Re: What about utility to calculate planner cost constants?

Greg Stark wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>>That's not really practical. There are currently 5 major query tuning
>>parameters, not counting the memory adjustments which really can't be left
>>out. You can't realistically test all combinations of 6 variables.
>
> I don't think it would be very hard at all actually.
[snip]
> What's needed is for the explain plan to total up the costing penalties
> independently. So the result would be something like
>
> 1000 * random_page_cost + 101 * sequential_page_cost + 2000 * index_tuple_cost
> + ...
>
> In other words a tuple like <1000,101,2000,...>
>
> And explain analyze would produce the above tuple along with the resulting
> time.
>
> Some program would have to gather these values from the log or stats data and
> gather them up into a large linear system and solve for values that minimize
> the divergence from the observed times.

You'd only need to log them if they diverged from expected anyway. That
should result in fairly low activity pretty quickly (or we're wasting
our time). Should they go to the stats collector rather than logs?

> (Also, currently explain analyze has overhead that makes this impractical.
> Ideally it could subtract out its overhead so the solutions would be accurate
> enough to be useful)

Don't we only need the top-level figures though? There's no need to
record timings for each stage, just work completed.

--
Richard Huxton
Archonet Ltd