Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > 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.
>
> Oh, so you're thinking of an internal table that provides a mapping back
> to the replaced xmin? Ugh. Perhaps it could be made to work, but it's
> a lot of mechanism, and it will slow down visibility checks (since
> AFAICS you'd have to check every subxid against the phantoms table).
My idea was to have a tuple bit indicating the xid is a phantom.
> If we go with a global CID counter then we don't have to add that step.
Seems Alvaro is already using a global counter.
> A global CID counter would also simplify other visibility tests. Alvaro
> hasn't said anything about how he's doing visibility checks across
> different subxacts of the same main xact, but without global CID there
> would need to be some pretty ugly checks to determine whether a subxact
> happened before or after the CID cutoff your outer xact is interested
> in.
If a global counter will reduce the number of phantom checks, then good.
However, I assume Alvaro has to access the creation/expire xid to
determine if the subtransaction committed, so I didn't think a global
counter would help reduce the number of lookups.
-- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610)
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