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Re: NOLOGGING option, or ?

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Neil Conway <neilc(at)samurai(dot)com>,Alvaro Herrera <alvherre(at)surnet(dot)cl>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: NOLOGGING option, or ?
Date: 2005-06-01 13:39:07
Message-ID: 200506011339.j51Dd7W14937@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs wrote:
> On Wed, 2005-06-01 at 04:44 -0400, Greg Stark wrote:
> > Greg Stark <gsstark(at)MIT(dot)EDU> writes:
> > 
> > > For CREATE TABLE AS in the non-PITR case you don't really need to WAL log the
> > > records at all. If it fails in the middle you just drop the table. When it
> > > completes you do a checkpoint before acknowledging the COMMIT.
> > > 
> > > I think this is already done for CREATE INDEX/REINDEX, also only in the
> > > non-PITR case.
> > 
> > Sorry to followup to my own message, but it occurs to me that COPY could be
> > made to automatically do this for the case of an empty destination table too.
> > 
> > I'm not sure if it should automatically check for an empty table or if there
> > should be an option for the user to indicate he wants COPY to replace the
> > current contents entirely. The latter might actually be more useful. .
> > 
> > But either way, you just WAL log a record indicating that the table should be
> > entirely empty. Then you fill it up without logging anything. Do a checkpoint
> > and then WAL log that the COPY is finished. If any failure occurs replay
> > leaves it empty.
> > 
> > Again this sadly only works in the non-PITR case.
> 
> Yes, all of the above could work.
> 
> It would use essentially the same functionality that Manfred suggested
> for handling truncated tables. Ignore the first LOAD DATA started
> message until recovery completes, then truncate table if the LOAD DATA
> complete message was not logged in wal.

Well, why not just add this functionality to COPY rather than create a
new command?  One optimization is to write the dirty shared buffers to
the kernel then fsync that relation, rather than do a checkpoint.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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