Re: question on TRUNCATE vs VACUUM FULL

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Bill,
Thanks for your quick response.
We are at version 8.2.5 - just recently upgraded from 7.4.5.
This strategy using truncate was just implemented yesterday.
Now I will revisit the vacuum full strategy. Does seem to
Be redundant.  
Is there a procedure to begin reporting a bug?  Is there
Someone or an email address that I could bring evidence to?


Mark Steben
Senior Database Administrator
@utoRevenueT 
A Dominion Enterprises Company
480 Pleasant Street
Suite B200
Lee, MA 01238
413-243-4800 Home Office 
413-243-4809 Corporate Fax
msteben@xxxxxxxxxxxxxxx

Visit our new website at 
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-----Original Message-----
From: Bill Moran [mailto:wmoran@xxxxxxxxxxxxxxxxxxxxxxx] 
Sent: Wednesday, March 19, 2008 9:35 AM
To: Mark Steben
Cc: 'Chris'; pgsql-performance@xxxxxxxxxxxxxx
Subject: Re:  question on TRUNCATE vs VACUUM FULL

In response to "Mark Steben" <msteben@xxxxxxxxxxxxxxx>:
> 
> 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@xxxxxxxxxxxxxx
> [mailto:pgsql-performance-owner@xxxxxxxxxxxxxx] On Behalf Of Chris
> Sent: Tuesday, March 18, 2008 9:11 PM
> To: Mark Steben
> Cc: pgsql-performance@xxxxxxxxxxxxxx
> Subject: Re:  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.
> 
> -- 
> Postgresql & php tutorials
> http://www.designmagick.com/
> 
> -- 
> Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx)
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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@xxxxxxxxxxxxxxxxxxxxxxx
Phone: 412-422-3463x4023

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