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Re: FSM search modes

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Simon Riggs" <simon(at)2ndQuadrant(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "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 18:36:34
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> Yes, as Tom points out, this must be done with bias away from the
>> very end of the table.
>> I meant that we should start from the beginning of large spaces and
>> that we shouldn't assume that all space worth filling is at start
>> of relation.
OK, so I did misunderstand you; we agree after all.  :-)
> So for example we might try resetting the search to the start of the
> relation with probability 0.01.
If I understand the heuristic you propose, and my math skill haven't
eroded too badly from lack of use, every 229 spots considered would
cause a 90% chance of reset.  That means that the odds of getting past
50,000 spots (the number of pages with available free space at which I
generally start to get worried) without resetting is about 1 in 10^218
-- which is a risk I'm willing to accept.  ;-)

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