On Tue, Jan 24, 2023 at 1:25 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > "David G. Johnston" <david.g.johns...@gmail.com> writes: > > On Tue, Jan 24, 2023 at 12:31 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > >> select ... from t1 left join t2 on (t1.x = t2.y and t1.x = 1); > >> > >> If we turn the generic equivclass.c logic loose on these clauses, > >> it will deduce t2.y = 1, which is good, and then apply t2.y = 1 at > >> the scan of t2, which is even better (since we might be able to turn > >> that into an indexscan qual). However, it will also try to apply > >> t1.x = 1 at the scan of t1, and that's just wrong, because that > >> will eliminate t1 rows that should come through with null extension. > > > Is there a particular comment or README where that last conclusion is > > explained so that it makes sense. > > Hm? It's a LEFT JOIN, so it must not eliminate any rows from t1. > A row that doesn't have t1.x = 1 will appear in the output with > null columns for t2 ... but it must still appear, so we cannot > filter on t1.x = 1 in the scan of t1. > > Ran some queries, figured it out. Sorry for the noise. I had turned the behavior of the RHS side appearing in the ON clause into a personal general rule then tried to apply it to the LHS (left join mental model) without working through the rules from first principles.
David J.
