fix: handle aggregate operations on empty selections

tests/system/small/test_dataframe.py

@@ -6296,3 +6296,10 @@ def test_agg_with_dict_containing_non_existing_col_raise_key_error(scalars_dfs):
 
     with pytest.raises(KeyError):
         bf_df.agg(agg_funcs)
+
+
+def test_dataframe_count_empty_selection_succeeds(session):
+    # Tests that aggregate ops on empty selections don't trigger invalid empty SELECT syntax
+    empty_df = bpd.DataFrame()
+    count_series = empty_df.count().to_pandas()
+    assert len(count_series) == 0

third_party/bigframes_vendored/ibis/backends/sql/compilers/base.py

@@ -1394,9 +1394,17 @@ def _generate_groups(groups):
         return map(sge.convert, range(1, len(groups) + 1))
 
     def visit_Aggregate(self, op, *, parent, groups, metrics):
-        sel = sg.select(
-            *self._cleanup_names(groups), *self._cleanup_names(metrics), copy=False
-        ).from_(parent, copy=False)
+        exprs = []
+        if groups:
+            exprs.extend(self._cleanup_names(groups))
+        if metrics:
+            exprs.extend(self._cleanup_names(metrics))
+
+        if not exprs:
+            # Empty aggregated projections are invalid in BigQuery
+            exprs = [sge.Literal.number(1)]
+
+        sel = sg.select(*exprs, copy=False).from_(parent, copy=False)
 
         if groups:
             sel = sel.group_by(*self._generate_groups(groups.values()), copy=False)

third_party/bigframes_vendored/ibis/backends/sql/compilers/bigquery/__init__.py

@@ -540,6 +540,15 @@ def visit_TimestampFromUNIX(self, op, *, arg, unit):
 
     def visit_Cast(self, op, *, arg, to):
         from_ = op.arg.dtype
+        if to.is_null():
+            return sge.Null()
+        if arg is NULL or (
+            isinstance(arg, sge.Cast)
+            and getattr(arg, "to", None) is not None
+            and str(arg.to).upper() == "NULL"
+        ):
+            if to.is_struct() or to.is_array():
+                return sge.Cast(this=NULL, to=self.type_mapper.from_ibis(to))
         if from_.is_timestamp() and to.is_integer():
             return self.f.unix_micros(arg)
         elif from_.is_integer() and to.is_timestamp():