Re: ALTER TABLE lock strength reduction patch is unsafe

On 7 July 2013 14:24, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> 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.

Rebased to v14

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