| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Josh Berkus <josh(at)agliodbs(dot)com> |
| Cc: | Andres Freund <andres(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: removing PD_ALL_VISIBLE |
| Date: | 2013-05-31 17:44:58 |
| Message-ID: | 20130531174458.GC1728@momjian.us |
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| Lists: | pgsql-hackers |
On Fri, May 31, 2013 at 10:28:12AM -0700, Josh Berkus wrote:
>
> >> Isn't the visibility map already required for proper return results as
> >> we use it for index-only scans. I think the optimization-only ship has
> >> sailed.
> >
> > At the moment we can remove it without causing corruption. If we were to
> > use it for freezing we couldn't anymore. So there's a difference - how
> > big it is I am not sure.
>
> Depends on your definition of corruption, really.
>
> But yes, right now, the vismap can lose bits without causing any
> corruption, and making all-frozen depend on it would eliminate that.
Roberts statement was:
> Loss or corruption of a single visibility map page means possible loss
> of half a gigabyte of data.
Certainly unidentified corruption of a visibility map page could easily
cause incorrect results. So, technically, _adding_ bits would cause
corruption.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
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