Horton

Thread

Commits

  1. Prevent overly-aggressive collapsing of joins to RTE_RESULT relations.

  1. Errors "failed to construct the join relation" and "failed to build any 2-way joins"

    Will Leinweber <will@bitfission.com> — 2019-12-12T23:32:05Z

    On 12.1, fresh initdb the following query gives me the error
"ERROR:  failed to construct the join relation"

  SELECT FROM (
    SELECT FROM pg_catalog.pg_stat_bgwriter AS ref_0
    LEFT JOIN pg_catalog.pg_stat_bgwriter AS ref_1 ON (true), LATERAL (
      SELECT FROM pg_catalog.pg_publication AS ref_2, LATERAL (
        SELECT FROM pg_catalog.pg_class
        WHERE ref_1.buffers_alloc IS NOT NULL
      ) AS subq_0
      WHERE true
      LIMIT 1
    ) AS subq_1
    WHERE true
  ) AS subq_2

If you move the limit up into subq_0, then the error changes to
"ERROR:  failed to build any 2-way joins"

  SELECT FROM (
    SELECT FROM pg_catalog.pg_stat_bgwriter AS ref_0
    LEFT JOIN pg_catalog.pg_stat_bgwriter AS ref_1 ON (true), LATERAL (
      SELECT FROM pg_catalog.pg_publication AS ref_2, LATERAL (
        SELECT FROM pg_catalog.pg_class
        WHERE ref_1.buffers_alloc IS NOT NULL
        LIMIT 1
      ) AS subq_0
      WHERE true
    ) AS subq_1
    WHERE true
  ) AS subq_2

I'm unable to reproduce either of the errors on 11.6 or 11.4. I haven't tried
any other versions. The actual value of the limit doesn't appear to matter,
just if it's present or not.

— Will

  2. Re: Errors "failed to construct the join relation" and "failed to build any 2-way joins"

    Tom Lane <tgl@sss.pgh.pa.us> — 2019-12-13T06:26:11Z

    Will Leinweber <will@bitfission.com> writes:
> On 12.1, fresh initdb the following query gives me the error
> "ERROR:  failed to construct the join relation"

I'm getting an assertion failure in an assert-enabled build, here:

(gdb) f 3
#3  0x00000000006f382a in create_lateral_join_info (root=0x2d380c8)
    at initsplan.c:637
637                     Assert(!bms_is_member(rti, lateral_relids));

Eyeing the plan produced by v11, I'm suspecting some oversight in
the RTE_RESULT changes (4be058fe9); but I haven't actually bisected.
Too tired to look closer right now.

			regards, tom lane

  3. Re: Errors "failed to construct the join relation" and "failed to build any 2-way joins"

    Tom Lane <tgl@sss.pgh.pa.us> — 2019-12-13T23:09:21Z

    I wrote:
> Will Leinweber <will@bitfission.com> writes:
>> On 12.1, fresh initdb the following query gives me the error
>> "ERROR:  failed to construct the join relation"

> Eyeing the plan produced by v11, I'm suspecting some oversight in
> the RTE_RESULT changes (4be058fe9); but I haven't actually bisected.

Yup: it's folding the join tree to the point where a PlaceHolderVar ends
up marked as to be evaluated by the same relation that uses it, and then
things go all pear-shaped.  Here's a proposed patch for that.

			regards, tom lane