MySQL选择查询非常慢 [英] MySQL select query is terribly slow

问题描述

我有一张桌子，上面有2 196 998条记录:

I have a table with 2 196 998 records:

CREATE TABLE price (
    dt TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    marketId INT,
    buy DOUBLE,
    sell DOUBLE,
    PRIMARY KEY (dt, marketId),
    FOREIGN KEY fk_price_market(marketId) REFERENCES market(id) ON UPDATE CASCADE ON DELETE CASCADE
)  ENGINE=INNODB;

查询

select max(buy) from price;

花费1.92秒，这是一个合理的时间，如果我在购买"列上创建索引，则需要0.00秒:

takes 1.92 sec that is a reasonable time and it takes 0.00 sec if I create an index on 'buy' column:

CREATE INDEX idx_price_buy ON price (buy);

和查询

select count(*) from price where marketId=309;

花费0.05秒并返回160570.

takes 0.05 sec and returns 160 570.

但是查询

select max(buy) from price where marketId=309;

花费15.49秒(这是非常大的)，即使我同时创建了两个示例:

takes 15.49 sec (that is terribly huge) even if I create both idices:

CREATE INDEX idx_price_market ON price (marketId);
CREATE INDEX idx_price_buy ON price (buy);

(我不确定，但是索引idx_price_market可能已经存在，因为在外键约束中需要marketId列)

(I am not sure, but probably index idx_price_market already exists because marketId column is needed in a foreign key constraint)

1)有没有优化的方法?

1) Is there a way to optimize it?

2)如果否，那么其他数据库呢?他们表现更好吗?

2) If no, what about other databases? Do they perform better?

创建复合索引后

创建索引idx_price_market_buy开价(marketId，购买)；

CREATE INDEX idx_price_market_buy ON price (marketId, buy);

查询需要0.00秒.

desc select max(buy) from price where marketId=309;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 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 (0.01 sec)

推荐答案

select max(buy) from price where marketId=309;

在每列上创建单独的索引可能不允许MySQL优化查询.

Creating individual indexes on each column probably does not allow MySQL to optimize the query.

对于此查询，您需要(marketId, buy)上的化合物索引.

For this query, you want a compound index on (marketId, buy).

create index idx_price_market_buy ON price (marketId, buy);

索引中列的顺序很重要:首先在marketId上使用查询过滤器(因此您希望此列位于Coumpound索引中的第一位置)，然后 计算最大buy

The ordering of columns in the index matters: first the query filters on marketId (so you want this column in first position in the coumpound index), then it computes the maximum buy.

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