On Wed, Feb 5, 2025 at 5:55 AM Tom Lane <t...@sss.pgh.pa.us> wrote: > Right now, if we have four tables to join, we have a joinlist > (A B C D). (Really they're integer relids, but let's use names here.) > If we decide to force C to be joined last, it should be sufficient to > convert this to ((A B D) C). If C and D both look like candidates for > this treatment, we can make it be (((A B) C) D) or (((A B) D) C). > This is pretty much the same thing that happens if you set > join_collapse_limit to 1 and use JOIN syntax to force a join order. > In fact, IIRC we start out with nested joinlists and there is some > code that normally flattens them until it decides it'd be creating > too large a sub-problem. I'm suggesting selectively reversing the > flattening.
This should not be too difficult to implement. Outer joins seem to add some complexity, though. We need to ensure that the resulting joins in each sub-list are legal given the query's join order constraints. For example, if we make the joinlist be (((A B) C) D), we need to ensure that the A/B join and the (A/B)/C join are legal. Thanks Richard
