On Fri, Jul 29, 2022 at 12:55 AM Tom Lane <t...@sss.pgh.pa.us> wrote: > It would not be profitable to flatten the range table before we've > done remove_useless_joins. We'd end up with useless entries from > subqueries that ultimately aren't there. We could perhaps do it > after we finish that phase, but I don't really see the point: it > wouldn't be better than what we do now, just the same work at a > different time.
That's not quite my question, though. Why do we ever build a non-flat range table in the first place? Like, instead of assigning indexes relative to the current subquery level, why not just assign them relative to the whole query from the start? It can't really be that we've done it this way because of remove_useless_joins(), because we've been building separate range tables and later flattening them for longer than join removal has existed as a feature. What bugs me is that it's very much not free. By building a bunch of separate range tables and combining them later, we generate extra work: we have to go back and adjust RT indexes after-the-fact. We pay that overhead for every query, not just the ones that end up with some unused entries in the range table. And why would it matter if we did end up with some useless entries in the range table, anyway? If there's some semantic difference, we could add a flag to mark those entries as needing to be ignored, which seems way better than crawling all over the whole tree adjusting RTIs everywhere. I don't really expect that we're ever going to change this -- and certainly not on this thread. The idea of running around and replacing RT indexes all over the tree is deeply embedded in the system. But are we really sure we want to add a second kind of index that we have to run around and adjust at the same time? If we are, so be it, I guess. It just looks really ugly and unnecessary to me. -- Robert Haas EDB: http://www.enterprisedb.com
