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Editorial Team
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Asked: 2026-05-24T12:09:42+00:00

I’ve been having an odd problem with some queries that depend on a sub

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I’ve been having an odd problem with some queries that depend on a sub query. They run lightning fast, until I use a UNION statement in the sub query. Then they run endlessly, I’ve given after 10 minutes. The scenario I’m describing now isn’t the original one I started with, but I think it cuts out a lot of possible problems yet yields the same problem. So even though it’s a pointless query, bear with me!

I have a table: 

tblUser - 100,000 rows
tblFavourites - 200,000 rows

If I execute:

SELECT COUNT(*) 
FROM tblFavourites 
WHERE userID NOT IN (SELECT uid FROM tblUser);

… then it runs in under a second. However, if I modify it so that the sub query has a UNION, it will run for at least 10 minutes (before I give up!)

SELECT COUNT(*) 
FROM tblFavourites 
WHERE userID NOT IN (SELECT uid FROM tblUser UNION SELECT uid FROM tblUser);

A pointless change, but it should yield the same result and I don’t see why it should take any longer?

Putting the sub-query into a view and calling that instead has the same effect.

Any ideas why this would be? I’m using SQL Azure.

Problem solved. See my answer below.

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1 Answer

  1. Editorial Team

    It turns out the problem was due to one of the indexes … tblFavourites contained two foreign keys to the primary key (uid) in tblUser:

    userId
otherUserId

    both columns had the same definition and same indexes, but I discovered that swapping userId for otherUserId in the original query solved the problem.

    I ran:

    ALTER INDEX ALL ON tblFavourites REBUILD

    … and the problem went away. The query now executes almost instantly.

    I don’t know too much about what goes on behind the scenes in Sql Server/Azure … but I can only imagine that it was a damaged index or something? I update statistics frequently, but that had no effect.

    Thanks!

    —- UPDATE

    The above was not fully correct. It did fix the problem for around 20 minutes, then it returned. I have been in touch with Microsoft support for several days and it seems the problem is to do with the tempDB. They are working on a solution at their end.

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