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I am running into some performance issue and I have a query as follow:
SELECT * FROM Foo
UNION
SELECT * FROM Boo
UNION
SELECT * FROM Koo
I know for sure that Koo is not returning any duplicates. I was considering to use UNION ALL at the end and therefore save the sorting and distinct selection time for Koo. The query should look like:
SELECT * FROM Foo
UNION
SELECT * FROM Boo
UNION ALL
SELECT * FROM Koo
Will that help or it will be affected bu the first UNION?
Always use UNION ALL if you know that there will not be duplicates.
It’s a bit gray here, but still worthwhile – although practically marginal.
If it’s a straight UNION-UNION, SQL Server can optimize this to collate all 3 result sets and perform a single sort across both. Since sorting is mostly O(n log n), it amounts to very little difference between that and [(A distinct B) add C].
update
Although it is possible to perform a single merge-sort, SQL Server doesn’t seem to do it (at least not always) – so the wisdom of using UNION ALL is definitely worth it here. Compare the plans here: SQLFiddle (click on the “View Execution Plan” links)