SQL left outer join with nested select optimisation

Question

I am querying SQL Server 2012.

I have a left outer join onto a large result set, that looks like the following:

Data
LEFT OUTER JOIN
(
    SELECT
        MemberID
        ,MIN(Date)
    FROM
        Tables
    GROUP BY
        MemberID
) T
ON Data.MemberID = T.MemberID

Where Data is already a large query (tens of thousands of rows returned).

My question is: Is this nested select query evaluated only for the MemberIDs I already have in "Data"? This seems to have slowed the sproc down fairly significantly, is there a more standard way of doing this? Or is this just to be expected?

Thanks in advance.

Answer 1

No, the subquery is evaluated for all the rows.

If you want the first row for something, then do:

row_number() over (partition by Memberid order by Date) as seqnum

And then use:

where seqnum = 1