Markus Demleitner <msdemlei@xxxxxxxxxxxxxxxxxxxxx> writes: > Maximally stripped down, my problem is that > select distinct 300 from <bigtable> > seqscans <bigtable> (at least in PostgreSQL 11.14). To me, it seems > obvious that this ought be be just one row containing 300 once > Postgres has established that <bigtable> is nonempty. > Why do things not work like this? Am I missing something major? That seems like the sort of optimization that we absolutely should not spend cycles looking for. If it were a trivial change consuming no detectable number of planning cycles, maybe it would be worth the development and maintenance effort; though I'd be dubious about the value. But the fact that it'd have to be transformed into something testing whether the table is nonempty makes it fairly nontrivial. I doubt it's worth the development cost plus the cycles imposed on every other query. regards, tom lane
