Robert Haas <robertmhaas@gmail.com> writes:
> ... how
> important is stability to ANALYZE? If you *either* retake your MVCC
> snapshots periodically as you re-scan the table *or* use a non-MVCC
> snapshot for the scan, you can get those same kinds of artifacts: you
> might see two copies of a just-updated row, or none. Maybe this would
> actually *break* something - e.g. could there be code that would get
> confused if we sample multiple rows for the same value in a column
> that has a UNIQUE index? But I think mostly the consequences would be
> that you might get somewhat different results from the statistics.
Yeah, that's an excellent point. I can imagine somebody complaining
"this query clearly matches a unique index, why is the planner estimating
multiple rows out?". But most of the time it wouldn't matter much.
(And I think you can get cases like that anyway today.)
> It's not clear to me that it would even be correct to categorize those
> somewhat-different results as "less accurate."
Estimating two rows where the correct answer is one row is clearly
"less accurate". But I suspect you'd have to be quite unlucky to
get such a result in practice from Simon's proposal, as long as we
weren't super-aggressive about changing ANALYZE's snapshot a lot.
regards, tom lane
