On Thu, Aug 26, 2021 at 9:13 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Thu, Aug 26, 2021 at 3:41 PM Peter Smith <smithpb2...@gmail.com> wrote: > > > > On Thu, Aug 26, 2021 at 3:00 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > > > On Thu, Aug 26, 2021 at 9:51 AM Peter Smith <smithpb2...@gmail.com> wrote: > > > > > > > > On Thu, Aug 26, 2021 at 1:20 PM Amit Kapila <amit.kapil...@gmail.com> > > > > wrote: > > > > > > > > > > On Thu, Aug 26, 2021 at 7:37 AM Peter Smith <smithpb2...@gmail.com> > > > > > wrote: > > > > > > > > > > > > On Wed, Aug 25, 2021 at 3:28 PM Amit Kapila > > > > > > <amit.kapil...@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.
OK. Per your suggestion, I will try to move as much of the row-filter cache code as possible out of the get_rel_sync_entry function and into the pgoutput_row_filter function. ------ Kind Regards, Peter Smith. Fujitsu Australia
