Re: MVCC catalog access

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2013-06-05 15:28:09 +0100, Greg Stark wrote:
>> I thought there were many call sites that were specifically depending
>> on seeing dirty reads to avoid race conditions with other backends --
>> which probably just narrowed the race condition or created different
>> ones.

> But SnapshotNow doesn't allow you to do actual dirty reads?

Yeah. I believe the issue is that we can't simply do MVCC catalog reads
with a snapshot taken at transaction start time or statement start time,
as we would do if executing an MVCC scan for a user query. Rather, the
snapshot has to be recent enough to ensure we see the current definition
of any table we've just acquired lock on, *even if that's newer than the
snapshot prevailing for the user's purposes*. Otherwise we might be
using the wrong rowtype definition or failing to enforce a just-added
constraint.

The last time we talked about this, we were batting around ideas of
keeping a "current snapshot for catalog purposes", which we'd update
or at least invalidate anytime we acquired a new lock. (In principle,
if that isn't new enough, we have a race condition that we'd better fix
by adding some more locking.) Robert's results seem to say that that
might be unnecessary optimization, and that it'd be sufficient to just
take a new snap each time we need to do a catalog scan. TBH I'm not
sure I believe that; it seems to me that this approach is surely going
to create a great deal more contention from concurrent GetSnapshotData
calls. But at the very least, this says we can experiment with the
behavioral aspects without bothering to build infrastructure for
tracking an appropriate catalog snapshot.

regards, tom lane