Horton

Re: unlogged tables vs. GIST

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>, Jeevan Chalke <jeevan.chalke@enterprisedb.com>, pgsql-hackers@postgresql.org
Date: 2013-01-15T18:48:09Z
Lists: pgsql-hackers

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  1. The GiST scan algorithm uses LSNs to detect concurrent pages splits, but

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jan 15, 2013 at 1:10 PM, Heikki Linnakangas
> <hlinnakangas@vmware.com> wrote:
>> Could we stash the counter e.g. in the root page of the index?

> That would require maintaining a counter per table rather than a
> single global counter, which would be bad because then we'd need to
> store one counter in shared memory for every table, rather than just
> one, period, which runs up against the fixed sizing of shared memory.

I think what Heikki had in mind was that the copy in the index would be
the authoritative one, not some image in shared memory.  This'd imply
dirtying the root page on every insert, as well as increased contention
for the root page, so it might have performance problems.

I think a bigger issue is where we'd find any space for it.  There's no
easily-spare space in a GIST page.  This reminds me again that the lack
of a metapage in GIST was a serious design error, which we should
correct if we ever break on-disk compatibility again.

I concur that adding such a counter to pg_control is a nonstarter,
though.

Given that we don't need crash recovery for an unlogged table, could
we get away with some variant of NSN that has weaker semantics than
XLOG LSNs?

			regards, tom lane