Re: LIMIT confuses the planner - Mailing list pgsql-performance

From Tom Lane
Subject Re: LIMIT confuses the planner
Date
Msg-id 3562.1235401789@sss.pgh.pa.us
Whole thread Raw
In response to Re: LIMIT confuses the planner  (Robert Haas <robertmhaas@gmail.com>)
Responses Re: LIMIT confuses the planner  (Kouber Saparev <kouber@saparev.com>)
List pgsql-performance
Robert Haas <robertmhaas@gmail.com> writes:
> If you left seq_page_cost (which isn't mentioned here) at the default
> value but reduced random_page_cost to 0.1, then you have
> random_page_cost < seq_page_cost.  That's probably Bad.

... well, it's certainly going to push the planner to believe indexscans
are cheaper than sorts no matter what.

The previously noted rowcount estimation problem might be a bigger issue
in this particular case, but I agree this is a Bad Idea.

            regards, tom lane

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