[英]Why my mysql doesn't use index?
I'm tuning my query for mysql. 我正在调整mysql查询。 the schema has index of user_id (following..) but the index is not used.
该架构具有user_id的索引(以下..),但未使用该索引。 why?
为什么?
Env: MySQL4.0.27,MyISAM 环境:MySQL4.0.27,MyISAM
SQL is the following : SQL如下:
SELECT type,SUM(value_a) A, SUM(value_b) B, SUM(value_c) C
FROM big_record_table
WHERE user_id='<user_id>'
GROUP BY type
Explain: 说明:
|table |type |possible_keys |key |key_len |ref |rows |Extra|
|big_record_table| ALL| user_id_key|||| 1059756 |Using where; Using temporary; Using filesort|
could you describe detail? 你能描述细节吗?
scheme is following: 方案如下:
CREATE TABLE `big_record_table` (
`user_id` int(11) NOT NULL default '0',
`type` enum('type_a','type_b','type_c') NOT NULL default 'type_a',
`value_a` bigint(20) NOT NULL default '0',
`value_b` bigint(20) default NULL,
`value_c` bigint(20) NOT NULL default '0',
KEY `user_id_key` (`user_id`)
) TYPE=MyISAM
My guess is that type
and user_id
are not indexed
. 我的猜测是
type
和user_id
没有indexed
。
Just a will run. 只是将运行。 You're not giving much to play with.
您没有太多可玩的东西。
First, we don't see how your indexes are declared. 首先,我们看不到如何声明您的索引。 Can you get a dump of the tables?
你能把桌子丢掉吗? In PostgreSQL you'd use
pg_dump
but I don't know how in MySQL. 在PostgreSQL中,您将使用
pg_dump
但在MySQL中我不知道如何使用。 Have you done an ANALYZE on the table? 您在桌子上做过分析吗?
It could be that implicit type conversion is preventing your index from being used. 隐式类型转换可能会阻止您的索引被使用。 You have defined user_id as an int, but specified a string in the query.
您已将user_id定义为一个int,但在查询中指定了一个字符串。 This gives MySQL the option of either converting the string in the query into an int (which might not be accurate) - or convert every user_id in the database into a string to compare against the string in the query.
这使MySQL可以选择将查询中的字符串转换为int(可能不准确)-或将数据库中的每个user_id转换为字符串以与查询中的字符串进行比较。
Short answer: try removing the quotes in the query 简短答案:尝试删除查询中的引号
SELECT type,SUM(value_a) A, SUM(value_b) B, SUM(value_c) C
FROM big_record_table
WHERE user_id=123
GROUP BY type
(where 123 is replaced with the correct user-id). (其中123替换为正确的用户ID)。
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