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Disallow joins whose lateral references need an unformable outer join

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If a LATERAL subquery references a PlaceHolderVar that must be
evaluated at an outer join, the subquery's lateral_relids include that
outer join's relid, since the lateral parameter is the outer join's
output.  join_is_legal() verified that the rels named by a proposed
join's minimum parameterization could still be joined to from outside,
but it did not consider such outer-join relids.  As a result, when
identity 3 permitted a commuted join order, we could approve a join
that includes part of that outer join's required input, even though
the outer join could then be completed only above the proposed join,
leaving the lateral parameter forever unsatisfiable.

In assert-enabled builds this tripped the backstop Assert in
try_nestloop_path().  In production builds the bogus join generated
only paths whose parameterization can never be satisfied, so they
could never appear in a complete plan.  This not only wasted planning
effort, but also could mislead the clauseless-join heuristics into
thinking that legality of this join means that some other join rel
need not be formed, and that could lead to failure to find any plan at
all.

Such joins used to be rejected by the have_dangerous_phv() check in
join_is_legal(), which refused any join where a PlaceHolderVar's eval
set overlapped the other side of the join without being contained in
it.  Commit a16ef313f removed that restriction, exposing this
oversight.

To fix, teach join_is_legal() to reject a proposed join if its minimum
parameterization includes an outer-join relid whose outer join cannot
be formed strictly outside the join, that is, when any rel needed to
form that outer join must become part of the proposed join's own join
tree.

Back-patch to v18, where commit a16ef313f removed the
have_dangerous_phv() restriction.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAMbWs49ZveAYx9T5b0=gxn0pnvqq=MbN-Z4KLbLAE+kH06mWVA@mail.gmail.com
Backpatch-through: 18
This commit is contained in:
Richard Guo
2026-09-04 10:33:39 +09:00
parent bb238161f9
commit 10a84539b9
3 changed files with 86 additions and 0 deletions
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26
src/backend/optimizer/path/joinrels.c
View File

@@ -640,6 +640,32 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
} while (more);
if (bms_overlap(join_plus_rhs, join_lateral_rels))
return false; /* will not be able to join to some RHS rel */
/*
* Furthermore, the minimum parameterization can include
* outer-join relids as well as baserel relids. In such a case
* the value laterally needed is an output of that outer join, so
* the outer join must be formed strictly outside this join for
* the value to be supplied to it. That's not possible if any rel
* needed to form the outer join is within join_plus_rhs, since
* all such rels must join into this join's own join tree. If we
* accepted this join anyway, every path for it would require a
* parameter that can never be supplied, so it could never appear
* in a complete plan.
*/
if (bms_overlap(join_lateral_rels, root->outer_join_rels))
{
foreach(l, root->join_info_list)
{
SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);
if (!bms_is_member(sjinfo->ojrelid, join_lateral_rels))
continue;
if (bms_overlap(join_plus_rhs, sjinfo->min_lefthand) ||
bms_overlap(join_plus_rhs, sjinfo->min_righthand))
return false; /* OJ can't be formed outside join */
}
}
}
}

40
src/test/regress/expected/join.out
View File

@@ -5880,6 +5880,46 @@ where ss1.c2 = 0;
----+----+----+----+----+----
(0 rows)
--
-- check that a join is disallowed when a lateral reference to an outer-join
-- output would have to be passed down into that outer join's own input
--
explain (costs off)
select count(*) from int4_tbl t1 left join
(select b.q1 as bx, 1 as one from int4_tbl a left join int8_tbl b on a.f1 = b.q2) t2
on true
left join lateral
(select c.f1 as cnt from int4_tbl c where c.f1 = t2.one offset 0) t3
on t2.bx = t3.cnt;
QUERY PLAN
-------------------------------------------------------------------------
Aggregate
-> Nested Loop Left Join
-> Seq Scan on int4_tbl t1
-> Materialize
-> Nested Loop Left Join
-> Hash Left Join
Hash Cond: (a.f1 = b.q2)
-> Seq Scan on int4_tbl a
-> Hash
-> Seq Scan on int8_tbl b
-> Subquery Scan on t3
Filter: ((b.q1 = t3.cnt) AND (b.q1 = t3.cnt))
-> Seq Scan on int4_tbl c
Filter: (f1 = (1))
(14 rows)
select count(*) from int4_tbl t1 left join
(select b.q1 as bx, 1 as one from int4_tbl a left join int8_tbl b on a.f1 = b.q2) t2
on true
left join lateral
(select c.f1 as cnt from int4_tbl c where c.f1 = t2.one offset 0) t3
on t2.bx = t3.cnt;
count
-------
25
(1 row)
--
-- test successful handling of full join underneath left join (bug #14105)
--

20
src/test/regress/sql/join.sql
View File

@@ -2093,6 +2093,26 @@ select ss2.* from
lateral (select i41.*, i8.*, ss1.* from text_tbl limit 1) ss2
where ss1.c2 = 0;
--
-- check that a join is disallowed when a lateral reference to an outer-join
-- output would have to be passed down into that outer join's own input
--
explain (costs off)
select count(*) from int4_tbl t1 left join
(select b.q1 as bx, 1 as one from int4_tbl a left join int8_tbl b on a.f1 = b.q2) t2
on true
left join lateral
(select c.f1 as cnt from int4_tbl c where c.f1 = t2.one offset 0) t3
on t2.bx = t3.cnt;
select count(*) from int4_tbl t1 left join
(select b.q1 as bx, 1 as one from int4_tbl a left join int8_tbl b on a.f1 = b.q2) t2
on true
left join lateral
(select c.f1 as cnt from int4_tbl c where c.f1 = t2.one offset 0) t3
on t2.bx = t3.cnt;
--
-- test successful handling of full join underneath left join (bug #14105)
--