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Re: nested transactions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: Manfred Koizar <mkoi-pg(at)aon(dot)at>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: nested transactions
Date: 2002-11-29 02:46:09
Message-ID: 19032.1038537969@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> Also, I should point out that balooning pg_clog by 16x is going to mean
> we are perhaps 4-8x more likely to need extra pages to mark all
> subtransactions.

So?  The critical point is that we don't need to serialize the pg_clog
operations if we do it that way.  Also, we can certainly expand the
number of pg_clog pages held in memory by some amount.  Right now it's
only 4, IIRC.  We could make it 64 and probably no one would even
notice.

> Isn't there some other way we can link these subtransactions together
> rather than mucking with pg_clog, as we only need the linkage while we
> mark them all committed?

You *cannot* expect to do it all in shared memory; you will be blown out
of the water by the first long transaction that comes along, if you try.
So the question is not whether we put the status into a file, it is only
what representation we choose.

Manfred suggested a separate log file ("pg_subclog" or some such) but
I really don't see any operational advantage to that.  You still end up
with 4 bytes per transaction, you're just assuming that putting them
in a different file makes it better.  I don't see how.

			regards, tom lane

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