MySQL8.0中的降序索引
前言
相信大家都知道,索引是有序的;不過,在MySQL之前版本中,只支援升序索引,不支援降序索引,這會帶來一些問題;在最新的MySQL 8.0版本中,終於引入了降序索引,接下來我們就來看一看。
降序索引
單列索引
(1)檢視測試表結構
mysql> show create table sbtest1\G *************************** 1. row *************************** Table: sbtest1 Create Table: CREATE TABLE `sbtest1` ( `id` int unsigned NOT NULL AUTO_INCREMENT,`k` int unsigned NOT NULL DEFAULT '0',`c` char(120) NOT NULL DEFAULT '',`pad` char(60) NOT NULL DEFAULT '',PRIMARY KEY (`id`),KEY `k_1` (`k`) ) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci MAX_ROWS=1000000 1 row in set (0.00 sec)
(2)執行SQL語句order by ... limit n,預設是升序,可以使用到索引
mysql> explain select * from sbtest1 order by k limit 10; +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | k_1 | 4 | NULL | 10 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ 1 row in set,1 warning (0.00 sec)
(3)執行SQL語句order by ... desc limit n,如果是降序的話,無法使用索引,雖然可以相反順序掃描,但效能會受到影響
mysql> explain select * from sbtest1 order by k desc limit 10; +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+---------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+---------------------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | k_1 | 4 | NULL | 10 | 100.00 | Backward index scan | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+---------------------+ 1 row in set,1 warning (0.00 sec)
(4)建立降序索引
mysql> alter table sbtest1 add index k_2(k desc); Query OK,0 rows affected (6.45 sec) Records: 0 Duplicates: 0 Warnings: 0
(5)再次執行SQL語句order by ... desc limit n,可以使用到降序索引
mysql> explain select * from sbtest1 order by k desc limit 10; +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | k_2 | 4 | NULL | 10 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-------+ 1 row in set,1 warning (0.00 sec)
多列索引
(1)檢視測試表結構
mysql> show create table sbtest1\G *************************** 1. row *************************** Table: sbtest1 Create Table: CREATE TABLE `sbtest1` ( `id` int unsigned NOT NULL AUTO_INCREMENT,KEY `k_1` (`k`),KEY `idx_c_pad_1` (`c`,`pad`) ) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci MAX_ROWS=1000000 1 row in set (0.00 sec)
(2)對於多列索引來說,如果沒有降序索引的話,那麼只有SQL 1才能用到索引,SQL 4能用相反順序掃描,其他兩條SQL語句只能走全表掃描,效率非常低
SQL 1:select * from sbtest1 order by c,pad limit 10;
SQL 2:select * from sbtest1 order by c,pad desc limit 10;
SQL 3:select * from sbtest1 order by c desc,pad limit 10;
SQL 4:explain select * from sbtest1 order by c desc,pad desc limit 10;
mysql> explain select * from sbtest1 order by c,pad limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | idx_c_pad_1 | 720 | NULL | 10 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set,1 warning (0.00 sec) mysql> explain select * from sbtest1 order by c,pad desc limit 10; +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | SIMPLE | sbtest1 | NULL | ALL | NULL | NULL | NULL | NULL | 950738 | 100.00 | Using filesort | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set,1 warning (0.00 sec) mysql> explain select * from sbtest1 order by c desc,pad limit 10; +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | SIMPLE | sbtest1 | NULL | ALL | NULL | NULL | NULL | NULL | 950738 | 100.00 | Using filesort | +----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set,1 warning (0.01 sec) mysql> explain select * from sbtest1 order by c desc,pad desc limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | idx_c_pad_1 | 720 | NULL | 10 | 100.00 | Backward index scan | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------+ 1 row in set,1 warning (0.00 sec)
(3)建立相應的降序索引
mysql> alter table sbtest1 add index idx_c_pad_2(c,pad desc); Query OK,0 rows affected (1 min 11.27 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> alter table sbtest1 add index idx_c_pad_3(c desc,pad); Query OK,0 rows affected (1 min 14.22 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> alter table sbtest1 add index idx_c_pad_4(c desc,0 rows affected (1 min 8.70 sec) Records: 0 Duplicates: 0 Warnings: 0
(4)再次執行SQL,均能使用到降序索引,效率大大提升
mysql> explain select * from sbtest1 order by c,pad desc limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | idx_c_pad_2 | 720 | NULL | 10 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set,pad limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | idx_c_pad_3 | 720 | NULL | 10 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set,pad desc limit 10; +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ | 1 | SIMPLE | sbtest1 | NULL | index | NULL | idx_c_pad_4 | 720 | NULL | 10 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+-------+ 1 row in set,1 warning (0.00 sec)
總結
MySQL 8.0引入的降序索引,最重要的作用是,解決了多列排序可能無法使用索引的問題,從而可以覆蓋更多的應用場景。
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