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Re: Index Bloat Problem

From: Greg Williamson <gwilliamson39(at)yahoo(dot)com>
To:
Cc: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Index Bloat Problem
Date: 2012-08-18 08:01:44
Message-ID: 1345276904.17534.YahooMailNeo@web125902.mail.ne1.yahoo.com (view raw or flat)
Thread:
Lists: pgsql-performance
Thanks for this description--we have index bloat problems on a massively active (but small) database.This may help shed light on our problems.

Sorry for top-posting--challenged email reader.

Greg W.



>________________________________
> From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
>To: Strahinja Kustudić <strahinjak(at)nordeus(dot)com> 
>Cc: pgsql-performance(at)postgresql(dot)org 
>Sent: Friday, August 17, 2012 7:33 PM
>Subject: Re: [PERFORM] Index Bloat Problem
> 
>On Thu, Aug 16, 2012 at 12:57 PM, Strahinja Kustudić
><strahinjak(at)nordeus(dot)com> wrote:
>>
>> @Jeff I'm not sure if I understand what you mean? I know that we never reuse
>> key ranges. Could you be more clear, or give an example please.
>
>If an index leaf page is completely empty because every entry on it
>were deleted, it will get recycled to be used in some other part of
>the index.  (Eventually--it can take a while, especially if you have
>long-running transactions).
>
>But if the leaf page is only mostly empty, because only most of
>entries on it were deleted, than it can never be reused, except for
>entries that naturally fall into its existing key range (which will
>never happen, if you never reuse key ranges)
>
>So if you have a million records with keys 1..1000000, and do a
>"delete from foo where key between 1 and 990000", then 99% of those
>old index pages will become completely empty and eligible for reuse.
>But if you do "delete from foo where key%100>0", then all of the pages
>will become 99% empty, and none will be eligible for reuse (except the
>very last one, which can still accept 1000001 and so on)
>
>There has  been talk of allowing logically adjacent, mostly empty
>pages to be merged so that one of them becomes empty, but the way
>concurrent access to btree indexes was designed this is extremely hard
>to do safely.
>
>Cheers,
>
>Jeff
>
>
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