On 2014-09-30 14:51:57 -0700, Kevin Grittner wrote:
> Josh Berkus <josh@agliodbs.com> wrote:
> > On 09/30/2014 02:39 PM, Kevin Grittner wrote:
> >> Josh Berkus <josh@agliodbs.com> wrote:
> >>> On 09/30/2014 07:15 AM, Kevin Grittner wrote:
> >>>
> >>>> At the risk of pushing people away from this POV, I'll point out
> >>>> that this is somewhat similar to what we do for unlogged bulk loads
> >>>> -- if all the conditions for doing it the fast way are present, we
> >>>> do it the fast way; otherwise it still works, but slower.
> >>>
> >>> Except that switching between fast/slow bulk loads affects *only* the
> >>> speed of loading, not the locking rules. Having a statement silently
> >>> take a full table lock when we were expecting it to be concurrent
> >>> (because, for example, the index got rebuilt and someone forgot the
> >>> UNIQUE) violates POLA from my perspective.
> >>
> >> I would not think that an approach which took a full table lock to
> >> implement the more general case would be accepted.
> >
> > Why not? There are certainly cases ... like bulk loading ... where
> > users would find it completely acceptable. Imagine that you're merging
> > 3 files into a single unlogged table before processing them into
> > finished data.
>
> So the expectation is that when we implement MERGE it will, by
> default, take out an EXCLUSIVE lock for the entire target table for
> the entire duration of the command? I would have expected a bit
> more finesse.
I think it'd be acceptable. Alternatively we'll just accept that you can
get uniqueness violations under concurrency. I many cases that'll be
fine.
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services