On Mon, 31 Mar 2025 at 16:21, Alena Rybakina <a.rybak...@postgrespro.ru> wrote: > However, is it necessary to check that extra->inner_unique must be false for > SEMI/ANTI joins here, or am I missing something? It looks a little confusing > at this point.
If it is necessary, I don't see the reason for it. It was me that worked on unique joins and I see no reason why a SEMI or ANTI join couldn't be marked as unique. The reason they're not today is that the only point of the unique join optimisation is so that during execution, the join nodes could skip to the next outer tuple after matching the current outer to an inner. If the join is unique, then there are no more join partners to find for the current outer after matching it up once. With SEMI and ANTI joins, we skip to the next outer tuple after finding a match anyway, so there's no point in going to the trouble of setting the inner_unique flag. I can't say definitively that we won't find a reason in the future that we should set inner_unique for SEMI/ANTI joins, so I don't follow the need for the Assert. Maybe you're seeing something that I'm not. What do you think will break if both flags are true? David
