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什么是MySQL索引提示-創(chuàng)新互聯(lián)
本篇內容介紹了“什么是MySQL索引提示”的有關知識，在實際案例的操作過程中，不少人都會遇到這樣的困境，接下來就讓小編帶領大家學習一下如何處理這些情況吧！希望大家仔細閱讀，能夠學有所成！
SQL提示，是優(yōu)化數據庫的一個重要手段，簡單來說，就是在SQL語句中加入一些人為的提示來達到優(yōu)化操作的目的。MySQL數據庫支持索引提示(INDEX HINT)顯式的告訴優(yōu)化器使用了哪個索引。有以下幾種情況可能用到索引提示：
1、MySQL數據庫的優(yōu)化器錯誤的選擇了某個索引，導致SQL運行很慢。這個在情況比較少見。優(yōu)化器在絕大部分情況下工作的非常有效和正確。
2、某些SQL語句可以選擇的索引非常多，這時優(yōu)化器選擇執(zhí)行計劃時間的開銷可能會大于SQL語句本身。例如優(yōu)化器分析Range查詢本身就是比較耗時的操作。這時DBA或開發(fā)人員分析最優(yōu)的索引選擇，通過index hint來強制使優(yōu)化器不進行各個路徑的成本分析直接選擇指定的索引來完成查詢。
index hint種類
MySql共有三種索引提示，分別是：USE INDEX、IGNORE INDEX和FORCE INDEX，他們之間的區(qū)別是：
1、use index：use index告訴MySql用列表中的其中一個索引去做本次查詢，就可以讓MySQL不再考慮其他可用的索引建議MySQL用這些索引，但是MySQL不一定會用。
MySQL > show create table test2 \G *************************** 1. row *************************** Table: test2 Create Table: CREATE TABLE `test2` ( `id` bigint(16) NOT NULL AUTO_INCREMENT, `order_seq` bigint(16) NOT NULL, `order_type` int(11) DEFAULT NULL, `order_flag` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_id` (`id`), KEY `idx_id_orderseq` (`id`,`order_seq`), KEY `idx_order_seq` (`order_seq`) ) ENGINE=InnoDB AUTO_INCREMENT=15002212 DEFAULT CHARSET=utf8mb4 1 row in set (0.00 sec) MySQL > explain select * from test2 where id>10000 and id<1000000; +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ | 1 | SIMPLE | test2 | NULL | range | PRIMARY,idx_id,idx_id_orderseq | PRIMARY | 8 | NULL | 2021716 | 100.00 | Using where | +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) MySQL > explain select * from test2 use index(idx_id) where id>10000 and id<100000; +----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-----------------------+ | 1 | SIMPLE | test2 | NULL | range | idx_id | idx_id | 8 | NULL | 180580 | 100.00 | Using index condition | +----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) MySQL > explain select * from test2 use index(idx_order_seq) where id=10000; +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | test2 | NULL | ALL | NULL | NULL | NULL | NULL | 14611349 | 0.00 | Using where | +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+ 1 row in set, 1 warning (0.01 sec)
2、ignore index：ignore index告訴mysql不要使用某些索引去做本次查詢
MySQL > show create table test2 \G *************************** 1. row *************************** Table: test2 Create Table: CREATE TABLE `test2` ( `id` bigint(16) NOT NULL AUTO_INCREMENT, `order_seq` bigint(16) NOT NULL, `order_type` int(11) DEFAULT NULL, `order_flag` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_id` (`id`), KEY `idx_id_orderseq` (`id`,`order_seq`), KEY `idx_order_seq` (`order_seq`) ) ENGINE=InnoDB AUTO_INCREMENT=15002212 DEFAULT CHARSET=utf8mb4 1 row in set (0.00 sec) MySQL > explain select * from test2 where id>10000 and id<1000000; +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ | 1 | SIMPLE | test2 | NULL | range | PRIMARY,idx_id,idx_id_orderseq | PRIMARY | 8 | NULL | 2021716 | 100.00 | Using where | +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) MySQL > explain select * from test2 ignore index(primary) where id>10000 and id<1000000; +----+-------------+-------+------------+-------+------------------------+--------+---------+------+---------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+------------------------+--------+---------+------+---------+----------+-----------------------+ | 1 | SIMPLE | test2 | NULL | range | idx_id,idx_id_orderseq | idx_id | 8 | NULL | 1971862 | 100.00 | Using index condition | +----+-------------+-------+------------+-------+------------------------+--------+---------+------+---------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec)
3、force index：強制MySQL使用一個特定的索引
MySQL > show create table test2 \G *************************** 1. row *************************** Table: test2 Create Table: CREATE TABLE `test2` ( `id` bigint(16) NOT NULL AUTO_INCREMENT, `order_seq` bigint(16) NOT NULL, `order_type` int(11) DEFAULT NULL, `order_flag` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_id` (`id`), KEY `idx_id_orderseq` (`id`,`order_seq`), KEY `idx_order_seq` (`order_seq`) ) ENGINE=InnoDB AUTO_INCREMENT=15002212 DEFAULT CHARSET=utf8mb4 1 row in set (0.00 sec) MySQL > explain select * from test2 where id>10000 and id<1000000; +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ | 1 | SIMPLE | test2 | NULL | range | PRIMARY,idx_id,idx_id_orderseq | PRIMARY | 8 | NULL | 2021716 | 100.00 | Using where | +----+-------------+-------+------------+-------+--------------------------------+---------+---------+------+---------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) MySQL > explain select * from test2 force index(idx_id) where id>10000 and id<1000000; +----+-------------+-------+------------+-------+---------------+--------+---------+------+---------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+---------+----------+-----------------------+ | 1 | SIMPLE | test2 | NULL | range | idx_id | idx_id | 8 | NULL | 1971862 | 100.00 | Using index condition | +----+-------------+-------+------------+-------+---------------+--------+---------+------+---------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec)
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