Horton

Re: Wrong rows estimations with joins of CTEs slows queries by more than factor 500

Tom Lane <tgl@sss.pgh.pa.us>

View thread
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Guo <guofenglinux@gmail.com>
Cc: Jian Guo <gjian@vmware.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Hans Buschmann <buschmann@nidsa.net>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, Zhenghua Lyu <zlyu@vmware.com>
Date: 2023-11-20T17:45:58Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Allow examine_simple_variable() to work on INSERT RETURNING Vars.

  2. Extract column statistics from CTE references, if possible.

  3. Remove SQL regression tests for GUCs related to NO_SHOW_ALL

Richard Guo <guofenglinux@gmail.com> writes:
> On Fri, Nov 17, 2023 at 11:38 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> That line of argument also leads to the conclusion that it'd be
>> okay to expose info about the ordering of the CTE result to the
>> upper planner.

> In the light of this conclusion, I had a go at propagating the pathkeys
> from CTEs up to the outer planner and came up with the attached.

Oh, nice!  I remembered we had code already to do this for regular
SubqueryScans, but I thought we'd need to do some refactoring to
apply it to CTEs.  I think you are right though that
convert_subquery_pathkeys can be used as-is.  Some thoughts:

* Do we really need to use make_tlist_from_pathtarget?  Why isn't
the tlist of the cteplan good enough (indeed, more so)?

* I don't love having this code assume that it knows how to find
the Path the cteplan was made from.  It'd be better to make
SS_process_ctes save that somewhere, maybe in a list paralleling
root->cte_plan_ids.

Alternatively: maybe it's time to do what the comments in
SS_process_ctes vaguely speculate about, and just save the Path
at that point, with construction of the plan left for createplan()?
That might be a lot of refactoring for not much gain, so not sure.

			regards, tom lane