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Re: cost_nonsequential_access()

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Manfred Koizar <mkoi-pg(at)aon(dot)at>
Cc: pgsql-patches(at)postgresql(dot)org
Subject: Re: cost_nonsequential_access()
Date: 2004-06-08 17:13:01
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-patches
Manfred Koizar <mkoi-pg(at)aon(dot)at> writes:
> On Tue, 08 Jun 2004 11:13:43 -0400, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> This patch seems to do considerably more violence to the equations than
>> is needed to cover that oversight, though.  The old behavior was
>> intentionally nonlinear in relsize; this is not.

> The comment says "entirely ad-hoc" and I didn't see a particular reason
> why the lower half should be nonlinear in relsize while the upper half
> is linear in 1/relsize.

Incremental changes in the relsize fraction are not going to change the
cost much except near 1, so I was after a curve that went like this
(pardon the crude artwork):

rpc                                            ***********************
cost                             *
1.0 *************
    0                           1                                large
                          relsize fraction

I don't think replacing the lower half of this with a straight line
segment is an improvement.

Possibly the relsize axis ought to be measured on a log scale, or
something like that, but that didn't seem to work nicely when relsize
approaches zero.

			regards, tom lane

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Subject: Re: cost_nonsequential_access()
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