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Re: Nested transactions and tuple header info

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Alvaro Herrera <alvherre(at)dcc(dot)uchile(dot)cl>,Manfred Koizar <mkoi-pg(at)aon(dot)at>,David Blasby <dblasby(at)refractions(dot)net>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Nested transactions and tuple header info
Date: 2004-06-02 14:57:05
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Lists: pgsql-hackers
Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Tom Lane wrote:
> >> I've been trying to think of ways to solve these problems by having a
> >> main xact and all its subxacts share a common CID sequence (ie, a
> >> subxact would have its own xid but would not start CID over at one).
> >> If you assume that, then Bruce's idea may indeed work, since you would
> >> never replace xmin in a way that would shift the interpretation of cmin
> >> into a different CID sequence.  But I suspect there is a simpler way to
> >> solve it given that constraint.
> > I thought about using a global command counter.  The problem there is
> > that there is no way to control the visibility of tuples by other
> > transactions on commit except going back end fixing up tuples, which is
> > unacceptable.
> No, I said own xid --- so the "phantom xid" part is still there.  But
> your idea definitely does *not* work unless you use a single CID
> sequence for the whole main xact; and I'm still wondering if there's
> not a simpler implementation possible given that assumption.

I don't understand why a single counter is needed for phantom xids.  We
keep the cmin/cmax on the tuple already, and our own backend can look up
the xmin/xmax that goes with the phantom.

  Bruce Momjian                        |
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
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