Query cost

.github/workflows/python-integration.yml

@@ -31,6 +31,12 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install -r requirements/test.txt
+    - name: Install multicorn2
+      run: |
+        git clone https://github.com/pgsql-io/multicorn2.git
+        cd multicorn2
+        git checkout v2.5
+        pip install .
     - name: Test with pytest
       env:
         SHILLELAGH_ADAPTER_KWARGS: ${{ secrets.SHILLELAGH_ADAPTER_KWARGS }}

src/shillelagh/backends/multicorn/fdw.py

@@ -129,6 +129,26 @@ def update(self, oldvalues: Row, newvalues: Row) -> Row:
     def rowid_column(self):
         return "rowid"
 
+    def get_rel_size(self, quals: List[Qual], columns: List[str]) -> Tuple[int, int]:
+        """
+        Estimate query cost.
+        """
+        all_bounds = get_all_bounds(quals)
+        filtered_columns = [
+            (column, operator[0])
+            for column, operators in all_bounds.items()
+            for operator in operators
+        ]
+
+        # the adapter returns an arbitrary cost that takes in consideration filtering and
+        # sorting; let's use that as an approximation for rows
+        rows = int(self.adapter.get_cost(filtered_columns, []))
+
+        # same assumption as the parent class
+        row_width = len(columns) * 100
+
+        return (rows, row_width)
+
     @classmethod
     def import_schema(  # pylint: disable=too-many-arguments
         cls,

tests/backends/multicorn/fdw_test.py

@@ -194,3 +194,19 @@ def test_update(mocker: MockerFixture, registry: AdapterLoader) -> None:
         {"rowid": 1, "name": "Bob", "age": 23, "pets": 3},
         {"rowid": 0, "name": "Alice", "age": 20, "pets": 1},
     ]
+
+
+def test_get_rel_Size(mocker: MockerFixture, registry: AdapterLoader) -> None:
+    """
+    Test the ``get_rel_size`` method.
+    """
+    mocker.patch("shillelagh.backends.multicorn.fdw.registry", registry)
+
+    registry.add("dummy", FakeAdapter)
+
+    wrapper = MulticornForeignDataWrapper(
+        {"adapter": "dummy", "args": "qQA="},
+        {},
+    )
+
+    assert wrapper.get_rel_size([Qual("age", ">", 21)], ["name", "age"]) == (666, 200)