Add select_common_typmod()

This accompanies select_common_type() and select_common_collation().
Typmods were previously combined using hand-coded logic in several
places.  The logic in select_common_typmod() isn't very exciting, but
it makes the code more compact and readable in a few locations, and in
the future we can perhaps do more complicated things if desired.

As a small enhancement, the type unification of the direct and
aggregate arguments of hypothetical-set aggregates now unifies the
typmod as well using this new function, instead of just dropping it.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/97df3af9-8b5e-fb7f-a029-3eb7e80d7af9@2ndquadrant.com

1 files changed, 37 insertions, 0 deletions

diff --git a/src/backend/parser/parse_coerce.c b/src/backend/parser/parse_coerce.c
index 2ffe47026b9..a2924e3d1ce 100644
--- a/src/backend/parser/parse_coerce.c
+++ b/src/backend/parser/parse_coerce.c
@@ -1523,6 +1523,43 @@ coerce_to_common_type(ParseState *pstate, Node *node,
 }
 
 /*
+ * select_common_typmod()
+ *		Determine the common typmod of a list of input expressions.
+ *
+ * common_type is the selected common type of the expressions, typically
+ * computed using select_common_type().
+ */
+int32
+select_common_typmod(ParseState *pstate, List *exprs, Oid common_type)
+{
+	ListCell   *lc;
+	bool		first = true;
+	int32		result = -1;
+
+	foreach(lc, exprs)
+	{
+		Node   *expr = (Node *) lfirst(lc);
+
+		/* Types must match */
+		if (exprType(expr) != common_type)
+			return -1;
+		else if (first)
+		{
+			result = exprTypmod(expr);
+			first = false;
+		}
+		else
+		{
+			/* As soon as we see a non-matching typmod, fall back to -1 */
+			if (result != exprTypmod(expr))
+				return -1;
+		}
+	}
+
+	return result;
+}
+
+/*
  * check_generic_type_consistency()
  *		Are the actual arguments potentially compatible with a
  *		polymorphic function?