Re: Automatic free space map filling

Tom Lane wrote:
> Christopher Browne <cbbrowne@acm.org> writes:
> > What is unclear to me in the discussion is whether or not this is
> > invalidating the item on the TODO list...
> 
> No, I don't think any of this is an argument against the
> dirty-page-bitmap idea.  The amount of foreground effort needed to set a
> dirty-page bit is minimal (maybe even zero, if we can make the bgwriter
> do it, though I'm pretty suspicious of that idea because I think it
> needs to be done immediately when the page is dirtied).  I don't see the
> dirty-page bitmap as changing the way that VACUUM works in any
> fundamental respect --- it will just allow the vacuum process to skip
> reading pages that certainly don't need to change.

See the email I just posted.  I am questioning how big a win it is to
skip heap pages if we have to sequentially scan all indexes.

> One point that does need to be considered though is what about
> anti-wraparound processing (ie, replacing old XIDs with FrozenXID before
> they wrap around)?  VACUUM currently is a safe way to handle that,
> but if its normal mode of operation stops looking at every tuple then
> we're going to have an issue there.

We would need to do sequential scan occasionally and somehow track that.

-- 
  Bruce Momjian   http://candle.pha.pa.us
  SRA OSS, Inc.   http://www.sraoss.com

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