Re: advancing snapshot's xmin

"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Gregory Stark <stark(at)enterprisedb(dot)com> writes:
>> Uhm, yeah, I somehow didn't write was I was thinking. I didn't mean to say we
>> would be taking a new snapshot for each INSERT but that we would be resetting
>> xmin for each INSERT. Whereas currently we only set xmin once when we set the
>> serializable snapshot.
>
> Right, but setting xmin within GetSnapshotData is essentially free.
> What I'm envisioning is that we lose the notion of "this is a
> serializable snapshot" that that function currently has, and just
> give it the rule "if MyProc->xmin is currently zero, then set it".
> Then the only additional mechanism needed is for the snapshot
> manager to detect when all snapshots are gone and zero out
> MyProc->xmin --- that would happen sometime during command shutdown,
> and per current discussion it shouldn't need a lock.

It would be nice if there was some way to notice that no other transactions
have committed since last we calculated a snapshot and just reuse that
snapshot.

I would say ideally before we throw out our xmin but I suspect the point of
synchronization needed to notice this condition would be tantamount to that
same lock anyways.

--
Gregory Stark
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