Re: ALTER TABLE lock strength reduction patch is unsafe

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Jan 3, 2012 at 12:55 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 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. Hence, more contention
>> in GetSnapshotData can be expected. A single-threaded test case doesn't
>> prove anything at all about what that might cost under load.

> This is obviously true at some level, but I'm not sure that it really
> matters. It's not that difficult to construct a test case where we
> have lots of people concurrently reading a table, or reading many
> tables, or writing a table, or writing many tables, but what kind of
> realistic test case involves enough DDL for any of this to matter?

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.

None of this is necessarily grounds to reject a patch along the proposed
lines. I'm just asking for some benchmarking effort to establish what
the costs might be, rather than naively hoping they are negligible.

regards, tom lane