>> I'd be happy to yank this in favor of my holistic solution to this >> problem I posted recently on the mailing list [1]. > > [1] > https://www.postgresql.org/message-id/flat/CAAaqYe8yKSvzbyu8w-dThRs9aTFMwrFxn_BkTYeXgjqe3CbNjg%40mail.gmail.com > > Not precisely sure what you mean - are you saying that we can just have an > overall test for NOT NULL, which thereby avoids the need to expand the array > and therefore dispenses with the GUC completely? > > Having indexes defined using WHERE NOT NULL is a very important use case.
I don't think we can avoid expanding the array for other cases (for example, being able to infer that "foo < 5" for "foo IN (1,2,3,4)". If we wanted to keep that inference without expanding the array we'd have to (at minimum, I think) duplicate a lot of the existing inference logic, but I haven't investigated it much. So keeping the GUC could allow someone to tune how large an array can be and still guarantee inferences like "foo < 5". But I'm not sure that that is as valuable. At least I haven't run into cases where I've noticed a need for it. My patch only addresses the IS NOT NULL inference precisely for the reason you state: we have lots of plans that (unless you tack on an explicit "foo IS NOT NULL" to your query) the planner decides it can't use WHERE NOT NULL indexes because it can't currently infer the correctness of that for large arrays.
