In UPDATE deparse, check for a subscript before processing the targets. (#8155)

DESCRIPTION: Checking first for the presence of subscript ops avoids a
shallow copy of the target list for target lists where there are no
array or json subscripts.

Commit 0c1b31c fixed a bug in UPDATE statements with array or json
subscripting in the target list. This commit modifies that to first
check that the target list has a subscript and avoid a shallow copy of
the target list for UPDATE statements with no array/json subscripting.

src/backend/distributed/deparser/citus_ruleutils.c

@@ -1837,6 +1837,62 @@ ensure_update_targetlist_in_param_order(List *targetList)
 }
 
 
+/*
+ * isSubsRef checks if a given node is a SubscriptingRef or can be
+ * reached through an implicit coercion.
+ */
+static
+bool
+isSubsRef(Node *node)
+{
+	if (node == NULL)
+	{
+		return false;
+	}
+
+	if (IsA(node, CoerceToDomain))
+	{
+		CoerceToDomain *coerceToDomain = (CoerceToDomain *) node;
+		if (coerceToDomain->coercionformat != COERCE_IMPLICIT_CAST)
+		{
+			/* not an implicit coercion, cannot reach to a SubscriptingRef */
+			return false;
+		}
+
+		node = (Node *) coerceToDomain->arg;
+	}
+
+	return (IsA(node, SubscriptingRef));
+}
+
+
+/*
+ * checkTlistForSubsRef - checks if any target entry in the list contains a
+ * SubscriptingRef or can be reached through an implicit coercion. Used by
+ * ExpandMergedSubscriptingRefEntries() to identify if any target entries
+ * need to be expanded - if not the original target list is preserved.
+ */
+static
+bool
+checkTlistForSubsRef(List *targetEntryList)
+{
+	ListCell *tgtCell = NULL;
+
+	foreach(tgtCell, targetEntryList)
+	{
+		TargetEntry *targetEntry = (TargetEntry *) lfirst(tgtCell);
+		Expr *expr = targetEntry->expr;
+
+		if (isSubsRef((Node *) expr))
+		{
+			return true;
+		}
+	}
+
+	return false;
+}
+
+
 /*
  * ExpandMergedSubscriptingRefEntries takes a list of target entries and expands
  * each one that references a SubscriptingRef node that indicates multiple (field)
@@ -1848,6 +1904,12 @@ ExpandMergedSubscriptingRefEntries(List *targetEntryList)
 	List *newTargetEntryList = NIL;
 	ListCell *tgtCell = NULL;
 
+	if (!checkTlistForSubsRef(targetEntryList))
+	{
+		/* No subscripting refs found, return original list */
+		return targetEntryList;
+	}
+
 	foreach(tgtCell, targetEntryList)
 	{
 		TargetEntry *targetEntry = (TargetEntry *) lfirst(tgtCell);