Message-ID: <4E79E091.2030401@peak6.com>
Date: Wed, 21 Sep 2011 08:03:13 -0500
From: Shaun Thomas <sthomas@peak6.com>
Reply-To: <sthomas@peak6.com>
Organization: Peak6
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To: Venkat Balaji <venkat.balaji@verse.in>
CC: PGSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: : Performance Improvement Strategy
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On 09/20/2011 11:22 AM, Venkat Balaji wrote:

> Please help me understand how to calculate free space in Tables and
> Indexes even after vacuuming and analyzing is performed.

Besides the query Mark gave you using freespacemap, there's also the 
pgstattuple contrib module. You'd use it like this:

SELECT pg_size_pretty(free_space) AS mb_free
   FROM pgstattuple('some_table');

Query must be run as a super-user, and I wouldn't recommend running it 
on huge tables, since it scans the actual data files to get its 
information. There's a lot of other useful information in that function, 
such as the number of dead rows.

> What i understand is that, even if we perform VACUUM ANALYZE
> regularly, the free space generated is not filled up.

VACUUM does not actually generate free space. It locates and marks 
reusable tuples. Any future updates or inserts on that table will be put 
in those newly reclaimed spots, instead of being bolted onto the end of 
the table.

> I see lot of free spaces or free pages in Tables and Indexes. But, I
> need to give an exact calculation on how much space will be reclaimed
> after VACUUM FULL and RE-INDEXING.

Why? If your database is so desperate for space, VACUUM and REINDEX 
won't really help you. A properly maintained database will still have a 
certain amount of "bloat" equal to the number of rows that change 
between maintenance intervals. One way or another, that space is going 
to be used by *something*.

It sounds more like you need to tweak your autovacuum settings to be 
more aggressive if you're seeing significant enough turnover that your 
tables are bloating significantly. One of our tables, for instance, gets 
vacuumed more than once per hour because it experiences 1,000% turnover 
daily.

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 800 | Chicago IL, 60604
312-676-8870
sthomas@peak6.com

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