Re: Choosing parallel_degree

On 8 April 2016 at 17:27, Paul Ramsey <pramsey(at)cleverelephant(dot)ca> wrote:

> PostGIS is just one voice...
>

We're listening.

> >> Functions have very unequal CPU costs, and we're talking here about
> >> using CPUs more effectively, why are costs being given the see-no-evil
> >> treatment? This is as true in core as it is in PostGIS, even if our
> >> case is a couple orders of magnitude more extreme: a filter based on a
> >> complex combination of regex queries will use an order of magnitude
> >> more CPU than one that does a little math, why plan and execute them
> >> like they are the same?
> >
> > Functions have user assignable costs.
>
> We have done a relatively bad job of globally costing our functions
> thus far, because it mostly didn't make any difference. In my testing
> [1], I found that costing could push better plans for parallel
> sequence scans and parallel aggregates, though at very extreme cost
> values (1000 for sequence scans and 10000 for aggregates)
>
> Obviously, if costs can make a difference for 9.6 and parallelism
> we'll rigorously ensure we have good, useful costs. I've already
> costed many functions in my parallel postgis test branch [2]. Perhaps
> the avoidance of cost so far is based on the relatively nebulous
> definition it has: about the only thing in the docs is "If the cost is
> not specified, 1 unit is assumed for C-language and internal
> functions, and 100 units for functions in all other languages. Larger
> values cause the planner to try to avoid evaluating the function more
> often than necessary."
>
> So what about C functions then? Should a string comparison be 5 and a
> multiplication 1? An image histogram 1000?
>

We don't have a clear methodology for how to do this.

It's a single parameter to allow you to achieve the plans that work
optimally. Hopefully that is simple enough for everyone to use and yet
flexible enough to make a difference.

If its not what you need, show us and it may make the case for change.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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