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Re: Really dumb planner decision

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Merlin Moncure <mmoncure(at)gmail(dot)com>, Matthew Wakeling <matthew(at)flymine(dot)org>, Grzegorz Jaśkiewicz <gryzman(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Really dumb planner decision
Date: 2009-04-16 15:49:14
Message-ID: 603c8f070904160849j104689dbvdd8fbd5b126e0e26@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Thu, Apr 16, 2009 at 9:49 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
>> On Thu, Apr 16, 2009 at 8:05 AM, Matthew Wakeling <matthew(at)flymine(dot)org> wrote:
>>> That solves the problem. So, a view is treated as a subquery then?
>
>> no...the view is simply inlined into the query (think C macro) using
>> the rules.  You just bumped into an arbitrary (and probably too low)
>> limit into the number of tables the planner can look at in terms of
>> optimizing certain types of plans.
>
> Bear in mind that those limits exist to keep you from running into
> exponentially increasing planning time when the size of a planning
> problem gets big.  "Raise 'em to the moon" isn't really a sane strategy.
> It might be that we could get away with raising them by one or two given
> the general improvement in hardware since the values were last looked
> at; but I'd be hesitant to push the defaults further than that.

I hasten to point out that I only suggested raising them to the moon
as a DEBUGGING strategy, not a production configuration.

I do however suspect that raising the defaults would be a good idea.
It seems that the limit has been 8 since those parameters were added
back in January of 2003, and yes, hardware is a lot better now.  We
should probably raise geqo_threshold at the same time, since that's
supposed to be larger than these parameters and the default is only
12.

...Robert

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Subject: Re: Shouldn't the planner have a higher cost for reverse index scans?
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