mysql - 如何在计算字段上优化 GROUP BY(以使用索引)？

Question

我有一个很大(近10M记录)的数据表，出于性能原因，它有一个辅助聚合伴随表。聚合表定期填充迄今为止未聚合的数据:

REPLACE INTO aggregate (channel_id, type, timestamp, value, count)
SELECT channel_id, 'day' AS type, MAX(timestamp) AS timestamp, SUM(value) AS value, COUNT(timestamp) AS count FROM data 
WHERE timestamp < UNIX_TIMESTAMP(DATE_FORMAT(NOW(), "%Y-%m-%d")) * 1000 
AND timestamp >= IFNULL((SELECT UNIX_TIMESTAMP(DATE_ADD(FROM_UNIXTIME(MAX(timestamp)/1000, "%Y-%m-%d"), 
    INTERVAL 1 day)) * 1000 FROM aggregate WHERE type = 'day'), 0) 
GROUP BY channel_id, YEAR(FROM_UNIXTIME(timestamp/1000)), DAYOFYEAR(FROM_UNIXTIME(timestamp/1000));

我发现即使没有返回数据，语句的 SELECT 部分也相当慢(在快速 PC 上需要 2 秒以上)。由于聚合需要在嵌入式设备上运行，这是一个问题。计划如下:

id  select_type table       type        key     key_len rows    Extra
1   PRIMARY     data        ALL                         9184560 Using where; Using temporary; Using filesort
2   SUBQUERY    aggregate   index       ts_uniq 22      1940    Using where; Using index

子查询本身是即时的。显然，由于 GROUP BY 子句中的计算，data 没有使用 channel_id/timestamp 索引:

CREATE TABLE `data` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `channel_id` int(11) DEFAULT NULL,
  `timestamp` bigint(20) NOT NULL,
  `value` double NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `ts_uniq` (`channel_id`,`timestamp`),
  KEY `IDX_ADF3F36372F5A1AA` (`channel_id`)
) ENGINE=MyISAM AUTO_INCREMENT=10432870 DEFAULT CHARSET=latin1;

查询可以进一步优化吗？

更新:添加请求的信息

SHOW INDEXES FROM data;

Table   Non_unique  Key_name    Seq_in_index    Column_name Collation   Cardinality Null    Index_type
data    0           PRIMARY     1               id          A           9184560             BTREE       
data    0           ts_uniq     1               channel_id  A           164         YES     BTREE       
data    0           ts_uniq     2               timestamp   A           9184560             BTREE       
data    1           IDX_ADF3..  1               channel_id  A           164         YES     BTREE       

CREATE TABLE `aggregate` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `channel_id` int(11) NOT NULL,
  `type` varchar(8) NOT NULL,
  `timestamp` bigint(20) NOT NULL,
  `value` double NOT NULL,
  `count` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `ts_uniq` (`channel_id`,`type`,`timestamp`)
) ENGINE=MyISAM AUTO_INCREMENT=1941 DEFAULT CHARSET=latin1;

我还注意到，当将 GROUP BY 更改为channel_id、时间戳时，查询变得即时。不幸的是，将数据计算添加为列是不可取的，因为分组是动态计算的。

我无法理解为什么当甚至没有任何数据要分组时 GROUP BY 索引会成为这样的问题。我尝试过运行

SELECT channel_id, 'day' AS type, MAX(timestamp) AS timestamp, SUM(value) AS value, COUNT(timestamp) AS count FROM data 
WHERE timestamp < UNIX_TIMESTAMP(DATE_FORMAT(NOW(), "%Y-%m-%d")) * 1000 
AND timestamp >= IFNULL((SELECT UNIX_TIMESTAMP(DATE_ADD(FROM_UNIXTIME(MAX(timestamp)/1000, "%Y-%m-%d"), INTERVAL 1 day)) * 1000 
    FROM aggregate WHERE type = 'day'), 0) 

同样慢，所以 GROUP 似乎不是问题所在？

更新2

进一步挖掘这条路表明

SELECT channel_id, 'day' AS type, timestamp, value, 1 FROM data 
WHERE timestamp >= (SELECT UNIX_TIMESTAMP(DATE_ADD(FROM_UNIXTIME(MAX(timestamp)/1000, "%Y-%m-%d"), 
    INTERVAL 1 day)) * 1000 FROM aggregate WHERE type = 'day');

仍然很慢(1.4秒) - 所以根本不是GROUP BY问题。

更新3

这仍然很慢:

SELECT channel_id, 'day' AS type, timestamp, value, 1 FROM data WHERE timestamp >= 1380837600000;

所以问题是内部比较是针对时间戳的，它不能使用channel_id、时间戳索引，尽管这是GROUP BY子句的一部分。这就引出了如何强制该索引的问题？

Answer 1

向数据表添加年份和日期列，并在 (channel_id,year,dayofyear) 上建立索引。插入行时填充两个新列。