fix: handle aggregate operations on empty selections (#2510)

SQL generator output fallbacks (SELECT 1 placeholder).
Fixes #<452681068> 🦕

Author: shuoweil

tests/system/small/test_dataframe.py

@@ -6296,3 +6296,20 @@ def test_agg_with_dict_containing_non_existing_col_raise_key_error(scalars_dfs):
 
 withpytest.raises(KeyError):
 bf_df.agg(agg_funcs)
+
+
+deftest_empty_agg_projection_succeeds():
+# Tests that the compiler generates a SELECT 1 fallback for empty aggregations,
+# protecting against BigQuery syntax errors when both groups and metrics are empty.
+importimportlib
+
+bq=importlib.import_module(
+"bigframes_vendored.ibis.backends.sql.compilers.bigquery"
+ )
+sg=importlib.import_module("bigframes_vendored.sqlglot")
+
+compiler=bq.BigQueryCompiler()
+res=compiler.visit_Aggregate(
+"op", parent=sg.table("parent_table"), groups=[], metrics=[]
+ )
+assert"SELECT 1"inres.sql()

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

@@ -1394,9 +1394,17 @@ def _generate_groups(groups):
 returnmap(sge.convert, range(1, len(groups) +1))
 
 defvisit_Aggregate(self, op, *, parent, groups, metrics):
-sel=sg.select(
-*self._cleanup_names(groups), *self._cleanup_names(metrics), copy=False
- ).from_(parent, copy=False)
+exprs= []
+ifgroups:
+exprs.extend(self._cleanup_names(groups))
+ifmetrics:
+exprs.extend(self._cleanup_names(metrics))
+
+ifnotexprs:
+# Empty aggregated projections are invalid in BigQuery
+exprs= [sge.Literal.number(1)]
+
+sel=sg.select(*exprs, copy=False).from_(parent, copy=False)
 
 ifgroups:
 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):
 
 defvisit_Cast(self, op, *, arg, to):
 from_=op.arg.dtype
+ifto.is_null():
+returnsge.Null()
+ifargisNULLor (
+isinstance(arg, sge.Cast)
+andgetattr(arg, "to", None) isnotNone
+andstr(arg.to).upper() =="NULL"
+ ):
+ifto.is_struct() orto.is_array():
+returnsge.Cast(this=NULL, to=self.type_mapper.from_ibis(to))
 iffrom_.is_timestamp() andto.is_integer():
 returnself.f.unix_micros(arg)
 eliffrom_.is_integer() andto.is_timestamp():