On 27/05/2010, at 02.48, Robert Haas <robertmh...@gmail.com> wrote:
On Wed, May 26, 2010 at 8:06 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
Josh Berkus <j...@agliodbs.com> writes:
How does that get us out of reading and writing old pages, though?
Yeah. Neither PD_ALL_VISIBLE nor the visibility map are going to
solve
your problem, because they cannot become set without having visited
the
page.
Well, maybe I'm confused here, but arranging things so that we NEVER
have to visit the page after initially writing it seems like it's
setting the bar almost impossibly high. Consider a table that is
regularly written but append-only. Every time autovacuum kicks in,
we'll go and remove any dead tuples and then mark the pages
PD_ALL_VISIBLE and set the visibility map bits, which will cause
subsequent vacuums to ignore the all-visible portions of the table...
until anti-wraparound kicks in, at which point we'll vacuum the entire
table and freeze everything.
Just a thought. Wouldn't a All-visible bit also enable index only
scans to some degree?
Jesper
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