diff --git a/meta/sql-columns.json b/meta/sql-columns.json
index 8f2afaa..4fc5c15 100644
--- a/meta/sql-columns.json
+++ b/meta/sql-columns.json
@@ -1,7 +1,17 @@
 {
   "$schema": "./sql-columns.schema.json",
   "description": "The columns of rows a SQL function returns whose value MEOS does not state. parser/sqlfn.py feeds every column of a returned row from the C value of its type: the value or array the function returns, or an out-parameter. A column its wrapper computes rather than reads is declared here, keyed by the composite type the function returns, or by its SQL name when it returns record; a row with a column neither fed nor declared stops the catalog.",
+  "composites": {
+    "geomval": {
+      "note": "PostGIS's composite of a geometry and a value, created by postgis_raster, which MobilityDB requires when built with RASTER; dumpAsPolygons returns a set of them.",
+      "columns": [["geom", "geometry"], ["val", "double precision"]]
+    }
+  },
   "rows": {
+    "geomval": {
+      "note": "A GeomVal holds a polygon and the value of the pixels it covers; the rows of geomval read its geom and its val, as PostGIS's ST_DumpAsPolygons does.",
+      "columns": {"geom": {"field": "geom"}, "val": {"field": "val"}}
+    },
     "index_tbox": {
       "note": "The tiles of a temporal box: index numbers them from 1 in the order the MEOS array returns them.",
       "columns": {"index": {"from": "ordinal"}}
diff --git a/meta/sql-columns.schema.json b/meta/sql-columns.schema.json
index f9a91d5..800afff 100644
--- a/meta/sql-columns.schema.json
+++ b/meta/sql-columns.schema.json
@@ -9,6 +9,33 @@
   "properties": {
     "$schema": { "type": "string" },
     "description": { "type": "string" },
+    "composites": {
+      "type": "object",
+      "description": "The composite types of another extension a SQL function returns, which the SQL the catalog reads does not create, keyed by type name.",
+      "additionalProperties": {
+        "type": "object",
+        "required": ["note", "columns"],
+        "additionalProperties": false,
+        "properties": {
+          "note": {
+            "type": "string",
+            "description": "The extension creating the type, and the functions returning it.",
+            "minLength": 1
+          },
+          "columns": {
+            "type": "array",
+            "description": "The (name, SQL type) of each member, in order.",
+            "minItems": 1,
+            "items": {
+              "type": "array",
+              "minItems": 2,
+              "maxItems": 2,
+              "items": { "type": "string", "minLength": 1 }
+            }
+          }
+        }
+      }
+    },
     "rows": {
       "type": "object",
       "description": "Keyed by the composite type a function returns, or by its SQL name when it returns record.",
diff --git a/parser/sqlfn.py b/parser/sqlfn.py
index 2c90830..6193d94 100644
--- a/parser/sqlfn.py
+++ b/parser/sqlfn.py
@@ -321,8 +321,9 @@ def sql_statements(sql_src):
     """(statements, type vocabulary, composite types) of the CREATE FUNCTION statements
     under `sql_src`, each statement as #_create_fn_stmts yields it with its body. The
     vocabulary holds every RETURNS type and every single-token argument type, so an
-    argument's type is read against it (#_arg_type)."""
-    stmts, vocab, composites = [], set(), {}
+    argument's type is read against it (#_arg_type). The composite types are those the
+    SQL creates and those of another extension a function returns (#declared_composites)."""
+    stmts, vocab, composites = [], set(), dict(declared_composites())
     sql_src = Path(sql_src)
     if not sql_src.exists():
         return stmts, vocab, composites
@@ -514,6 +515,17 @@ def declared_columns(path=_COLUMNS_META):
     return {key: entry["columns"] for key, entry in doc["rows"].items()}
 
 
+def declared_composites(path=_COLUMNS_META):
+    """The composite types of another extension a SQL function returns, stated in
+    `meta/sql-columns.json` beside the rows #declared_columns reads, since the SQL the
+    catalog reads creates none of them; {typeName: [(column, type), ...]} as
+    #_composite_types yields the ones it creates: PostGIS's `geomval`, the rows of
+    `dumpAsPolygons`."""
+    doc = json.loads(Path(path).read_text())
+    return {key: [tuple(c) for c in entry["columns"]]
+            for key, entry in (doc.get("composites") or {}).items()}
+
+
 def _c_base(ctype):
     """A C type without `const`, `struct` and its pointer levels, with the
     `<stdint.h>` spellings read as MEOS's own (`int64_t` is `int64`), and the number of
diff --git a/tests/test_sqlfn_rows.py b/tests/test_sqlfn_rows.py
index d68b619..55a9265 100644
--- a/tests/test_sqlfn_rows.py
+++ b/tests/test_sqlfn_rows.py
@@ -23,7 +23,7 @@
 from pathlib import Path
 
 from parser.sqlfn import (_wrapper_sql_sigs, attach_row_sources, attach_sqlfn_map,
-                          declared_columns)
+                          declared_columns, declared_composites)
 
 ROOT = Path(__file__).resolve().parents[1]
 COLUMNS = ROOT / "meta" / "sql-columns.json"
@@ -75,6 +75,11 @@
   RETURNS SETOF integer
   AS 'MODULE_PATHNAME', 'Set_unnest'
   LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
+CREATE FUNCTION dumpAsPolygons(rast raster, band integer DEFAULT 1,
+    exclude_nodata_value boolean DEFAULT true)
+  RETURNS SETOF geomval
+  AS 'MODULE_PATHNAME', 'Raster_dump_as_polygons'
+  LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
 """
 
 
@@ -105,6 +110,13 @@ def test_a_composite_return_type_gives_its_members(self):
         self.assertEqual(s["columns"], [("index", "integer"), ("tile", "tbox")])
         self.assertEqual(s["argDefaults"], [None, None, "'2000-01-03'"])
 
+    def test_a_composite_of_another_extension_gives_its_declared_members(self):
+        """As #test_a_composite_return_type_gives_its_members, for a type the SQL does
+        not create: geomval's members are those meta/sql-columns.json states for it."""
+        s = _sigs()["Raster_dump_as_polygons"][0]
+        self.assertEqual((s["ret"], s["retSet"]), ("geomval", True))
+        self.assertEqual(s["columns"], [("geom", "geometry"), ("val", "double precision")])
+
     def test_one_value_has_no_columns(self):
         self.assertIsNone(_sigs()["Set_unnest"][0]["columns"])
 
@@ -409,6 +421,19 @@ def test_the_declarations_are_read_by_row(self):
         self.assertEqual(declared["index_tbox"], {"index": {"from": "ordinal"}})
         self.assertEqual(declared["tDisjointPairs"], PAIRS_OFFSET)
 
+    def test_the_composites_are_read_by_type(self):
+        """As #test_the_declarations_are_read_by_row reads the rows."""
+        self.assertEqual(declared_composites(COLUMNS)["geomval"],
+                         [("geom", "geometry"), ("val", "double precision")])
+
+    def test_a_composite_member_without_a_type_is_refused(self):
+        """As #test_an_unknown_source_is_refused refuses a row's column."""
+        import jsonschema
+        doc = json.loads(COLUMNS.read_text())
+        doc["composites"]["geomval"]["columns"][0] = ["geom"]
+        with self.assertRaises(jsonschema.ValidationError):
+            jsonschema.validate(doc, json.loads(SCHEMA.read_text()))
+
 
 if __name__ == "__main__":
     unittest.main()