Date: Wed, 18 Oct 2006 17:33:08 -0500
From: "Jim C. Nasby" <jim@nasby.net>
To: Graham Davis <gdavis@refractions.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: index growth problem
Message-ID: <20061018223307.GC56874@nasby.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)