Re: row filtering for logical replication

On Thu, Aug 26, 2021 at 9:51 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> On Thu, Aug 26, 2021 at 1:20 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Aug 26, 2021 at 7:37 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Aug 25, 2021 at 3:28 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > ...
> > > >
> > > > Hmm, I think the gain via caching is not visible because we are using
> > > > simple expressions. It will be visible when we use somewhat complex
> > > > expressions where expression evaluation cost is significant.
> > > > Similarly, the impact of this change will magnify and it will also be
> > > > visible when a publication has many tables. Apart from performance,
> > > > this change is logically correct as well because it would be any way
> > > > better if we don't invalidate the cached expressions unless required.
> > >
> > > Please tell me what is your idea of a "complex" row filter expression.
> > > Do you just mean a filter that has multiple AND conditions in it? I
> > > don't really know if few complex expressions would amount to any
> > > significant evaluation costs, so I would like to run some timing tests
> > > with some real examples to see the results.
> > >
> >
> > I think this means you didn't even understand or are convinced why the
> > patch has cache in the first place. As per your theory, even if we
> > didn't have cache, it won't matter but that is not true otherwise, the
> > patch wouldn't have it.
>
> I have never said there should be no caching. On the contrary, my
> performance test results [1] already confirmed that caching ExprState
> is of benefit for the millions of times it may be used in the
> pgoutput_row_filter function. My only doubts are in regard to how much
> observable impact there would be re-evaluating the filter expression
> just a few extra times by the get_rel_sync_entry function.
>

I think it depends but why in the first place do you want to allow
re-evaluation when there is a way for not doing that?

--
With Regards,
Amit Kapila.