使用索引查詢

MariaDB [test]> explain select * from te where id=22; #在沒有增加索引情況下，rows為7，即查詢行數 +------+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | te | ALL | NULL | NULL | NULL | NULL | 7 | Using where | +------+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.01 sec)MariaDB [test]> alter table te add index tindex(id); #增加索引 Query OK, 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0MariaDB [test]> explain select * from te where id=22; #rows變為1 +------+-------------+-------+------+---------------+--------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+--------+---------+-------+------+-------+ | 1 | SIMPLE | te | ref | tindex | tindex | 5 | const | 1 | | +------+-------------+-------+------+---------------+--------+---------+-------+------+-------+ 1 row in set (0.00 sec)MariaDB [test]>

索引使用注意事項
使用like關鍵字查詢時，%號位于首位會導致無法使用索引查詢，否則可以正常使用索引查詢，如下

多列索引問題

MariaDB [test]> alter table te add index indextwo(id,name); #創建索引 Query OK, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0MariaDB [test]> show create table te\G; *************************** 1. row *************************** Table: te Create Table: CREATE TABLE `te` ( `id` int(11) DEFAULT NULL, `name` varchar(20) DEFAULT NULL, `se` varchar(3) DEFAULT 'man', `city` varchar(3) DEFAULT 'gx', KEY `indextwo` (`id`,`name`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 1 row in set (0.00 sec)ERROR: No query specifiedMariaDB [test]> explain select * from te where name='ds'; #rows為7 +------+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | te | ALL | NULL | NULL | NULL | NULL | 7 | Using where | +------+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)MariaDB [test]> explain select * from te where id=3 and name='ds'; #增加多列索引的首列后，可以正常使用索引 +------+-------------+-------+------+---------------+----------+---------+-------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+----------+---------+-------------+------+-----------------------+ | 1 | SIMPLE | te | ref | indextwo | indextwo | 28 | const,const | 1 | Using index condition | +------+-------------+-------+------+---------------+----------+---------+-------------+------+-----------------------+ 1 row in set (0.00 sec)MariaDB [test]>

轉載于:https://www.cnblogs.com/biaopei/p/7730497.html

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