| From: | Merlin Moncure <mmoncure(at)gmail(dot)com> |
|---|---|
| To: | Edson Richter <edsonrichter(at)hotmail(dot)com> |
| Cc: | David Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>, "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: Re : Re : Query "top 10 and others" |
| Date: | 2014-07-08 21:40:20 |
| Message-ID: | CAHyXU0xdvo0GesOxcAPGz=g+1b0MiED_4bRyxEWk2Pn27aqeDA@mail.gmail.com |
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| Lists: | pgsql-general |
On Sat, Jul 5, 2014 at 8:12 AM, Edson Richter <edsonrichter(at)hotmail(dot)com> wrote:
> Thanks!
>
> I'll investigate (explain) performance for both versions.
also be advised that in most cases when you use SQL 'UNION' you really
should be using 'UNION ALL'. It's a very common mistake:
UNION: form proper set union, combine set and remove tuple duplicates
UNION ALL: append two sets
For large sets, the duplicate removal can be expensive and possibly
introduce subtle data dependent bugs in the worst case.
merlin
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