Fix mis-optimization of semijoins with more than one LHS relation.

The inner-unique patch (commit 9c7f5229) supposed that if we're
considering a JOIN_UNIQUE_INNER join path, we can always set inner_unique
for the join, because the inner path produced by create_unique_path should
be unique relative to the outer relation.  However, that's true only if
we're considering joining to the whole outer relation --- otherwise we may
be applying only some of the join quals, and so the inner path might be
non-unique from the perspective of this join.  Adjust the test to only
believe that we can set inner_unique if we have the whole semijoin LHS on
the outer side.

There is more that can be done in this area, but this commit is only
intended to provide the minimal fix needed to get correct plans.

Per report from Teodor Sigaev.  Thanks to David Rowley for preliminary
investigation.

Discussion: https://postgr.es/m/f994fc98-389f-4a46-d1bc-c42e05cb43ed@sigaev.ru

src/backend/optimizer/path/joinpath.c

@@ -126,8 +126,11 @@ add_paths_to_joinrel(PlannerInfo *root,
 	 *
 	 * We have some special cases: for JOIN_SEMI and JOIN_ANTI, it doesn't
 	 * matter since the executor can make the equivalent optimization anyway;
-	 * we need not expend planner cycles on proofs.  For JOIN_UNIQUE_INNER, we
-	 * know we're going to force uniqueness of the innerrel below.  For
+	 * we need not expend planner cycles on proofs.  For JOIN_UNIQUE_INNER, if
+	 * the LHS covers all of the associated semijoin's min_lefthand, then it's
+	 * appropriate to set inner_unique because the path produced by
+	 * create_unique_path will be unique relative to the LHS.  (If we have an
+	 * LHS that's only part of the min_lefthand, that is *not* true.)  For
 	 * JOIN_UNIQUE_OUTER, pass JOIN_INNER to avoid letting that value escape
 	 * this module.
 	 */
@@ -138,7 +141,8 @@ add_paths_to_joinrel(PlannerInfo *root,
 			extra.inner_unique = false; /* well, unproven */
 			break;
 		case JOIN_UNIQUE_INNER:
-			extra.inner_unique = true;
+			extra.inner_unique = bms_is_subset(sjinfo->min_lefthand,
+											   outerrel->relids);
 			break;
 		case JOIN_UNIQUE_OUTER:
 			extra.inner_unique = innerrel_is_unique(root, outerrel, innerrel,

src/test/regress/expected/join.out

@@ -5634,3 +5634,32 @@ reset enable_sort;
 drop table j1;
 drop table j2;
 drop table j3;
+-- check that semijoin inner is not seen as unique for a portion of the outerrel
+explain (verbose, costs off)
+select t1.unique1, t2.hundred
+from onek t1, tenk1 t2
+where exists (select 1 from tenk1 t3
+              where t3.thousand = t1.unique1 and t3.tenthous = t2.hundred)
+      and t1.unique1 < 1;
+                                   QUERY PLAN                                    
+---------------------------------------------------------------------------------
+ Nested Loop
+   Output: t1.unique1, t2.hundred
+   ->  Hash Join
+         Output: t1.unique1, t3.tenthous
+         Hash Cond: (t3.thousand = t1.unique1)
+         ->  HashAggregate
+               Output: t3.thousand, t3.tenthous
+               Group Key: t3.thousand, t3.tenthous
+               ->  Index Only Scan using tenk1_thous_tenthous on public.tenk1 t3
+                     Output: t3.thousand, t3.tenthous
+         ->  Hash
+               Output: t1.unique1
+               ->  Index Only Scan using onek_unique1 on public.onek t1
+                     Output: t1.unique1
+                     Index Cond: (t1.unique1 < 1)
+   ->  Index Only Scan using tenk1_hundred on public.tenk1 t2
+         Output: t2.hundred
+         Index Cond: (t2.hundred = t3.tenthous)
+(18 rows)
+

src/test/regress/sql/join.sql

@@ -1856,3 +1856,11 @@ reset enable_sort;
 drop table j1;
 drop table j2;
 drop table j3;
+
+-- check that semijoin inner is not seen as unique for a portion of the outerrel
+explain (verbose, costs off)
+select t1.unique1, t2.hundred
+from onek t1, tenk1 t2
+where exists (select 1 from tenk1 t3
+              where t3.thousand = t1.unique1 and t3.tenthous = t2.hundred)
+      and t1.unique1 < 1;