Re: Proposal for Implenting read-only queries during wal replay (SoC 2007)

"Florian G. Pflug" <fgp@phlo.org> writes:
> My line of reasoning is that stopping wal replay at a arbitrary point,
> and then starting a read-only transaction with an "empty snapshot" (meaning
> that all exactly those transactions marked as comitted in the clog are 
> assumed to be visible to the transaction) is exactly the same as sending
> the backend a SIGKILL when it just wrote the wal record in question,
> and then restarting postgres, and starting a transaction.

The hole in that reasoning is that no one would be satisfied with the
behavior of a Postgres database that was being forcibly restarted every
few seconds.  Yeah, we won't lose transactions that have been promised
committed, but losing a large fraction of transactions-in-progress won't
please anyone.  Nor will queries on a slave that's behaving like that
provide an accurate model of what the same queries would produce if issued
on the master.
        regards, tom lane