【原創】MySQL 模擬條件索引薦

我們知道，MySQL 不支援條件索引。 什麼是條件索引呢？ 條件索引就是在索引列上根據WHERE條件進行一定的過濾後產生的索引。 這樣的索引有以下優勢：

第一點， 比基於這個列的全部索引佔用空間來的小。

第二點， 特別是基於FULL INDEX SCAN 的時候，佔用空間小的索引對記憶體佔用也小很多。

PostgreSQL,SqlServer等都支援條件索引，所以我們先來看下條件索引的實際情況。

表結構如下，記錄大概有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
執行的查詢語句為：
select * from girl1 where rank between 20 and 60 limit 20;
用了全部索引的查詢計劃：
                                                           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
用了條件索引的查詢計劃：
                                                           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，在掃描的記錄數以及時間上，條件索引的優勢都很明顯。

接下來，我們在MySQL 模擬下這樣的過程。

由於MySQL 不支援這樣的索引， 在SQL層面上，只能建立一個索引表來儲存對應條件的主鍵以及索引鍵。

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)
接下來，對基礎表的更新操作做下修改，建立了三個觸發器。
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，我們匯入測試資料。
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)

這裡，我們把查詢語句修改成基礎表和條件索引表的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;

當然這只是功能上的一個演示。 最終實現得靠MySQL 5.8了。^____^