b
缺點：type也就是index級別
索引b，a
explain select b,a from ygzt_test order by b
缺點：與強制索引同，只能達到index級別，還加大了索引建立的復雜度
結論：使用強制索引
結論：
1.單列索引中——<,<=,=,>,>=,between,like（右邊模糊）適用索引
2.索引中有范圍的，有序性失效，解決方案以實際為准