From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Andrew Dunstan <andrew(at)dunslane(dot)net>, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: WIP: extensible enums |
Date: | 2010-10-24 04:26:07 |
Message-ID: | 23598.1287894367@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I suppose you could fix this by always updating every row, and storing
> in each row the total count of elements (or a random number). Then
> it'd be obvious if you'd read an inconsistent view of the world.
Well, the easy way to read a consistent view of the world is to load the
cache using an MVCC snapshot instead of SnapshotNow. The current code
structure isn't amenable to that because it's relying on a syscache to
fetch the data for it, but that seems pretty inefficient anyway. I'm
thinking of changing it around so that the enum cache gets loaded with
a regular systable_beginscan() scan, and then we could load with an
MVCC snapshot.
I'm kind of inclined to go to the float-based representation anyway,
though, just because not having to update other rows to do an insert
seems like a good thing. But we could combine that with an MVCC
snapshot on the read side, which would make renumbering safe, which
would mean that we could auto-renumber when we ran out of code space
and not otherwise. Is that getting too complicated?
regards, tom lane
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