Remove bogus Assert and dead code in remove_useless_results_recurse().

The JOIN_SEMI case Assert'ed that there are no PlaceHolderVars that
need to be evaluated at the semijoin's RHS, which is wrong because
there could be some in the semijoin's qual condition.  However, there
could not be any references further up than that, and within the qual
there is not any way that such a PHV could have gone to null yet, so
we don't really need the PHV and there is no need to avoid making the
RHS-removal optimization.  The upshot is that there's no actual bug
in production code, and we ought to just remove this misguided Assert.

While we're here, also drop the JOIN_RIGHT case, which is dead code
because reduce_outer_joins() already got rid of JOIN_RIGHT.

Per bug #17700 from Xin Wen.  Uselessness of the JOIN_RIGHT case
pointed out by Richard Guo.  Back-patch to v12 where this code
was added.

Discussion: https://postgr.es/m/17700-2b5c10d917c30687@postgresql.org

diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index f4cdb879c2ce8dacc6f17a0870caa857355829c1..2ea3ca734ecea5a3463259e53ab071cb5aa84685 100644 (file)
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -3251,16 +3251,6 @@ remove_useless_results_recurse(PlannerInfo *root, Node *jtnode)
                    jtnode = j->larg;
                }
                break;
-           case JOIN_RIGHT:
-               /* Mirror-image of the JOIN_LEFT case */
-               if ((varno = get_result_relid(root, j->larg)) != 0 &&
-                   (j->quals == NULL ||
-                    !find_dependent_phvs(root, varno)))
-               {
-                   remove_result_refs(root, varno, j->rarg);
-                   jtnode = j->rarg;
-               }
-               break;
            case JOIN_SEMI:
 
                /*
@@ -3269,14 +3259,17 @@ remove_useless_results_recurse(PlannerInfo *root, Node *jtnode)
                 * LHS, since we should either return the LHS row or not.  For
                 * simplicity we inject the filter qual into a new FromExpr.
                 *
-                * Unlike the LEFT/RIGHT cases, we just Assert that there are
-                * no PHVs that need to be evaluated at the semijoin's RHS,
-                * since the rest of the query couldn't reference any outputs
-                * of the semijoin's RHS.
+                * There is a fine point about PHVs that are supposed to be
+                * evaluated at the RHS.  Such PHVs could only appear in the
+                * semijoin's qual, since the rest of the query cannot
+                * reference any outputs of the semijoin's RHS.  Therefore,
+                * they can't actually go to null before being examined, and
+                * it'd be OK to just remove the PHV wrapping.  We don't have
+                * infrastructure for that, but remove_result_refs() will
+                * relabel them as to be evaluated at the LHS, which is fine.
                 */
                if ((varno = get_result_relid(root, j->rarg)) != 0)
                {
-                   Assert(!find_dependent_phvs(root, varno));
                    remove_result_refs(root, varno, j->larg);
                    if (j->quals)
                        jtnode = (Node *)
@@ -3290,6 +3283,7 @@ remove_useless_results_recurse(PlannerInfo *root, Node *jtnode)
                /* We have no special smarts for these cases */
                break;
            default:
+               /* Note: JOIN_RIGHT should be gone at this point */
                elog(ERROR, "unrecognized join type: %d",
                     (int) j->jointype);
                break;

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 21ca7d04bc1529430356cb4d31ba3e1413a06e21..b8d43e4c14343c8a17481f73de785996c245fafa 100644 (file)
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -3550,6 +3550,26 @@ where b;
  0 | t | t
 (2 rows)
 
+-- Test PHV in a semijoin qual, which confused useless-RTE removal (bug #17700)
+explain (verbose, costs off)
+with ctetable as not materialized ( select 1 as f1 )
+select * from ctetable c1
+where f1 in ( select c3.f1 from ctetable c2 full join ctetable c3 on true );
+         QUERY PLAN         
+----------------------------
+ Result
+   Output: 1
+   One-Time Filter: (1 = 1)
+(3 rows)
+
+with ctetable as not materialized ( select 1 as f1 )
+select * from ctetable c1
+where f1 in ( select c3.f1 from ctetable c2 full join ctetable c3 on true );
+ f1 
+----
+  1
+(1 row)
+
 --
 -- test inlining of immutable functions
 --

diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 50c19d822bf117411559c80a256fa482c47f6867..65aab85c3548713c0e3f614cf60e84f7990b1fe2 100644 (file)
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -1156,6 +1156,16 @@ select * from
            select a as b) as t3
 where b;
 
+-- Test PHV in a semijoin qual, which confused useless-RTE removal (bug #17700)
+explain (verbose, costs off)
+with ctetable as not materialized ( select 1 as f1 )
+select * from ctetable c1
+where f1 in ( select c3.f1 from ctetable c2 full join ctetable c3 on true );
+
+with ctetable as not materialized ( select 1 as f1 )
+select * from ctetable c1
+where f1 in ( select c3.f1 from ctetable c2 full join ctetable c3 on true );
+
 --
 -- test inlining of immutable functions
 --