Re: why partition pruning doesn't work?

2018-06-01 17:53 GMT+02:00 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:

> Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> writes:
> > I think the patch is right if we were to handle only SQLValueFunction,
> > but the bigger picture here is that we aren't evaluating stable
> > functions before run-time partition pruning happens. I was under the
> > impression that the stable functions/expressions get evaluated and
> > folded into a constant just before the execution begins since a stable
> > function produces the same output for same input during one execution
> > invocation. But I am not able to find where we do that and probably we
> > don't do that at all.
>
> We don't; there was a patch floating around to make that happen, but
> it hasn't been updated lately.
>
> I agree though that it seems strange to special-case SQLValueFunction
> rather than any-stable-expression. As long as the evaluation happens
> at executor start (i.e. with the query's run-time snapshot) it should
> be reasonable to simplify any stable expression.
>
> It's worth questioning whether this is a bug fix or an improvement.
> If the latter, it probably ought to wait for v12.
>

The result is correct - but it was unpleasant surprise. I searched the most
simple demo for this feature, and it doesn't work. Filtering based on
CURRENT_DATE is often.

Regards

Pavel

> regards, tom lane
>