perf(memtables): avoid converting to pyarrow for all memtables

ibis/backends/duckdb/__init__.py

@@ -1578,7 +1578,7 @@ def _register_in_memory_table(self, op: ops.InMemoryTable) -> None:
             self.con.table(name)
         except (duckdb.CatalogException, duckdb.InvalidInputException):
             # only register if we haven't already done so
-            self.con.register(name, op.data.to_pyarrow(op.schema))
+            self.con.register(name, op.data.to_native())
 
     def _register_udfs(self, expr: ir.Expr) -> None:
         con = self.con

ibis/formats/__init__.py

@@ -254,6 +254,14 @@ def to_pyarrow(self, schema: Schema) -> pa.Table:  # pragma: no cover
     def to_polars(self, schema: Schema) -> pl.DataFrame:  # pragma: no cover
         """Convert this input to a Polars DataFrame."""
 
+    def to_native(self) -> T:
+        """Convert to underlying native format.
+
+        Useful when the backend can handle the in-memory data structure
+        directly.
+        """
+        return self.obj
+
     def to_pyarrow_bytes(self, schema: Schema) -> bytes:
         import pyarrow as pa
         import pyarrow_hotfix  # noqa: F401