Bruce,
> Roberts statement was:
>
>> Loss or corruption of a single visibility map page means possible loss
>> of half a gigabyte of data.
I fail to be alarmed at this; currently losing a single page of the clog
causes just as widespread corruption (worse, actually, since it's not
confined to one table). It does point to the eventual need to checksum
these things, though.
> Certainly unidentified corruption of a visibility map page could easily
> cause incorrect results. So, technically, _adding_ bits would cause
> corruption.
Yes, that's already true. I'm pointing out that if we depend on the
vismap for all-frozen, then losing bits *also* causes corruption, so
that's something we need to test for. Right now, there is no possible
corruption from losing bits; we simply end up scannning more pages than
we have to.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com