Performance difference if I UNION first, then put WHERE on unioned result set?
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Published on 2010-05-11T22:09:56Z
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In Microsoft SQL, is there any difference in performance between this:
SELECT columns FROM table1 WHERE cond
UNION
SELECT columns FROM table2 WHERE cond
and this:
SELECT columns FROM
(
SELECT columns FROM table1
UNION
SELECT columns FROM table2
) WHERE cond
?
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