| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: removing PD_ALL_VISIBLE |
| Date: | 2013-05-31 17:17:09 |
| Message-ID: | 20130531171709.GE4606@awork2.anarazel.de |
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| Lists: | pgsql-hackers |
On 2013-05-31 13:14:13 -0400, Bruce Momjian wrote:
> On Thu, May 30, 2013 at 09:47:22AM -0400, Robert Haas wrote:
> > Well, as Heikki points out, I think that's unacceptably dangerous.
> > Loss or corruption of a single visibility map page means possible loss
> > of half a gigabyte of data.
> >
> > Also, if we go that route, looking at the visibility map is no longer
> > an optimization; it's essential for correctness. We can't decide to
> > skip it when it seems expensive, for example, as Jeff was proposing.
>
> 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.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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