MySQL 模拟条件索引

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：
第一点， 比基于这个列的全部索引占用空间来的小。
第二点， 特别是基于FULL INDEX SCAN 的时候，占用空间小的索引对内存占用也小很多。


PostgreSQL,SqlServer等都支持条件索引，所以我们先来看下条件索引的实际情况。
表结构如下，记录大概有10W行：

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

可以看出，在扫描的记录数以及时间上，条件索引的优势都很明显。

接下来，我们在MySQL 模拟下这样的过程。
由于MySQL 不支持这样的索引， 在SQL层面上，只能创建一个索引表来保存对应条件的主键以及索引键。

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)

这里，我们把查询语句修改成基础表和条件索引表的JOIN。

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;

当然这只是功能上的一个演示。 最终实现得靠MySQL 5.8了。^____^