feat: allow empty structs

working towards ibis-project#8289

I'm not sure how useful empty structs are, since it seems like
only bigquery, dask, and pandas actually support them.
But still, if you stay in ibis-land, perhaps it is useful.
ie for doing type manipulations, or maybe you
only use them for intermediate calculations?
Not that hard for us to support it, so why not.

I'm not sure of the history of the specific disallowment
that I am removing from the type inference.

Relevant context:

- ibis-project#8876
- https://github.com/ibis-project/ibis/issues?q=empty+struct

ibis/backends/tests/errors.py

@@ -110,13 +110,14 @@
     from psycopg2.errors import OperationalError as PsycoPg2OperationalError
     from psycopg2.errors import ProgrammingError as PsycoPg2ProgrammingError
     from psycopg2.errors import SyntaxError as PsycoPg2SyntaxError
+    from psycopg2.errors import UndefinedFunction as PsycoPg2UndefinedFunction
     from psycopg2.errors import UndefinedObject as PsycoPg2UndefinedObject
 except ImportError:
     PsycoPg2SyntaxError = PsycoPg2IndeterminateDatatype = (
         PsycoPg2InvalidTextRepresentation
     ) = PsycoPg2DivisionByZero = PsycoPg2InternalError = PsycoPg2ProgrammingError = (
         PsycoPg2OperationalError
-    ) = PsycoPg2UndefinedObject = None
+    ) = PsycoPg2UndefinedFunction = PsycoPg2UndefinedObject = None
 
 try:
     from pymysql.err import NotSupportedError as MySQLNotSupportedError

ibis/backends/tests/test_struct.py

@@ -14,11 +14,16 @@
 import ibis.expr.datatypes as dt
 from ibis import util
 from ibis.backends.tests.errors import (
+    ClickHouseDatabaseError,
+    DuckDBParserException,
     PolarsColumnNotFoundError,
+    PolarsComputeError,
     PsycoPg2InternalError,
     PsycoPg2SyntaxError,
+    PsycoPg2UndefinedFunction,
     Py4JJavaError,
     PySparkAnalysisException,
+    TrinoUserError,
 )
 from ibis.common.exceptions import IbisError
 
@@ -29,6 +34,20 @@
 ]
 
 
+@pytest.mark.notimpl("clickhouse", raises=ClickHouseDatabaseError)
+@pytest.mark.notimpl("duckdb", raises=DuckDBParserException)
+@pytest.mark.notimpl("flink", raises=Py4JJavaError)
+@pytest.mark.notimpl("polars", raises=PolarsComputeError)
+@pytest.mark.notimpl("postgres", raises=PsycoPg2UndefinedFunction)
+@pytest.mark.notimpl("pyspark", raises=Py4JJavaError)
+@pytest.mark.notimpl("risingwave", raises=PsycoPg2InternalError)
+@pytest.mark.notimpl("trino", raises=TrinoUserError)
+def test_struct_factory_empty(con):
+    s = ibis.struct({})
+    result = con.execute(s)
+    assert result == {}
+
+
 @pytest.mark.notimpl(["dask"])
 @pytest.mark.parametrize(
     ("field", "expected"),

ibis/expr/datatypes/value.py

@@ -42,8 +42,6 @@ def infer(value: Any) -> dt.DataType:
 @infer.register(collections.OrderedDict)
 def infer_struct(value: Mapping[str, Any]) -> dt.Struct:
     """Infer the [`Struct`](./datatypes.qmd#ibis.expr.datatypes.Struct) type of `value`."""
-    if not value:
-        raise TypeError("Empty struct type not supported")
     fields = {name: infer(val) for name, val in value.items()}
     return dt.Struct(fields)
 

ibis/expr/operations/structs.py

@@ -4,6 +4,7 @@
 
 from public import public
 
+import ibis.expr.datashape as ds
 import ibis.expr.datatypes as dt
 import ibis.expr.rules as rlz
 from ibis.common.annotations import ValidationError, attribute
@@ -38,8 +39,6 @@ class StructColumn(Value):
     names: VarTuple[str]
     values: VarTuple[Value]
 
-    shape = rlz.shape_like("values")
-
     def __init__(self, names, values):
         if len(names) != len(values):
             raise ValidationError(
@@ -52,3 +51,9 @@ def __init__(self, names, values):
     def dtype(self) -> dt.DataType:
         dtypes = (value.dtype for value in self.values)
         return dt.Struct.from_tuples(zip(self.names, dtypes))
+
+    @attribute
+    def shape(self) -> ds.DataShape:
+        if len(self.values) == 0:
+            return ds.scalar
+        return rlz.highest_precedence_shape(self.values)

ibis/tests/expr/test_struct.py

@@ -5,9 +5,12 @@
 import pytest
 
 import ibis
+import ibis.expr.datashape as ds
+import ibis.expr.datatypes as dt
 import ibis.expr.operations as ops
 import ibis.expr.types as ir
 from ibis import _
+from ibis.common.annotations import ValidationError
 from ibis.expr.tests.test_newrels import join_tables
 from ibis.tests.util import assert_pickle_roundtrip
 
@@ -22,6 +25,17 @@ def s():
     return ibis.table(dict(a="struct<f: float, g: string>"), name="s")
 
 
+@pytest.mark.parametrize("val", [{}, []])
+@pytest.mark.parametrize("typ", [None, "struct<>", dt.Struct.from_tuples([])])
+def test_struct_factory_empty(val, typ):
+    with pytest.raises(ValidationError):
+        ibis.struct(val, type="struct<a: float64, b: float64>")
+    s = ibis.struct(val, type=typ)
+    assert s.names == tuple()
+    assert s.type() == dt.Struct.from_tuples([])
+    assert s.op().shape == ds.scalar
+
+
 def test_struct_operations():
     value = OrderedDict(
         [