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Re: Visibility map, partial vacuums

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Visibility map, partial vacuums
Date: 2008-11-24 14:37:59
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Lists: pgsql-hackers
Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> I've been thinking that we could add one frozenxid field to each 
> visibility map page, for the oldest xid on the heap pages covered by the 
> visibility map page. That would allow more fine-grained anti-wraparound 
> vacuums as well.

This doesn't strike me as a particularly good idea.  Right now the map
is only hints as far as vacuum is concerned --- if you do the above then
the map becomes critical data.  And I don't really think you'll buy

> The visibility map won't be inquired unless you vacuum. This is a bit 
> tricky. In vacuum, we only know whether we can set a bit or not, after 
> we've acquired a cleanup lock on the page, and scanned all the tuples. 
> While we're holding a cleanup lock, we don't want to do I/O, which could 
> potentially block out other processes for a long time. So it's too late 
> to extend the visibility map at that point.

This is no good; I think you've made the wrong tradeoffs.  In
particular, even though only vacuum *currently* uses the map, you want
to extend it to be used by indexscans.  So it's going to uselessly
spring into being even without vacuums.

I'm not convinced that I/O while holding cleanup lock is so bad that we
should break other aspects of the system to avoid it.  However, if you
want to stick to that, how about
	* vacuum page, possibly set its header bit
	* release page lock (but not pin)
	* if we need to set the bit, fetch the corresponding map page
	  (I/O might happen here)
	* get share lock on heap page, then recheck its header bit;
	  if still set, set the map bit

			regards, tom lane

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