Optimal performing query for latest record for each N

Question

Here is the scenario I find myself in.

I have a reasonably big table that I need to query the latest records from. Here is the create for the essential columns for t

Answer 1

Depends on your data (how many rows are there per group?) and your indexes.

See Optimizing TOP N Per Group Queries for some performance comparisons of 3 approaches.

In your case with millions of rows for only a small number of Vehicles I would add an index on VehicleID, Timestamp and do

SELECT CA.*
FROM   Vehicles V
       CROSS APPLY (SELECT TOP 1 *
                    FROM   ChannelValue CV
                    WHERE  CV.VehicleID = V.VehicleID
                    ORDER  BY TimeStamp DESC) CA  

Answer 2

If your records are inserted sequentially, replacing TimeStamp in your query with ID may make a difference.

As a side note, how many records is this returning? Your delay could be network overhead if you are getting hundreds of thousands of rows back.

Answer 3

Try this:

SELECT SequencedChannelValue.* -- Specify only the columns you need, exclude the SequencedChannelValue
FROM
    (
        SELECT 
            ChannelValue.*,   -- Specify only the columns you need
            SeqValue = ROW_NUMBER() OVER(PARTITION BY VehicleID ORDER BY TimeStamp DESC)
        FROM ChannelValue
    ) AS SequencedChannelValue
WHERE SequencedChannelValue.SeqValue = 1

A table or index scan is expected, because you're not filtering data in any way. You're asking for the latest TimeStamp for all VehicleIDs - the query engine HAS to look at every row to find the latest TimeStamp.

You can help it out by narrowing the number of columns being returned (don't use SELECT *), and by providing an index that consists of VehicleID + TimeStamp.