Received: from malur.postgresql.org ([217.196.149.56]) by arkaria.postgresql.org with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1oHRXP-00011w-Ve for pgsql-hackers@arkaria.postgresql.org; Fri, 29 Jul 2022 15:04:27 +0000 Received: from localhost ([127.0.0.1] helo=malur.postgresql.org) by malur.postgresql.org with esmtp (Exim 4.92) (envelope-from ) id 1oHRXO-0000sD-5v for pgsql-hackers@arkaria.postgresql.org; Fri, 29 Jul 2022 15:04:26 +0000 Received: from magus.postgresql.org ([2a02:c0:301:0:ffff::29]) by malur.postgresql.org with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1oHRXN-0000s4-Ss for pgsql-hackers@lists.postgresql.org; Fri, 29 Jul 2022 15:04:25 +0000 Received: from sss.pgh.pa.us ([66.207.139.130]) by magus.postgresql.org with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1oHRXG-0001ol-UW for pgsql-hackers@postgresql.org; Fri, 29 Jul 2022 15:04:25 +0000 Received: from sss1.sss.pgh.pa.us (localhost [127.0.0.1]) by sss.pgh.pa.us (8.15.2/8.15.2) with ESMTP id 26TF4Gd63772668; Fri, 29 Jul 2022 11:04:16 -0400 From: Tom Lane To: Robert Haas cc: Amit Langote , Jacob Champion , Zhihong Yu , David Rowley , PostgreSQL-development Subject: Re: generic plans and "initial" pruning In-reply-to: References: <215356.1647286703@sss.pgh.pa.us> ! <3633494.1659070517@sss.pgh.pa.us> message dated "Fri, 29 Jul 2022 08:22:26 -0400" MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <3772666.1659107056.1@sss.pgh.pa.us> Content-Transfer-Encoding: quoted-printable Date: Fri, 29 Jul 2022 11:04:16 -0400 Message-ID: <3772667.1659107056@sss.pgh.pa.us> List-Id: List-Help: List-Subscribe: List-Post: List-Owner: List-Archive: Archived-At: Precedence: bulk Robert Haas writes: > 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? We could probably make that work, but I'm skeptical that it would really be an improvement overall, for a couple of reasons. (1) The need for merge-rangetables-and-renumber-Vars logic doesn't go away. It just moves from setrefs.c to the rewriter, which would have to do it when expanding views. This would be a net loss performance-wise, I think, because setrefs.c can do it as part of a parsetree scan that it has to perform anyway for other housekeeping reasons; but the rewriter would need a brand new pass over the tree. Admittedly that pass would only happen for view replacement, but it's still not open-and-shut that there'd be a performance win. (2) The need for varlevelsup and similar fields doesn't go away, I think, because we need those for semantic purposes such as discovering the query level that aggregates are associated with. That means that subquery flattening still has to make a pass over the tree to touch every Var's varlevelsup; so not having to adjust varno at the same time would save little. I'm not sure whether I think it's a net plus or net minus that varno would become effectively independent of varlevelsup. It'd be different from the way we think of them now, for sure, and I think it'd take awhile to flush out bugs arising from such a redefinition. > 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? You probably want to avert your eyes from [1], then ;-). Although I'm far from convinced that the cross-list index fields currently proposed there are actually necessary; the cost to adjust them during rangetable merging could outweigh any benefit. regards, tom lane [1] https://www.postgresql.org/message-id/flat/CA+HiwqGjJDmUhDSfv-U2qhKJjt= 9ST7Xh9JXC_irsAQ1TAUsJYg@mail.gmail.com