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@xxxxxxxxx

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