Re: ALTER TABLE lock strength reduction patch is unsafe

On Tue, Jan 3, 2012 at 6:24 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 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.

All of which is reasonable doubt.

So far, all I'm saying is that on a couple of heavy duty cases that
I've run, I haven't noticed a problem. That is sufficient for me to
push forwards with a patch that does very close to what we want, so we
can judge acceptability with wider tests. I'm not saying it will be
acceptable a priori, only that it is worth trying because the benefits
are high. The number of call points are also high.

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