Re: Searching for the cause of a bad plan - Mailing list pgsql-performance

From Tom Lane
Subject Re: Searching for the cause of a bad plan
Date
Msg-id 13644.1190397220@sss.pgh.pa.us
Whole thread Raw
In response to Re: Searching for the cause of a bad plan  (Simon Riggs <simon@2ndquadrant.com>)
Responses Re: Searching for the cause of a bad plan  (Simon Riggs <simon@2ndquadrant.com>)
List pgsql-performance
Simon Riggs <simon@2ndquadrant.com> writes:
> On Fri, 2007-09-21 at 12:08 -0400, Tom Lane wrote:
>> The reason you get a bad plan is that this rowcount estimate is so far
>> off:

> That's true, but its not relevant,

Yes it is --- the reason it wants to use a hashjoin instead of a
nestloop is exactly that it thinks the loop would iterate too many
times.  (Ten is already too many in this case --- if it had estimated
five rows out of the join, it'd have gone with the nestloop, since
the cost estimate difference at the top level is less than 2x.)

            regards, tom lane

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