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#1
September 22nd, 2005, 03:07 AM
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separate select's vs union
let S1, S2, S3 be selects against different tables but with property that they can be used in union, can I safely assume that union works no worst than issuing those 3 selects separately?
i.e. Performance(S1 and S2 and S3)<=Performance(S1 union S2 union S3) is true?
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#2
September 22nd, 2005, 04:29 AM
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IMHO not, because straight UNION suppresses duplicates with an addtional pass, what you said should apply to UNION ALL only.
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