我们知道，MySQL 不支持条件索引。 什么是条件索引呢? 条件索引就是在索引列上根据WHERE条件进行一定的过滤后产生的索引。 这样的索引有以下优势：
<strong>Table "ytt.girl1"
Column |Type|Modifiers
--------+---------+--------------------
id| integer | not null
rank| integer | not null default 0
Indexes:
"girl1_pkey" PRIMARY KEY, btree (id)
"idx_girl1_rank" btree (rank) WHERE rank >= 10 AND rank <= 100
执行的查询语句为：
select * from girl1 where rank between 20 and 60 limit 20;
用了全部索引的查询计划：
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------
Limit(cost=0.29..36.58 rows=20 width=8) (actual time=0.024..0.054 rows=20 loops=1)
->Index Scan using idx_girl1_rank on girl1(cost=0.29..421.26 rows=232 width=8) (actual time=0.023..0.044 rows=20 loops=1)
Index Cond: ((rank >= 20) AND (rank <= 60))
Total runtime: 0.087 ms
(4 rows)
Time: 1.881 ms
用了条件索引的查询计划：
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------
Limit(cost=0.28..35.54 rows=20 width=8) (actual time=0.036..0.068 rows=20 loops=1)
->Index Scan using idx_girl1_rank on girl1(cost=0.28..513.44 rows=291 width=8) (actual time=0.033..0.061 rows=20 loops=1)
Index Cond: ((rank >= 20) AND (rank <= 60))
Total runtime: 0.106 ms
(4 rows)
Time: 0.846 ms</strong>
<strong>ytt>show create table girl1_filtered_index; +----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | girl1_filtered_index | CREATE TABLE `girl1_filtered_index` ( `id` int(11) NOT NULL, `rank` int(11) NOT NULL DEFAULT '0', PRIMARY KEY (`id`), KEY `idx_rank` (`rank`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 | +----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) 接下来，对基础表的更新操作做下修改，创建了三个触发器。 DELIMITER $$ USE `t_girl`$$ DROP TRIGGER /*!50032 IF EXISTS */ `filtered_insert`$$ CREATE /*!50017 DEFINER = 'root'@'localhost' */ TRIGGER `filtered_insert` AFTER INSERT ON `girl1` FOR EACH ROW BEGIN IF new.rank BETWEEN 10 AND 100 THEN INSERT INTO girl1_filtered_index VALUES (new.id,new.rank); END IF; END; $$ DELIMITER ; DELIMITER $$ USE `t_girl`$$ DROP TRIGGER /*!50032 IF EXISTS */ `filtered_update`$$ CREATE /*!50017 DEFINER = 'root'@'localhost' */ TRIGGER `filtered_update` AFTER UPDATE ON `girl1` FOR EACH ROW BEGIN IF new.rank BETWEEN 10 AND 100 THEN REPLACE girl1_filtered_index VALUES (new.id,new.rank); ELSE DELETE FROM girl1_filtered_index WHERE id = old.id; END IF; END; $$ DELIMITER ; DELIMITER $$ USE `t_girl`$$ DROP TRIGGER /*!50032 IF EXISTS */ `filtered_delete`$$ CREATE /*!50017 DEFINER = 'root'@'localhost' */ TRIGGER `filtered_delete` AFTER DELETE ON `girl1` FOR EACH ROW BEGIN DELETE FROM girl1_filtered_index WHERE id = old.id; END; $$ DELIMITER ; OK，我们导入测试数据。 ytt>load data infile 'girl1.txt' into table girl1 fields terminated by ','; Query OK, 100000 rows affected (1.05 sec) Records: 100000 Deleted: 0 Skipped: 0 Warnings: 0 ytt>select count(*) from girl1; +----------+ | count(*) | +----------+ | 100000 | +----------+ 1 row in set (0.04 sec) ytt>select count(*) from girl1_filtered_index; +----------+ | count(*) | +----------+ | 640 | +----------+ 1 row in set (0.00 sec)</strong>
<strong>select a.id,a.rank from girl1 as a where a.id in (select b.id from girl1_filtered_index as b where b.rank between 20 and 60) limit 20;</strong>
