diff --git a/parser/sqlfn.py b/parser/sqlfn.py
index 5372606..4616bd2 100644
--- a/parser/sqlfn.py
+++ b/parser/sqlfn.py
@@ -63,6 +63,16 @@ def _split_top_commas(s):
 _ARGMODE = re.compile(r"^(?:IN|OUT|INOUT|VARIADIC)\s+", re.I)
 
 
+def _arg_default(decl):
+    """The literal default expression of an arg declaration, or None for a required arg.
+    Complement of `_bare_type` (same split, the other side): `integer DEFAULT 0` -> `0`,
+    `text DEFAULT NULL` -> `NULL`, `float` -> None. Kept verbatim from the SQL source
+    (no interpretation) so a consumer of an optional trailing arg has its omitted value."""
+    a = _ARGMODE.sub("", decl.strip())
+    parts = re.split(r"\bDEFAULT\b|=", a, maxsplit=1, flags=re.I)
+    return parts[1].strip() if len(parts) > 1 and parts[1].strip() else None
+
+
 def _bare_type(decl):
     """A CREATE FUNCTION arg declaration with its argmode and DEFAULT / `= expr` clause
     stripped — leaving `[argname] argtype`, argtype possibly multi-word (double precision)."""
@@ -140,9 +150,11 @@ def _wrapper_sql_sigs(sql_src):
         if wrapper is None:
             continue                                            # LANGUAGE SQL / $$ body — no C symbol
         args = [_arg_type(a, vocab) for a in argdecls]
+        arg_defaults = [_arg_default(a) for a in argdecls]
         required = sum(1 for a in argdecls if not re.search(r"\bDEFAULT\b", a, re.I))
         out.setdefault(wrapper, []).append(
-            {"sqlName": sqlname, "args": args, "required": required, "ret": ret})
+            {"sqlName": sqlname, "args": args, "required": required,
+             "argDefaults": arg_defaults, "ret": ret})
     return out
 
 
@@ -241,8 +253,18 @@ def attach_sqlfn_map(idl, meos_src, mdb_src, sql_src=None):
             # its {tint,tbigint,tfloat,ttext} overloads). One wrapper backs several @sqlfn
             # names (Temporal_to_tinstant <- tintInst/tgeometryInst/...), so keep only the
             # overloads whose CREATE FUNCTION name is this function's @sqlfn.
-            own = [{"args": s["args"], "ret": s["ret"]} for s in sigs
-                   if s["sqlName"] == f["sqlfn"]]
+            # `argDefaults` (the per-arg literal SQL default, None for a required arg) is
+            # attached ONLY for a signature that actually has an optional arg, so a binding
+            # can render the shorter overload of a SQL-optional argument with its omitted
+            # value; default-free signatures stay {args, ret} unchanged.
+            own = []
+            for s in sigs:
+                if s["sqlName"] != f["sqlfn"]:
+                    continue
+                entry = {"args": s["args"], "ret": s["ret"]}
+                if any(d is not None for d in s["argDefaults"]):
+                    entry["argDefaults"] = s["argDefaults"]
+                own.append(entry)
             if own:
                 f["sqlSignatures"] = own
         if pairs[0][1]:
diff --git a/tests/test_sql_defaults.py b/tests/test_sql_defaults.py
new file mode 100644
index 0000000..cf24b29
--- /dev/null
+++ b/tests/test_sql_defaults.py
@@ -0,0 +1,57 @@
+"""Regression tests for capturing per-arg SQL DEFAULT values in parser/sqlfn.py.
+
+An optional argument (`integer DEFAULT 0`, `text DEFAULT NULL`) is SQL-optional: a
+binding must render the shorter overload with the omitted value substituted. The parser
+records the literal default per arg as `argDefaults` (None for a required arg), attached
+to a signature only when it has an optional arg, so default-free signatures stay
+`{args, ret}` unchanged.
+
+Plain unittest, no pytest dependency; synthetic SQL via a temp dir.
+"""
+import tempfile
+import unittest
+from pathlib import Path
+
+from parser.sqlfn import _arg_default, _wrapper_sql_sigs
+
+
+class ArgDefaultTests(unittest.TestCase):
+    def test_arg_default_literal_kept_verbatim(self):
+        self.assertEqual(_arg_default("integer DEFAULT 0"), "0")
+        self.assertEqual(_arg_default("val integer DEFAULT 0"), "0")     # named arg
+        self.assertEqual(_arg_default("text DEFAULT NULL"), "NULL")
+        self.assertEqual(_arg_default("withbbox boolean DEFAULT FALSE"), "FALSE")
+        self.assertEqual(_arg_default("maxdd integer = 15"), "15")       # `=` form
+        self.assertEqual(_arg_default("VARIADIC opts text DEFAULT NULL"), "NULL")
+
+    def test_required_arg_has_no_default(self):
+        self.assertIsNone(_arg_default("floatset"))
+        self.assertIsNone(_arg_default("s floatset"))
+        self.assertIsNone(_arg_default("double precision"))
+
+    def test_wrapper_sql_sigs_records_arg_defaults(self):
+        # round(floatset, integer DEFAULT 0): the trailing precision is SQL-optional.
+        sql = ("CREATE FUNCTION round(floatset, integer DEFAULT 0)\n"
+               "  RETURNS floatset AS 'MODULE_PATHNAME', 'Set_round' LANGUAGE C;\n")
+        with tempfile.TemporaryDirectory() as d:
+            (Path(d) / "x.sql").write_text(sql)
+            sigs = _wrapper_sql_sigs(d)
+        self.assertIn("Set_round", sigs)
+        s = sigs["Set_round"][0]
+        self.assertEqual(s["args"], ["floatset", "integer"])
+        self.assertEqual(s["argDefaults"], [None, "0"])
+        self.assertEqual(s["required"], 1)
+
+    def test_all_required_args_have_all_none_defaults(self):
+        sql = ("CREATE FUNCTION set_union(floatset, floatset)\n"
+               "  RETURNS floatset AS 'MODULE_PATHNAME', 'Union_set_set' LANGUAGE C;\n")
+        with tempfile.TemporaryDirectory() as d:
+            (Path(d) / "x.sql").write_text(sql)
+            sigs = _wrapper_sql_sigs(d)
+        s = sigs["Union_set_set"][0]
+        self.assertEqual(s["argDefaults"], [None, None])
+        self.assertEqual(s["required"], 2)
+
+
+if __name__ == "__main__":
+    unittest.main()