nem*_*ems 17 mysql index primary-key mysql-5.5
我在将日期时间(甚至日期)作为主键的第一部分进行索引时遇到问题。
我使用 MySQL 5.5
这是我的两个表:
-- This is my standard table with dateDim as a dateTime
CREATE TABLE `stats` (
`dateDim` datetime NOT NULL,
`accountDim` mediumint(8) unsigned NOT NULL,
`execCodeDim` smallint(5) unsigned NOT NULL,
`operationTypeDim` tinyint(3) unsigned NOT NULL,
`junkDim` tinyint(3) unsigned NOT NULL,
`ipCountryDim` smallint(5) unsigned NOT NULL,
`count` int(10) unsigned NOT NULL,
`amount` bigint(20) NOT NULL,
PRIMARY KEY (`dateDim`,`accountDim`,`execCodeDim`,`operationTypeDim`,`junkDim`,`ipCountryDim`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
-- Here is a copy with datDim as an integer
CREATE TABLE `stats_todays` (
`dateDim` int(11) unsigned NOT NULL,
`accountDim` mediumint(8) unsigned NOT NULL,
`execCodeDim` smallint(5) unsigned NOT NULL,
`operationTypeDim` tinyint(3) unsigned NOT NULL,
`junkDim` tinyint(3) unsigned NOT NULL,
`ipCountryDim` smallint(5) unsigned NOT NULL,
`count` int(10) unsigned NOT NULL,
`amount` bigint(20) NOT NULL,
PRIMARY KEY (`dateDim`,`accountDim`,`execCodeDim`,`operationTypeDim`,`junkDim`,`ipCountryDim`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
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我用完全相同的数据填充两个表(接近 10 000 000)
但:
我的问题是:当索引的第一部分是日期时间时,为什么 MySQL 不使用 PRIMARY KEY ???这是非常奇怪的,因为使用相同的数据但用 INTEGER 和 TO_DAYS(dateDim) 合并相同的请求会晃动......
带有统计表(和日期时间)的示例:
SELECT *
FROM `stats`
WHERE
dateDim = '2014-04-03 00:00:00'
AND accountDim = 4
AND execCodeDim = 9
AND operationTypeDim = 1
AND junkDim = 5
AND ipCountryDim = 3
=> 1 result (4.5sec)
Explain:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE stats ALL NULL NULL NULL NULL 8832329 Using where
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对另一个表 stats_todays 的相同请求(使用 INTEGER 和 TO_DAYS() )
EXPLAIN SELECT *
FROM `stats_todays`
WHERE
dateDim = TO_DAYS('2014-04-03 00:00:00')
AND accountDim = 4
AND execCodeDim = 9
AND operationTypeDim = 1
AND junkDim = 5
AND ipCountryDim = 3
=> Result 1 row (0.0003 sec)
Explain:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE stats_todays const PRIMARY PRIMARY 13 const,const,const,const,const,const 1
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如果您阅读了完整的帖子,您就会明白这不是一个低基数问题,因为请求使用与 INTEGER dateDim 字段完全相同的基数....
以下是一些高级细节:
SELECT COUNT( DISTINCT dateDim )
FROM stats_todays
UNION ALL
SELECT COUNT( DISTINCT dateDim )
FROM stats;
Result:
COUNT(DISTINCT dateDim)
2192
2192
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这是索引描述:
SHOW INDEXES FROM `stats`
Table Non_unique Key_name Seq_in_index Column_name Collation Cardinality Sub_part Packed Null Index_type Comment Index_comment
stats 0 PRIMARY 1 dateDim A 6921 NULL NULL BTREE
stats 0 PRIMARY 2 accountDim A 883232 NULL NULL BTREE
stats 0 PRIMARY 3 execCodeDim A 8832329 NULL NULL BTREE
stats 0 PRIMARY 4 operationTypeDim A 8832329 NULL NULL BTREE
stats 0 PRIMARY 5 junkDim A 8832329 NULL NULL BTREE
stats 0 PRIMARY 6 ipCountryDim A 8832329 NULL NULL BTREE
SHOW INDEXES FROM `stats_todays`
Table Non_unique Key_name Seq_in_index Column_name Collation Cardinality Sub_part Packed Null Index_type Comment Index_comment
stats_todays 0 PRIMARY 1 dateDim A 7518 NULL NULL BTREE
stats_todays 0 PRIMARY 2 accountDim A 4022582 NULL NULL BTREE
stats_todays 0 PRIMARY 3 execCodeDim A 8045164 NULL NULL BTREE
stats_todays 0 PRIMARY 4 operationTypeDim A 8045164 NULL NULL BTREE
stats_todays 0 PRIMARY 5 junkDim A 8045164 NULL NULL BTREE
stats_todays 0 PRIMARY 6 ipCountryDim A 8045164 NULL NULL BTREE
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SELECT dateDim, COUNT(*) FROM stats GROUP BY dateDim WITH ROLLUP
小智 6
这是 5.5.x 中的错误。看这里
这表明您的查询应该是
SELECT *
FROM `stats`
WHERE
dateDim = CAST('2014-04-03 00:00:00' as datetime)
AND accountDim = 4
AND execCodeDim = 9
AND operationTypeDim = 1
AND junkDim = 5
AND ipCountryDim = 3
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