Re: ALTER TABLE lock strength reduction patch is unsafe

On Tue, Jan 3, 2012 at 1:42 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Um ... you're supposing that only DDL uses SnapshotNow, which is
>>> wrong.  I refer you to the parser, the planner, execution
>>> functions for arrays, records, enums, any sort of relcache
>>> reload, etc etc etc.  Yes, some of that is masked by
>>> backend-internal caching, some of the time, but it's folly to
>>> just assume that there are no SnapshotNow scans during normal
>>> queries.
>>
>> Hmm.  That's unfortunate, because it seems difficult to construct
>> a test case that will exercise every feature in the system.
>
> Would the result of IsMVCCSnapshot() change for these snapshots?  If
> so, it might require work in the SSI code to avoid a performance hit
> there.  In early profiling and stepping through execution I noticed
> that we had overhead in serializable transactions for the planner
> checks for the actual values at the beginning or end of an index.
> This went away when we avoided SSI work for reads using a non-MVCC
> snapshot.  If we're going to start using MVCC snapshots for such
> things, we need some other way to avoid unnecessary work in this
> area (and possibly others).
>
> At a minimum, some comparative benchmarks at the serializable
> isolation level would be in order when considering a patch like
> this.

Ugh. Yeah, that sounds like a problem. :-(

--
Robert Haas
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