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Re: crash-safe visibility map, take three

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Jesper Krogh <jesper(at)krogh(dot)cc>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2011-01-06 02:10:15
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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.

Robert Haas
The Enterprise PostgreSQL Company

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