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Date: Fri, 8 Jan 2010 14:55:00 -0500
Message-ID: <603c8f071001081155w3b7b8042s362837542cfbc42b@mail.gmail.com>
Subject: Re: Change query join order
From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Kaloyan Iliev Iliev <kaloyan@digsys.bg>, pgsql-performance@postgresql.org
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On Fri, Jan 8, 2010 at 2:23 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> If the other plan does turn out to be faster (and I agree with Tom
>> that there is no guarantee of that), then one thing to check is
>> whether seq_page_cost and random_page_cost are set too high. =A0If the
>> data is all cached, the default values of 4 and 1 are three orders of
>> magnitude too large, and they should also be set to equal rather than
>> unequal values.
>
> Tweaking the cost parameters to suit your local situation is the
> recommended cure for planner misjudgments; but I'd recommend against
> changing them on the basis of only one example. =A0You could easily
> find yourself making other cases worse. =A0Get a collection of common
> queries for your app and look at the overall effects.

No argument, and well said -- just trying to point out that the
default values really are FAR too high for people with databases that
fit in OS cache.

...Robert