mysql-按索引列分組+其中按索引列導致速度降低

Question

我有下一個結構的表statistics ：

+-------------------+----------------------+------+-----+---------+----------------+
| Field             | Type                 | Null | Key | Default | Extra          |
+-------------------+----------------------+------+-----+---------+----------------+
| id                | int(11)              | NO   | PRI | NULL    | auto_increment |
| created_at        | datetime             | YES  | MUL | NULL    |                |
| year_in_tz        | smallint(5) unsigned | YES  | MUL | NULL    |                |
| month_in_tz       | tinyint(3) unsigned  | YES  | MUL | NULL    |                |
+-------------------+----------------------+------+-----+---------+----------------+

使用created_at，year_in_tz，month_in_tz和（year_in_tz，month_in_tz）上的鍵：

 ALTER TABLE `statistics` ADD INDEX created_at (created_at);
 alter table statistics add index year_in_tz (year_in_tz);
 alter table statistics add index month_in_tz (month_in_tz);
 alter table statistics add index year_month_in_tz(year_in_tz,month_in_tz);

一些查詢示例...

mysql> SELECT COUNT(*) AS count_all, year_in_tz, month_in_tz 
       FROM `statistics` 
       GROUP BY year_in_tz, month_in_tz;
+-----------+------------+-------------+
| count_all | year_in_tz | month_in_tz |
+-----------+------------+-------------+
|    467890 |       2011 |          11 |
|   7339389 |       2011 |          12 |
+-----------+------------+-------------+
2 rows in set (5.04 sec)  

mysql> describe SELECT COUNT(*) AS count_all, year_in_tz, month_in_tz FROM `statistics` GROUP BY year_in_tz, month_in_tz;
 +----+-------------+--------------------+-------+---------------+------------------+---------+------+---------+-------------+
 | id | select_type | table              | type  | possible_keys | key              | key_len | ref  | rows    | Extra       |
 +----+-------------+--------------------+-------+---------------+------------------+---------+------+---------+-------------+
 |  1 | SIMPLE      | statistics         | index | NULL          | year_month_in_tz | 5       | NULL | 7797984 | Using index |
 +----+-------------+--------------------+-------+---------------+------------------+---------+------+---------+-------------+
 1 row in set (0.01 sec)

 mysql> SELECT COUNT(*) AS count_all, year_in_tz, month_in_tz 
        FROM `statistics` 
        WHERE (created_at BETWEEN '2011-10-31 20:00:00' AND '2011-12-31 19:59:59') 
        GROUP BY year_in_tz, month_in_tz;
 +-----------+------------+-------------+
 | count_all | year_in_tz | month_in_tz |
 +-----------+------------+-------------+
 |    467890 |       2011 |          11 |
 |   7339389 |       2011 |          12 |
 +-----------+------------+-------------+
 2 rows in set (1 min 33.46 sec)

 mysql> describe SELECT COUNT(*) AS count_all, year_in_tz, month_in_tz FROM `statistics` WHERE (created_at BETWEEN '2011-10-31 20:00:00' AND '2011-12-31 19:59:59') GROUP BY year_in_tz, month_in_tz;
 +----+-------------+--------------------+-------+---------------+------------------+---------+------+---------+-------------+
 | id | select_type | table              | type  | possible_keys | key              | key_len | ref  | rows    | Extra       |
 +----+-------------+--------------------+-------+---------------+------------------+---------+------+---------+-------------+
 |  1 | SIMPLE      | statistics         | index | created_at    | year_month_in_tz | 5       | NULL | 7797984 | Using where |
 +----+-------------+--------------------+-------+---------------+------------------+---------+------+---------+-------------+
 1 row in set (0.07 sec)

因此，如果我在索引列上使用where語句with子句+按索引列分組，則速度極低。 也許有人知道如何改進上一個查詢以使其更快 ？

PS在處理索引之后，我發現（created_at，year_in_tz，month_in_tz）上的新索引使查詢運行得更快，但我希望每個查詢0-1秒，而不是10秒：

alter table lending_statistics add index created_at_with_year_and_month_in_tz (created_at,year_in_tz,month_in_tz);

mysql> describe SELECT COUNT(*) AS count_all, year_in_tz, month_in_tz FROM `statistics`        WHERE (created_at BETWEEN '2011-10-31 20:00:00' AND '2011-12-31 19:59:59') GROUP BY year_in_tz, month_in_tz;
+----+-------------+--------------------+-------+-------------------------------------------------+--------------------------------------+---------+------+---------+-----------------------------------------------------------+
| id | select_type | table              | type  | possible_keys                                   | key                                  | key_len | ref  | rows    | Extra                                                     |
+----+-------------+--------------------+-------+-------------------------------------------------+--------------------------------------+---------+------+---------+-----------------------------------------------------------+
|  1 | SIMPLE      | statistics         | range | created_at,created_at_with_year_and_month_in_tz | created_at_with_year_and_month_in_tz | 9       | NULL | 3612208 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+--------------------+-------+-------------------------------------------------+--------------------------------------+---------+------+---------+-----------------------------------------------------------+

設置1行（0.05秒）

mysql> SELECT COUNT(*) AS count_all,        year_in_tz, month_in_tz        FROM `lending_statistics`        WHERE (created_at BETWEEN '2011-10-31 20:00:00' AND '2011-12-31 19:59:59')        GROUP BY year_in_tz, month_in_tz;
+-----------+------------+-------------+
| count_all | year_in_tz | month_in_tz |
+-----------+------------+-------------+
|    467890 |       2011 |          11 |
|   7339389 |       2011 |          12 |
+-----------+------------+-------------+
2 rows in set (10.62 sec)

Answer 1

將字段ID添加到您的索引created_at_with_year_and_month_in_tz，然后更改您的select語句以使用

select count(id) ....

在MySQL 5.6中，由於所有訪問的字段都是索引的一部分，因此ICP功能可能會有所幫助。 我相信，當您指定count（*）時，MySQL可能會讀取實際的數據記錄，因此它需要讀取索引文件以及數據文件。

Answer 2

試試這個，有一個日期時間索引的MySQL已知問題

    WHERE
        created_at BETWEEN 
               CAST('2011-10-31 20:00:00' AS datetime) AND 
               CAST('2011-12-31 19:59:59'  AS datetime)

Answer 3

緩慢的COUNT(*)查詢是MySQL和PostgreSQL（及其他RDBMS）經常遇到的麻煩，因為在查詢執行期間會執行順序表掃描。 嘗試考慮將聚合數據緩存到其他位置： memcached ， redis等。