Making The Most Of Temp Tables Part 3: More Opportune Indexes

I Know You


You have too many indexes on too many tables already, and the thought of adding more fills you with a dread that has a first, middle, last, and even a confirmation name.

This is another place where temp tables can save your bacon, because as soon as the query is done they basically disappear.

Forever. Goodbye.

Off to buy a pack of smokes.

That Yesterday


In yesterday’s post, we looked at how a temp table can help you materialize an expression that would otherwise be awkward to join on.

If we take that same query, we can see how using the temp table simplifies indexing.

SELECT
    p.OwnerUserId,
    SUM(p.Score) AS TotalScore,
    COUNT_BIG(*) AS records,
    CASE WHEN p.PostTypeId = 1 
         THEN p.OwnerUserId
         WHEN p.PostTypeId = 2
         THEN p.LastEditorUserId
    END AS JoinKey
INTO #Posts
FROM dbo.Posts AS p
WHERE p.PostTypeId IN (1, 2)
AND   p.Score > 100
GROUP BY CASE
             WHEN p.PostTypeId = 1 
             THEN p.OwnerUserId
             WHEN p.PostTypeId = 2 
             THEN p.LastEditorUserId
         END,
         p.OwnerUserId;

CREATE CLUSTERED INDEX c ON #Posts(JoinKey);

SELECT *
FROM #Posts AS p
WHERE EXISTS
(
    SELECT 1/0
    FROM dbo.Users AS u
    WHERE p.JoinKey = u.Id
);

Rather than have to worry about how to handle a bunch of columns across the where and join and select, we can just stick a clustered index on the one column we care about doing anything relational with to get the final result.

Thanks for reading!

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3 thoughts on “Making The Most Of Temp Tables Part 3: More Opportune Indexes

  1. Is it really necessary to write the #temp table twice?
    First as a HEAP, and then as a Clustered Index.

    If you only use the #temp table once, would it not be better just to let SQL Server sort whilst Selecting your data?

    I’ll try to test this, as soon as I get close to a database (traveling atm)

      1. well, I did re-read it.
        And the links.
        And then it dawned on me.
        As the temp tables are normally much smaller than the query that populates it, it helps if you can get it to go parallel.
        Then create the index, as it is small(ish).
        Thank you for making things clear.

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