Re: Window Function "Run Conditions"

On Mon, Aug 16, 2021 at 3:28 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Thu, 1 Jul 2021 at 21:11, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> > 1) Unsure of the API to the prosupport function. I wonder if the
> > prosupport function should just be able to say if the function is
> > either monotonically increasing or decreasing or neither then have
> > core code build a qual. That would make the job of building new
> > functions easier, but massively reduce the flexibility of the feature.
> > I'm just not sure it needs to do more in the future.
>
> I looked at this patch again today and ended up changing the API that
> I'd done for the prosupport functions. These just now set a new
> "monotonic" field in the (also newly renamed)
> SupportRequestWFuncMonotonic struct. This can be set to one of the
> values from the newly added MonotonicFunction enum, namely:
> MONOTONICFUNC_NONE, MONOTONICFUNC_INCREASING, MONOTONICFUNC_DECREASING
> or MONOTONICFUNC_BOTH.
>
> I also added handling for a few more cases that are perhaps rare but
> could be done with just a few lines of code. For example; COUNT(*)
> OVER() is MONOTONICFUNC_BOTH as it can neither increase nor decrease
> for a given window partition. I think technically all of the standard
> set of aggregate functions could have a prosupport function to handle
> that case. Min() and Max() could go a little further, but I'm not sure
> if adding handling for that would be worth it, and if someone does
> think that it is worth it, then I'd rather do that as a separate
> patch.
>
> I put the MonotonicFunction enum in plannodes.h. There's nothing
> specific about window functions or support functions. It could, for
> example, be reused again for something else such as monotonic
> set-returning functions.
>
> One thing which I'm still not sure about is where
> find_window_run_conditions() should be located. Currently, it's in
> allpaths.c but that does not really feel like the right place to me.
> We do have planagg.c in src/backend/optimizer/plan, maybe we need
> planwindow.c?
>
> David
>
Hi,

+ if ((res->monotonic & MONOTONICFUNC_INCREASING) ==
MONOTONICFUNC_INCREASING)

The above can be simplified as 'if (res->monotonic &
MONOTONICFUNC_INCREASING) '

Cheers