Re: ALTER TABLE lock strength reduction patch is unsafe

On 3 January 2012 18:42, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I wrote:
>>> Another point that requires some thought is that switching SnapshotNow
>>> to be MVCC-based will presumably result in a noticeable increase in each
>>> backend's rate of wanting to acquire snapshots.
>
> BTW, I wonder if this couldn't be ameliorated by establishing some
> ground rules about how up-to-date a snapshot really needs to be.
> Arguably, it should be okay for successive SnapshotNow scans to use the
> same snapshot as long as we have not acquired a new lock in between.
> If not, reusing an old snap doesn't introduce any race condition that
> wasn't there already.

Now that has been implemented using the above design, we can resubmit
the lock level reduction patch, with thanks to Robert.

Submitted patch passes original complaint/benchmark.

Changes
* various forms of ALTER TABLE, notably ADD constraint and VALIDATE
* CREATE TRIGGER

One minor coirrections to earlier thinking with respect to toast
tables. That might be later relaxed.

Full tests including proof of lock level reductions, plus docs.

--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services