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From: David G Johnston <david.g.johnston@gmail.com>
To: pgsql-performance@postgresql.org
Subject: Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions
Date: Mon, 21 Jul 2014 14:54:11 -0700 (PDT)
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johno wrote
> Thanks for the quick reply David!
> 
> However I am still unsure how these two queries are not relationally
> equivalent. I am struggling to find a counterexample where the first and
> third query (in email, not in gist) would yield different results. Any
> ideas?

Remove the outer LIMIT 100 from both queries...

The first query would return a maximal number of rows that meet the OR
criteria while the second query would return at most 200 rows since both
sub-queries would still have their own independent LIMIT 100 clauses.

David J.




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