如何有效地获得每个值的最近插入的记录

Question

I have a table TableA , and the data will be like below:

PostID   PostComments   PostTransDate                    UserID
-----------------------------------------------------------------
10000    VRDFHFGFTR     2013-10-26 21:08:19.817          43434
10000    GFDGDFSDFF     2013-10-26 21:12:32.323          67576
10000    HGFHGFBNBF     2013-10-26 21:43:43.545          3232
10000    JNFNGHFGHG     2013-10-26 21:45:46.656          768
10000    MJHJNGJHGH     2013-10-26 21:56:32.767          9897
10001    XCVGFDGDFG     2013-10-26 22:54:54.868          3424
10001    YTUGFGHHGF     2013-10-26 13:32:54.132          12313
10001    HGFHFGHGHF     2013-10-26 18:08:32.878          6565

Here, I want to get UserID,PostComments of each PostID of maximum PostTransDate value.

Output required:

--------------------------------------------------
PostID   PostComments   PostTransDate                    UserID
-----------------------------------------------------------------
10000    MJHJNGJHGH     2013-10-26 21:56:32.767          9897
10001    XCVGFDGDFG     2013-10-26 22:54:54.868          3424

I already have queries for getting this.

Query 1:

SELECT  TT.PostID,TT.PostComments,TT.UserID, TT.PostTransDate
FROM tableA TT WITH(NOLOCK) 
INNER JOIN
(
    SELECT PostID,MAX(PostTransDate)  PostTransDate
    FROM tableA T WITH(NOLOCK)
    GROUP BY PostID 
) T ON T.PostID = TT.PostID AND T.PostTransDate = TT.PostTransDate 

Query 2:

SELECT *
FROM
(
SELECT PostID,UserID,PostTransDate,T.PostComments,
        ROW_NUMBER() OVER(PARTITION BY PostID ORDER BY PostTransDate DESC) RNO
FROM tableA T

) N WHERE RNO = 1

I can't run these queries on production as these are very heavy. If anyone has a more simplified query than this, please post.

Answer 1

Having no idea about your underlying index structure, and whether or not you can even change it, I would suggest this index:

CREATE INDEX x ON dbo.TableA(PostID, PostTransDate DESC) 
  INCLUDE (UserID, PostComments);

This will still require a scan to solve the existing query, but it will at least scan this index, which will be more efficient than scanning the entire table (assuming there are other columns in the table that aren't referenced by this query).

;WITH x AS 
(
  SELECT PostID, UserID, PostTransDate, PostComments,
    rn = ROW_NUMBER() OVER (PARTITION BY PostID ORDER BY PostTransDate DESC)
  FROM dbo.TableA
)
SELECT PostID, UserID, PostTransDate, PostComments
  FROM x WHERE rn = 1;

If you want to include ties (multiple comments on the same post by different users at the same time), just change ROW_NUMBER() to DENSE_RANK() (well, actually, if you're only ever after the latest date, you could use RANK() just as effectively - I'm not sure they perform any different but it will save you 6 characters). And if you don't want to include ties, you could break them predictably by adding an additional column to the order by inside OVER() - for example, if you wanted the user with the longest tenure, you could order by UserID after the descending post date.

Another idea, if you can't change the indexing or this doesn't provide enough boost, is to materialize the results in another table. You can handle this pretty easily with a trigger, but it will affect your DML workload, so it's certainly something you shouldn't just do to fix this one query. It might actually make your application's performance worse. Of course unless you materialize all the data for this query in this table (and that would be quite redundant), it might not work so well, because in order to retrieve the data from the main table, you'll still have to join to it, and you'll still likely need a scan on the larger table to do so. If the main table has an IDENTITY column or some other primary key, that might make things both easier and more efficient, but I'm not going to start coding up solutions until I fully understand the underlying structure.

Answer 2

Try this out:

SELECT ta1.* FROM tableA ta1
LEFT JOIN tableA ta2
ON ta1.postId = ta2.postId AND ta1.postTransDate < ta2.postTransDate
WHERE ta2.postTransDate IS NULL

Output:

| POSTID | POSTCOMMENTS |                  POSTTRANSDATE | USERID |
|--------|--------------|--------------------------------|--------|
|  10000 |   MJHJNGJHGH | October, 26 2013 21:56:32+0000 |   9897 |
|  10001 |   XCVGFDGDFG | October, 26 2013 22:54:54+0000 |   3424 |

FIddle here