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Re: FSM search modes

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, decibel <decibel(at)decibel(dot)org>, Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: FSM search modes
Date: 2009-10-01 21:18:54
Message-ID: 603c8f070910011418n2b89d17ct94262a2d5005cfff@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Oct 1, 2009 at 5:08 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> The elephant in the room here is that if the relation is a million
>> pages of which 1-100,000 and 1,000,000 are in use, no amount of bias
>> is going to help us truncate the relation unless every tuple on page
>> 1,000,000 gets updated or deleted.
>
> Well, there is no way to move a tuple across pages in a user-invisible,
> non-blocking fashion, so our ability to do something automatic about the
> above scenario is limited.  The discussion at the moment is about ways
> of reducing the probability of getting into that situation in the first
> place.  That doesn't preclude also providing some more-invasive tools
> that people can use when they do get into that situation; but let's
> not let I-want-a-magic-pony syndrome prevent us from doing anything
> at all.

That's fair enough, but it's our usual practice to consider, before
implementing a feature or code change, what fraction of the people it
will actually help and by how much.  If there's a way that we can
improve the behavior of the system in this area, I am all in favor of
it, but I have pretty modest expectations for how much real-world
benefit will ensue.  I suspect that it's pretty common for large
tables to contain a core of infrequently-updated records, and even a
very light smattering of those, distributed randomly, will be enough
to stop table shrinkage before it can get very far.

...Robert

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Next:From: Alvaro HerreraDate: 2009-10-01 21:22:06
Subject: Re: TODO item: Allow more complex user/database default GUC settings
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Subject: Re: FSM search modes

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