Fix problems when a plain-inheritance parent table is excluded.

When an UPDATE/DELETE/MERGE's target table is an old-style
inheritance tree, it's possible for the parent to get excluded
from the plan while some children are not.  (I believe this is
only possible if we can prove that a CHECK ... NO INHERIT
constraint on the parent contradicts the query WHERE clause,
so it's a very unusual case.)  In such a case, ExecInitModifyTable
mistakenly concluded that the first surviving child is the target
table, leading to at least two bugs:

1. The wrong table's statement-level triggers would get fired.

2. In v16 and up, it was possible to fail with "invalid perminfoindex
0 in RTE with relid nnnn" due to the child RTE not having permissions
data included in the query plan.  This was hard to reproduce reliably
because it did not occur unless the update triggered some non-HOT
index updates.

In v14 and up, this is easy to fix by defining ModifyTable.rootRelation
to be the parent RTE in plain inheritance as well as partitioned cases.

While the wrong-triggers bug also appears in older branches, the
relevant code in both the planner and executor is quite a bit
different, so it would take a good deal of effort to develop and
test a suitable patch.  Given the lack of field complaints about the
trigger issue, I'll desist for now.  (Patching v11 for this seems
unwise anyway, given that it will have no more releases after next
month.)

Per bug #18147 from Hans Buschmann.

Amit Langote and Tom Lane

Discussion: https://postgr.es/m/18147-6fc796538913ee88@postgresql.org

src/include/nodes/pathnodes.h

@@ -2344,7 +2344,7 @@ typedef struct ModifyTablePath
 	CmdType		operation;		/* INSERT, UPDATE, DELETE, or MERGE */
 	bool		canSetTag;		/* do we set the command tag/es_processed? */
 	Index		nominalRelation;	/* Parent RT index for use of EXPLAIN */
-	Index		rootRelation;	/* Root RT index, if target is partitioned */
+	Index		rootRelation;	/* Root RT index, if partitioned/inherited */
 	bool		partColsUpdated;	/* some part key in hierarchy updated? */
 	List	   *resultRelations;	/* integer list of RT indexes */
 	List	   *updateColnosLists;	/* per-target-table update_colnos lists */

src/include/nodes/plannodes.h

@@ -216,11 +216,12 @@ typedef struct ProjectSet
  *		Apply rows produced by outer plan to result table(s),
  *		by inserting, updating, or deleting.
  *
- * If the originally named target table is a partitioned table, both
- * nominalRelation and rootRelation contain the RT index of the partition
- * root, which is not otherwise mentioned in the plan.  Otherwise rootRelation
- * is zero.  However, nominalRelation will always be set, as it's the rel that
- * EXPLAIN should claim is the INSERT/UPDATE/DELETE/MERGE target.
+ * If the originally named target table is a partitioned table or inheritance
+ * tree, both nominalRelation and rootRelation contain the RT index of the
+ * partition root or appendrel RTE, which is not otherwise mentioned in the
+ * plan.  Otherwise rootRelation is zero.  However, nominalRelation will
+ * always be set, as it's the rel that EXPLAIN should claim is the
+ * INSERT/UPDATE/DELETE/MERGE target.
  *
  * Note that rowMarks and epqParam are presumed to be valid for all the
  * table(s); they can't contain any info that varies across tables.
@@ -232,7 +233,7 @@ typedef struct ModifyTable
 	CmdType		operation;		/* INSERT, UPDATE, DELETE, or MERGE */
 	bool		canSetTag;		/* do we set the command tag/es_processed? */
 	Index		nominalRelation;	/* Parent RT index for use of EXPLAIN */
-	Index		rootRelation;	/* Root RT index, if target is partitioned */
+	Index		rootRelation;	/* Root RT index, if partitioned/inherited */
 	bool		partColsUpdated;	/* some part key in hierarchy updated? */
 	List	   *resultRelations;	/* integer list of RT indexes */
 	List	   *updateColnosLists;	/* per-target-table update_colnos lists */