Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

Kevin Grittner wrote:
> On Wed, Jun 15, 2016 at 1:59 PM, Alvaro Herrera
> <alvherre(at)2ndquadrant(dot)com> wrote:

> > We actually go quite some lengths to support this case, even when it's
> > the opinion of many that we shouldn't. For example VACUUM doesn't try
> > to find index entries using the values in each deleted tuple; instead we
> > remember the TIDs and then scan the indexes (possibly many times) to
> > find entries that match those TIDs -- which is much slower. Yet we do
> > it this way to protect the case that somebody is doing the
> > not-really-IMMUTABLE function.
> >
> > In other words, I don't think we consider the position you argued as
> > acceptable.
>
> What are you saying is unacceptable, and what behavior would be
> acceptable instead?

The answer "we don't support the situation where you have an index using
an IMMUTABLE function that isn't actually immutable" is not acceptable.
The acceptable solution would be a design that doesn't have that
property as a requisite.

I think having various executor(/heapam) checks that raise errors when
queries are executed from within ANALYZE is acceptable. I don't know
about the TOAST related angle Andres just raised.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services