本篇內容介紹了“MySQL數(shù)據(jù)庫max()函數(shù)的作用是什么”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!
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查看表結構:
mysql> show create table coupon_use_test \G *************************** 1. row *************************** Table: coupon_use_test Create Table: CREATE TABLE `coupon_use_test` ( `id` int(11) NOT NULL DEFAULT '0', `user_id` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `coupon_code` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT '', `status` varchar(2) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT '00', `use_time` datetime DEFAULT NULL, `remark1` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `remark2` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `remark3` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, `create_user_id` varchar(128) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL, `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP ) ENGINE=InnoDB DEFAULT CHARSET=utf8 1 row in set (0.00 sec)
測試查詢速度:
mysql> select max(create_time) from coupon_use_test; +---------------------+ | max(create_time) | +---------------------+ | 2016-06-25 16:44:25 | +---------------------+ 1 row in set (2.01 sec)
查看執(zhí)行計劃:
mysql> explain select max(create_time) from coupon_use_test; +----+-------------+-----------------+------------+------+---------------+------+---------+------+---------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------------+------------+------+---------------+------+---------+------+---------+----------+-------+ | 1 | SIMPLE | coupon_use_test | NULL | ALL | NULL | NULL | NULL | NULL | 1706101 | 100.00 | NULL | +----+-------------+-----------------+------------+------+---------------+------+---------+------+---------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
創(chuàng)建create_time字段索引
mysql> alter table coupon_use_test add index idx_create_time(create_time); Query OK, 0 rows affected (17.49 sec) Records: 0 Duplicates: 0 Warnings: 0
再次查詢:
mysql> select max(create_time) from coupon_use_test; +---------------------+ | max(create_time) | +---------------------+ | 2016-06-25 16:44:25 | +---------------------+ 1 row in set (0.00 sec)
查看執(zhí)行計劃:
mysql> explain select max(create_time) from coupon_use_test; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+ 1 row in set, 1 warning (0.00 sec)
索引是有序的,create_time字段加完索引之后取max(create_time)速度變快。
看到其他優(yōu)化方法,通過轉變SQL查詢方式實現(xiàn)
mysql> select create_time from coupon_use_test order by create_time desc limit 1; +---------------------+ | create_time | +---------------------+ | 2016-06-25 16:44:25 | +---------------------+ 1 row in set (0.00 sec)
查看執(zhí)行計劃:
mysql> explain select create_time from coupon_use_test order by create_time desc limit 1; +----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+------+----------+-------------+ | 1 | SIMPLE | coupon_use_test | NULL | index | NULL | idx_create_time | 4 | NULL | 1 | 100.00 | Using index | +----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec)
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