Re: question on TRUNCATE vs VACUUM FULL

In response to "Mark Steben" <msteben(at)autorevenue(dot)com>:
>
> I know what Vacuum full and truncate are supposed to do.

Then why do you keep doing the vacuum full? Doesn't really make
sense as a maintenance strategy.

> My confusion lies in the fact that we empty table C after
> Function D finishes. There aren't any current data or records
> To touch on the table. The MVCC leftovers are all purely dead
> Rows that should be deleted. Given this, I thought that
> Vacuum full and truncate should provide exactly the same result.

I would expect so as well. You may want to mention which version
of PostgreSQL you are using, because it sounds like a bug. If it's
an old version, you probably need to upgrade. If it's a recent
version and you can reproduce this behaviour, you probably need
to approach this like a bug report.

>
> I've attached my original memo to the bottom.
>
>
>
> -----Original Message-----
> From: pgsql-performance-owner(at)postgresql(dot)org
> [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Chris
> Sent: Tuesday, March 18, 2008 9:11 PM
> To: Mark Steben
> Cc: pgsql-performance(at)postgresql(dot)org
> Subject: Re: [PERFORM] question on TRUNCATE vs VACUUM FULL
>
>
> >
> > So my question is this: Shouldn't VACUUM FULL clean Table C and reclaim
> > all its space?
>
> You've got concepts mixed up.
>
> TRUNCATE deletes all of the data from a particular table (and works in
> all dbms's).
>
> http://www.postgresql.org/docs/8.3/interactive/sql-truncate.html
>
>
>
> VACUUM FULL is a postgres-specific thing which does work behind the
> scenes to clean up MVCC left-overs. It does not touch any current data
> or records in the table, it's purely behind the scenes work.
>
> http://www.postgresql.org/docs/current/interactive/sql-vacuum.html
>
>
> The two have completely different uses and nothing to do with each other
> what-so-ever.
>
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> [Mark Steben]
>
> Table A houses info on all emails that have ever been created for the
> purpose of being delivered to our end customers.
>
> Big table. About 23 million rows.
>
> Table B, the 'holding' table is populated with Table A key information via
> an after trigger whenever Table A is updated or inserted to.
>
> Table C, the 'work' table is populated by function D from table B. It is
> configured exactly like table B.
>
> PLPGSQL Function D inserts a predefined number of rows from table B to
> table C. For purposes of discussion, say 500.
>
> Function D, after it does its thing, then deletes the 500 rows it
> processed from table B, and ALL 500 rows from table C.
>
>
>
> This entire process, after a sleep period of 10 seconds, repeats itself all
> day.
>
>
>
> After each fifth iteration of function D, we perform a VACUUM FULL on both
> tables B and C.
>
> Takes less than 5 seconds.
>
>
>
> In terms of transaction processing:
>
> Table A is processed by many transactions (some read, some update),
>
> Table B is processed by
>
> - any transaction updating or inserting to Table A via the after
> trigger (insert, update)
>
> - Function D (insert, update, delete)
>
> Table C is processed ONLY by function D (insert, update, delete). Nothing
> else touches it;
>
> PG_LOCKS table verifies that that this table is totally free of any
> transaction
>
> Between iterations of function D.
>
>
>
>
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--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/

wmoran(at)collaborativefusion(dot)com
Phone: 412-422-3463x4023

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