为表中的每个ID选择最新的3条记录

Question

我有一个带有复合主键(ID,Date)的表,如下所示.

+------+------------+-------+
|  ID  |    Date    | Value |
+------+------------+-------+
|   1  | 1433419200 |   15  |
|   1  | 1433332800 |   23  |
|   1  | 1433246400 |   41  |
|   1  | 1433160000 |   55  |
|   1  | 1432900800 |   24  |
|   2  | 1433419200 |   52  |
|   2  | 1433332800 |   23  |
|   2  | 1433246400 |   39  |
|   2  | 1433160000 |   22  |
|   3  | 1433419200 |   11  |
|   3  | 1433246400 |   58  |
|  ... |    ...     |  ...  |
+------+------------+-------+

Date列上还有一个单独的索引.桌子大小适中,目前行约600k,每天增长约2k.

我想做一个SELECT查询,返回Date每个记录的最新3条记录(按时间戳排序)ID.对于每个给定ID的Date值,值始终是唯一的,因此无需担心Date这里的关系.

我尝试了一种自我加入的方法,受到这个答案的启发,但它花了几秒钟才运行并且什么也没有返回:

SELECT p1.ID, p1.Date, p1.Value FROM MyTable AS p1
LEFT JOIN MyTable AS p2 
ON p1.ID=p2.ID AND p1.Date<=p2.Date
GROUP BY p1.ID
HAVING COUNT(*)<=5
ORDER BY p1.ID, p1.Date DESC;

什么是快速解决方案？

Answer 1

您可以查找每个ID的最近三个日期:

SELECT ID, Date, Value
FROM MyTable
WHERE Date IN (SELECT Date
               FROM MyTable AS T2
               WHERE T2.ID = MyTable.ID
               ORDER BY Date DESC
               LIMIT 3)

或者,查找每个ID的第三个最近日期,并将其用作限制:

SELECT ID, Date, Value
FROM MyTable
WHERE Date >= IFNULL((SELECT Date
                      FROM MyTable AS T2
                      WHERE T2.ID = MyTable.ID
                      ORDER BY Date DESC
                      LIMIT 1 OFFSET 2),
                     0)

两个查询都应该从主键的索引中获得良好的性能.