Fix infer_arbiter_indexes() to not barf on system columns.

While it could be argued that rejecting system column mentions in the
ON CONFLICT list is an unsupported feature, falling over altogether
just because the table has a unique index on OID is indubitably a bug.

As far as I can tell, fixing infer_arbiter_indexes() is sufficient to
make ON CONFLICT (oid) actually work, though making a regression test
for that case is problematic because of the impossibility of setting
the OID counter to a known value.

Minor cosmetic cleanups along with the bug fix.

src/backend/optimizer/util/plancat.c

@@ -465,26 +465,18 @@ infer_arbiter_indexes(PlannerInfo *root)
 
 	/*
 	 * Build normalized/BMS representation of plain indexed attributes, as
-	 * well as direct list of inference elements.  This is required for
-	 * matching the cataloged definition of indexes.
+	 * well as a separate list of expression items.  This simplifies matching
+	 * the cataloged definition of indexes.
 	 */
 	foreach(l, onconflict->arbiterElems)
 	{
-		InferenceElem *elem;
+		InferenceElem *elem = (InferenceElem *) lfirst(l);
 		Var		   *var;
 		int			attno;
 
-		elem = (InferenceElem *) lfirst(l);
-
-		/*
-		 * Parse analysis of inference elements performs full parse analysis
-		 * of Vars, even for non-expression indexes (in contrast with utility
-		 * command related use of IndexElem).  However, indexes are cataloged
-		 * with simple attribute numbers for non-expression indexes.  Those
-		 * are handled later.
-		 */
 		if (!IsA(elem->expr, Var))
 		{
+			/* If not a plain Var, just shove it in inferElems for now */
 			inferElems = lappend(inferElems, elem->expr);
 			continue;
 		}
@@ -492,14 +484,13 @@ infer_arbiter_indexes(PlannerInfo *root)
 		var = (Var *) elem->expr;
 		attno = var->varattno;
 
-		if (attno < 0)
+		if (attno == 0)
 			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-					 errmsg("system columns cannot be used in an ON CONFLICT clause")));
-		else if (attno == 0)
-			elog(ERROR, "whole row unique index inference specifications are not valid");
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("whole row unique index inference specifications are not supported")));
 
-		inferAttrs = bms_add_member(inferAttrs, attno);
+		inferAttrs = bms_add_member(inferAttrs,
+								 attno - FirstLowInvalidHeapAttributeNumber);
 	}
 
 	/*
@@ -516,18 +507,18 @@ infer_arbiter_indexes(PlannerInfo *root)
 					 errmsg("constraint in ON CONFLICT clause has no associated index")));
 	}
 
-	indexList = RelationGetIndexList(relation);
-
 	/*
 	 * Using that representation, iterate through the list of indexes on the
 	 * target relation to try and find a match
 	 */
+	indexList = RelationGetIndexList(relation);
+
 	foreach(l, indexList)
 	{
 		Oid			indexoid = lfirst_oid(l);
 		Relation	idxRel;
 		Form_pg_index idxForm;
-		Bitmapset  *indexedAttrs = NULL;
+		Bitmapset  *indexedAttrs;
 		List	   *idxExprs;
 		List	   *predExprs;
 		AttrNumber	natt;
@@ -586,17 +577,15 @@ infer_arbiter_indexes(PlannerInfo *root)
 		if (!idxForm->indisunique)
 			goto next;
 
-		/* Build BMS representation of cataloged index attributes */
+		/* Build BMS representation of plain (non expression) index attrs */
+		indexedAttrs = NULL;
 		for (natt = 0; natt < idxForm->indnatts; natt++)
 		{
 			int			attno = idxRel->rd_index->indkey.values[natt];
 
-			/* XXX broken */
-			if (attno < 0)
-				elog(ERROR, "system column in index");
-
 			if (attno != 0)
-				indexedAttrs = bms_add_member(indexedAttrs, attno);
+				indexedAttrs = bms_add_member(indexedAttrs,
+								 attno - FirstLowInvalidHeapAttributeNumber);
 		}
 
 		/* Non-expression attributes (if any) must match */