From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | "Florian G(dot) Pflug" <fgp(at)phlo(dot)org> |
Cc: | Martijn van Oosterhout <kleptog(at)svana(dot)org>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: An Idea for planner hints |
Date: | 2006-08-09 13:42:13 |
Message-ID: | 19477.1155130933@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
"Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
> Image a complex, autogenerated query with looks something like this
> select ....
> from t1
> join t2 on ...
> join t3 on ...
> join t4 on ...
> ...
> ...
> where
> <big, complicated expression derived from some user input>.
> This big, complicated expression looks different for every query - and
> currently, postgres often vastly overestimates the selectivity of this
> expression.
This is a straw man. There is no way that your application can throw in
a chosen-at-random selectivity value for a join condition that it
doesn't understand and have that be more likely to be right than the
planner's guess.
regards, tom lane
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