Opinionated Software

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DISTINCT in UNIONs —

Developers absolutely love DISTINCT don’t they? They use them everywhere but often for the wrong reasons. My current pet hate in this area is DISTINCT in UNIONs. I’m guessing they:

  • Don’t realise UNION will give them the DISTINCT list
  • Somehow believe DISTINCT in that part of the UNION will make the statement miraculously faster

None of which are necessarily true – in fact the addition of a DISTINCT can adversely affect execution plans in some cases.


Categorised as: Rants | SQL



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