Re: random() (was Re: New GUC to sample log queries)

On Wed, Dec 26, 2018 at 1:21 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> One thing we'd have to think about if we want to take this seriously
> is whether a process-wide PRNG state is really adequate; if you're
> trying to make a particular call site be deterministic, you'd likely
> wish it weren't interfered with by other call sites.  On the flip
> side, having more call sites probably makes things more random and
> thus better for normal usage.  Not sure how to resolve that tension
> (but I don't want to build a whole lot of new infrastructure here).

I don't think that you need to resolve the tension -- I'd be fine with
continuing to have a process-wide PRNG that was cordoned-off for
internal use. I care about making behavior deterministic when running
an SQL script within a single backend. This can only be expected to
work when all inputs (non-random and random alike) are carefully
accounted for, including even the mutation of a seed value as the test
case runs. I also found myself disabling synchronized sequential
scanning within the same test case I mentioned, because they caused
false positive failures very occasionally. I can imagining a similar
test case where the tester has to worry about autovacuum running
ANALYZE, too (as it happens, these were all INSERT ... SELECT
statements, where that didn't seem to be an issue).

Trying to reason about the behavior of a call site in isolation seems
way too complicated to be practical. In general, there will never be a
standard way to think about this kind of debugging, and it would be
unreasonable to insist on providing any kind of standard framework.
It's a low-level tool, useful only to backend hackers.

-- 
Peter Geoghegan