Eric Ridge wrote:

> > Again, you'll probably need to put this low level requirement into
> > context if you want sound advice from this list.
> I'm just thinking out lout here, but the context is likely something
> along the lines of externally storing all transaction ids, and
> periodically asking Postgres if they're
> known-to-be-aborted-by-all-transactions -- one at a time.

I think if you just check the global xmin, then you're going to maintain
a very long list of transactions "potentially running" whenever there
are long-lived transactions (pg_dump, for example).  You could try to
add a TransactionIdIsInProgress() somewhere and discard the xact
downright (by DidCommit and DidAbort) if it returns false.  A single
long-running transaction can keep the global xmin down by hundreds of
millions of Xids.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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