Fix some issues with wrong placement of pseudo-constant quals.

initsplan.c figured that it could push Var-free qual clauses to
the top of the current JoinDomain, which is okay in the abstract.
But if the current domain is inside some outer join, and we later
commute an inside-the-domain outer join with one outside it,
we end up placing the pushed-up qual clause incorrectly.

In distribute_qual_to_rels, avoid this by using the syntactic scope
of the qual clause; with the exception that if we're in the top-level
join domain we can still use the full query relid set, ensuring the
resulting gating Result node goes to the top of the plan.  (This is
approximately as smart as the pre-v16 code was.  Perhaps we can do
better later, but it's not clear that such cases are worth a lot of
sweat.)

In process_implied_equality, we don't have a clear notion of syntactic
scope, but we do have the results of SpecialJoinInfo construction.
Thumb through those and remove any lower outer joins that might get
commuted to above the join domain.  Again, we can make an exception
for the top-level join domain.  It'd be possible to work harder here
(for example, by keeping outer joins that aren't shown as potentially
commutable), but I'm going to stop here for the moment.  This issue
has convinced me that the current representation of join domains
probably needs further refinement, so I'm disinclined to write
inessential dependent logic just yet.

In passing, tighten the qualscope passed to process_implied_equality
by generate_base_implied_equalities_no_const; there's no need for
it to be larger than the rel we are currently considering.

Tom Lane and Richard Guo, per report from Tender Wang.

Discussion: https://postgr.es/m/CAHewXNk9eJ35ru5xATWioTV4+xZPHptjy9etdcNPjUfY9RQ+uQ@mail.gmail.com

1 files changed, 61 insertions, 0 deletions

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 16318d9da2..5a2756b333 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -5119,6 +5119,67 @@ from int8_tbl t1
          ->  Seq Scan on onek t4
 (13 rows)
 
+-- More tests of correct placement of pseudoconstant quals
+-- simple constant-false condition
+explain (costs off)
+select * from int8_tbl t1 left join
+  (int8_tbl t2 inner join int8_tbl t3 on false
+   left join int8_tbl t4 on t2.q2 = t4.q2)
+on t1.q1 = t2.q1;
+              QUERY PLAN              
+--------------------------------------
+ Hash Left Join
+   Hash Cond: (t1.q1 = q1)
+   ->  Seq Scan on int8_tbl t1
+   ->  Hash
+         ->  Result
+               One-Time Filter: false
+(6 rows)
+
+-- deduce constant-false from an EquivalenceClass
+explain (costs off)
+select * from int8_tbl t1 left join
+  (int8_tbl t2 inner join int8_tbl t3 on (t2.q1-t3.q2) = 0 and (t2.q1-t3.q2) = 1
+   left join int8_tbl t4 on t2.q2 = t4.q2)
+on t1.q1 = t2.q1;
+              QUERY PLAN              
+--------------------------------------
+ Hash Left Join
+   Hash Cond: (t1.q1 = q1)
+   ->  Seq Scan on int8_tbl t1
+   ->  Hash
+         ->  Result
+               One-Time Filter: false
+(6 rows)
+
+-- pseudoconstant based on an outer-level Param
+explain (costs off)
+select exists(
+  select * from int8_tbl t1 left join
+    (int8_tbl t2 inner join int8_tbl t3 on x0.f1 = 1
+     left join int8_tbl t4 on t2.q2 = t4.q2)
+  on t1.q1 = t2.q1
+) from int4_tbl x0;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Seq Scan on int4_tbl x0
+   SubPlan 1
+     ->  Nested Loop Left Join
+           Join Filter: (t2.q2 = t4.q2)
+           ->  Nested Loop Left Join
+                 Join Filter: (t1.q1 = t2.q1)
+                 ->  Seq Scan on int8_tbl t1
+                 ->  Materialize
+                       ->  Result
+                             One-Time Filter: (x0.f1 = 1)
+                             ->  Nested Loop
+                                   ->  Seq Scan on int8_tbl t2
+                                   ->  Materialize
+                                         ->  Seq Scan on int8_tbl t3
+           ->  Materialize
+                 ->  Seq Scan on int8_tbl t4
+(16 rows)
+
 -- check that join removal works for a left join when joining a subquery
 -- that is guaranteed to be unique by its GROUP BY clause
 explain (costs off)