我有这样的选择查询的ItemType为varchar类型和ItemComments很慢为int类型:MySQL的选择查询获取与两种情况和降序
select * from ItemInfo where ItemType="item_type" order by ItemComments desc limit 1
你可以看到这个查询有3个条件:
- 其中'ItemType'等于特定值;
- 为了用降序排列
有趣的是,当我选择与所有三个条件的行,它变得非常缓慢“ItemComments”
select * from ItemInfo where ItemType="item_type" order by ItemComments desc limit 1;
/* Affected rows: 0 Found rows: 1 Warnings: 0 Duration for 1 query: 16.318 sec. */
select * from ItemInfo where ItemType="item_type" order by ItemComments limit 1;
/* Affected rows: 0 Found rows: 1 Warnings: 0 Duration for 1 query: 0.140 sec. */
select * from ItemInfo order by ItemComments desc limit 1;
/* Affected rows: 0 Found rows: 1 Warnings: 0 Duration for 1 query: 0.015 sec. */
另外,
- 我使用MySQL 5.7 InnoDB引擎。
- 我已经在ItemType和ItemComments上创建了索引,而表ItemInfo包含2百万行。
我已经搜索了许多可能的解释,如MySQL支持降序索引,复合索引等。但是这些仍然不能解释为什么查询#1运行缓慢而查询#2和#3运行良好。
如果有人能帮助我,我将非常感激。
更新:create table和解释信息
创建代码:
CREATE TABLE `ItemInfo` (
`ItemID` VARCHAR(255) NOT NULL,
`ItemType` VARCHAR(255) NOT NULL,
`ItemPics` VARCHAR(255) NULL DEFAULT '0',
`ItemName` VARCHAR(255) NULL DEFAULT '0',
`ItemComments` INT(50) NULL DEFAULT '0',
`ItemScore` DECIMAL(10,1) NULL DEFAULT '0.0',
`ItemPrice` DECIMAL(20,2) NULL DEFAULT '0.00',
`ItemDate` DATETIME NULL DEFAULT '1971-01-01 00:00:00',
PRIMARY KEY (`ItemID`, `ItemType`),
INDEX `ItemDate` (`ItemDate`),
INDEX `ItemComments` (`ItemComments`),
INDEX `ItemType` (`ItemType`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB;
解释结果:
mysql> explain select * from ItemInfo where ItemType="item_type" order by ItemComments desc limit 1;
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | i | NULL | index | ItemType | ItemComments | 5 | NULL | 83 | 1.20 | Using where |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
mysql> explain select * from ItemInfo where ItemType="item_type" order by ItemComments limit 1;
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | i | NULL | index | ItemType | ItemComments | 5 | NULL | 83 | 1.20 | Using where |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
mysql> explain select * from ItemInfo order by ItemComments desc limit 1;
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------+
| 1 | SIMPLE | i | NULL | index | NULL | ItemComments | 5 | NULL | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------+
从O.查询琼斯:
mysql> explain
-> SELECT a.*
-> FROM ItemInfo a
-> JOIN (
-> SELECT MAX(ItemComments) ItemComments, ItemType
-> FROM ItemInfo
-> GROUP BY ItemType
-> ) maxcomm ON a.ItemType = maxcomm.ItemType
-> AND a.ItemComments = maxcomm.ItemComments
-> WHERE a.ItemType = 'item_type';
+----+-------------+------------+------------+-------+----------------------------------------+-------------+---------+---------------------------+---------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+----------------------------------------+-------------+---------+---------------------------+---------+----------+--------------------------+
| 1 | PRIMARY | a | NULL | ref | ItemComments,ItemType | ItemType | 767 | const | 27378 | 100.00 | Using where |
| 1 | PRIMARY | <derived2> | NULL | ref | <auto_key0> | <auto_key0> | 772 | mydb.a.ItemComments,const | 10 | 100.00 | Using where; Using index |
| 2 | DERIVED | ItemInfo | NULL | index | PRIMARY,ItemDate,ItemComments,ItemType | ItemType | 767 | NULL | 2289466 | 100.00 | NULL |
+----+-------------+------------+------------+-------+----------------------------------------+-------------+---------+---------------------------+---------+----------+--------------------------+
我不确定我是否执行此查询是正确的,但我无法在相当长的时间内获取记录。
查询来自维杰。但我想补充的ItemType连接条件的原因,只有max_comnt返回来自其他ItemType的项目:
SELECT ifo.* FROM ItemInfo ifo
JOIN (SELECT ItemType, MAX(ItemComments) AS max_comnt FROM ItemInfo WHERE ItemType="item_type") inn_ifo
ON ifo.ItemComments = inn_ifo.max_comnt and ifo.ItemType = inn_ifo.ItemType
/* Affected rows: 0 Found rows: 1 Warnings: 0 Duration for 1 query: 7.441 sec. */
explain result:
+----+-------------+------------+------------+-------------+-----------------------+-----------------------+---------+-------+-------+----------+-----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------------+-----------------------+-----------------------+---------+-------+-------+----------+-----------------------------------------------------+
| 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 1 | PRIMARY | ifo | NULL | index_merge | ItemComments,ItemType | ItemComments,ItemType | 5,767 | NULL | 88 | 100.00 | Using intersect(ItemComments,ItemType); Using where |
| 2 | DERIVED | ItemInfo | NULL | ref | ItemType | ItemType | 767 | const | 27378 | 100.00 | NULL |
+----+-------------+------------+------------+-------------+-----------------------+-----------------------+---------+-------+-------+----------+-----------------------------------------------------+
而且我想解释一下为什么要放在第一位使用以便与限制:我打算从表中随机地取记录一个特定的概率。从python生成的随机索引作为变量发送给MySQL。但后来我发现它花费了很多时间,所以我决定只使用我得到的第一张唱片。
经过O的启发。琼斯和维杰,我尝试使用最大功能,但它表现不佳:
select max(ItemComments) from ItemInfo where ItemType='item_type'
/* Affected rows: 0 Found rows: 1 Warnings: 0 Duration for 1 query: 6.225 sec. */
explain result:
+----+-------------+------------+------------+------+---------------+----------+---------+-------+-------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+----------+---------+-------+-------+----------+-------+
| 1 | SIMPLE | ItemInfo | NULL | ref | ItemType | ItemType | 767 | const | 27378 | 100.00 | NULL |
+----+-------------+------------+------------+------+---------------+----------+---------+-------+-------+----------+-------+
谢谢你所有的这个问题。希望您可以根据上述信息提供更多解决方案。
你能展示一个解释吗? –