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Re: crash-safe visibility map, take three

From: Jim Nasby <jim(at)nasby(dot)net>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: PostgreSQL-development Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: crash-safe visibility map, take three
Date: 2011-01-07 18:28:54
Message-ID: 7502A9A8-B5C8-4C82-8843-4AE0F638D207@nasby.net (view raw or flat)
Thread:
Lists: pgsql-hackers
On Jan 5, 2011, at 8:10 PM, Robert Haas wrote:
> On Wed, Jan 5, 2011 at 3:22 PM, Jesper Krogh <jesper(at)krogh(dot)cc> wrote:
>> Given a crash-safe visibility map, what purpuse does the PD_ALL_VISIBLE bit
>> serve?
> 
> If we modify a page on which PD_ALL_VISIBLE isn't set, we don't
> attempt to update the visibility map.  In theory, this is an important
> optimization to reduce contention on the visibility map page, since
> there are something like 64K heap pages per visibility map page.  In
> practice, I'm not sure in what workloads it matters or by how much.

What specific locking are you worried about? The page locks themselves? Isn't changing the bit essentially a single instruction operation?

This is sounding like premature optimization... ;)
--
Jim C. Nasby, Database Architect                   jim(at)nasby(dot)net
512.569.9461 (cell)                         http://jim.nasby.net



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