Re: row filtering for logical replication - Mailing list pgsql-hackers

On Thu, Aug 26, 2021 at 3:41 PM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Thu, Aug 26, 2021 at 3:00 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Aug 26, 2021 at 9:51 AM Peter Smith <smithpb2250@gmail.com> wrote:
> > >
> > > On Thu, Aug 26, 2021 at 1:20 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Thu, Aug 26, 2021 at 7:37 AM Peter Smith <smithpb2250@gmail.com> wrote:
> > > > >
> > > > > On Wed, Aug 25, 2021 at 3:28 PM Amit Kapila <amit.kapila16@gmail.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?
>
> Because the current code logic of having the "delayed" ExprState
> evaluation does come at some cost.
>

So, now you mixed it with the second point. Here, I was talking about
the need for correct invalidation but you started discussing when to
first time evaluate the expression, both are different things.

>  And the cost is -
> a. Needing an extra condition and more code in the function pgoutput_row_filter
> b. Needing to maintain the additional Node list
>

I am not sure you need (b) above and I think (a) should make the
overall code look clean.

> If we chose not to implement a delayed ExprState cache evaluation then
> there would still be a (one-time) ExprState cache evaluation but it
> would happen whenever get_rel_sync_entry is called (regardless of if
> pgoputput_row_filter is subsequently called). E.g. there can be some
> rebuilds of the ExprState cache if the user calls TRUNCATE.
>

Apart from Truncate, it will also be a waste if any error happens
before actually evaluating the filter, tomorrow there could be other
operations like replication of sequences (I have checked that proposed
patch for sequences uses get_rel_sync_entry) where we don't need to
build ExprState (as filters might or might not be there). So, it would
be better to avoid cache lookups in those cases if possible. I still
think doing expensive things like preparing expressions should ideally
be done only when it is required.
--
With Regards,
Amit Kapila.