On Sat, Mar 25, 2023 at 11:41 PM Tom Lane <t...@sss.pgh.pa.us> wrote:
> Richard Guo <guofengli...@gmail.com> writes: > > Should we instead mark the constant-TRUE clause with required_relids > > plus the OJ relid? > > I do not think it matters. Yeah, I agree that it makes no difference currently. One day if we want to replace the is_pushed_down flag with checking to see if a clause's required_relids includes the OJ being formed in order to tell whether it's a filter or join clause, I think we'd need to make this change. > > > Even if the join does become clauseless, it will end up being an > > unqualified nestloop. I think the join ordering algorithm will force > > this join to be formed when necessary. > > We would find *some* valid plan, but not necessarily a *good* plan. > The point of the dummy clause is to ensure that the join is considered > as soon as possible. That might not be the ideal join order of course, > but we'll consider it among other join orders and arrive at a cost-based > decision. With no dummy clause, the join order heuristics would always > delay this join as long as possible; so even if another ordering is > better, we'd not find it. I understand it now. Thanks for the explanation. Thanks Richard
