From: Jim C. Nasby <jim@nasby.net>
To: Graham Davis <gdavis@refractions.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: index growth problem
Date: Wed, 18 Oct 2006 17:33:08 -0500
Message-ID: <20061018223307.GC56874@nasby.net> (raw)
In-Reply-To: <4536A8A3.1050409@refractions.net>
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On Wed, Oct 18, 2006 at 03:20:19PM -0700, Graham Davis wrote:
> I have a question about index growth.
>
> The way I understand it, dead tuples in indexes were not reclaimed by
> VACUUM commands in the past. However, I've read in a few forum posts
> that this was changed somewhere between 7.4 and 8.0.
There was a change to indexes that made vacuum more effective; I don't
remember the details off-hand.
> I'm having an issue where my GIST indexes are growing quite large, and
> running a VACUUM doesn't appear to remove the dead tuples. For example,
> if I check out the size an index before running any VACUUM :
>
> select pg_relation_size('asset_positions_position_idx');
> pg_relation_size
> ------------------
> 11624448
> (1 row)
>
> The size is about 11Mb. If I run a VACUUM command in verbose, I see
> this about the index:
>
> INFO: index "asset_positions_position_idx" now contains 4373 row
> versions in 68 pages
> DETAIL: 0 index pages have been deleted, 0 are currently reusable.
> CPU 0.00s/0.00u sec elapsed 0.16 sec.
>
> When I run the same command to find the size after the VACUUM, it hasn't
> changed. However, if I drop and then recreate this index, the size
> becomes much smaller (almost half the size):
>
> drop index asset_positions_position_idx;
> DROP INDEX
>
> CREATE INDEX asset_positions_position_idx ON asset_positions USING GIST
> (position GIST_GEOMETRY_OPS);
> CREATE INDEX
>
> select pg_relation_size('asset_positions_position_idx');
> pg_relation_size
> ------------------
> 6225920
> (1 row)
>
> Is there something I am missing here, or is the reclaiming of dead
> tuples for these indexes just not working when I run a VACUUM? Is it
> suppose to work?
That's not really a useful test to see if VACUUM is working. VACUUM can
only trim space off the end of a relation (index or table), where by
'end' I mean the end of the last file for that relation on the
filesystem. This means it's pretty rare for VACUUM to actually shrink
files on-disk for tables. This can be even more difficult for indexes (I
think it's virtually impossible to shrink a B-tree index file).
--
Jim Nasby jim@nasby.net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
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To: pgsql-performance@postgresql.org
Cc: jim@nasby.net, gdavis@refractions.net
Subject: Re: index growth problem
In-Reply-To: <20061018223307.GC56874@nasby.net>
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