[OmniSci/MapD] Add support for arbitrary aggregate

This fixes #1680 by adding support for the `arbitrary` aggregate in
MapD by mapping it to the `SAMPLE` function.
Author: Saul Shanabrook <s.shanabrook@gmail.com>

Closes #1683 from saulshanabrook/omnisci-sample and squashes the following commits:

f8d9ee1 [Saul Shanabrook] Add support for arbitrary aggregate in OmniSci/MapD (fixes #1680)

ibis/bigquery/compiler.py

@@ -277,7 +277,7 @@ def _arbitrary(translator, expr):
     if where is not None:
         arg = where.ifelse(arg, ibis.NA)
 
-    if how != 'first':
+    if how not in (None, 'first'):
         raise com.UnsupportedOperationError(
             '{!r} value not supported for arbitrary in BigQuery'.format(how)
         )

ibis/clickhouse/operations.py

@@ -129,7 +129,9 @@ def varargs_formatter(translator, expr):
 
 def _arbitrary(translator, expr):
     arg, how, where = expr.op().args
-    functions = {'first': 'any',
+    functions = {
+                 None: 'any',
+                 'first': 'any',
                  'last': 'anyLast',
                  'heavy': 'anyHeavy'}
     return _aggregate(translator, functions[how], arg, where=where)

ibis/expr/api.py

@@ -481,7 +481,7 @@ def group_concat(arg, sep=',', where=None):
     return ops.GroupConcat(arg, sep, where).to_expr()
 
 
-def arbitrary(arg, where=None, how='first'):
+def arbitrary(arg, where=None, how=None):
     """
     Selects the first / last non-null value in a column
 

ibis/expr/operations.py

@@ -835,7 +835,7 @@ def output_type(self):
 
 class Arbitrary(Reduction):
     arg = Arg(rlz.column(rlz.any))
-    how = Arg(rlz.isin({'first', 'last', 'heavy'}), default='first')
+    how = Arg(rlz.isin({'first', 'last', 'heavy'}), default=None)
     where = Arg(rlz.boolean, default=None)
     output_type = rlz.scalar_like('arg')
 

ibis/mapd/operations.py

@@ -573,6 +573,20 @@ def _table_column(translator, expr):
 count = _reduction('count')
 
 
+def _arbitrary(translator, expr):
+    arg, how, where = expr.op().args
+
+    if how not in (None, 'last'):
+        raise com.UnsupportedOperationError(
+            '{!r} value not supported for arbitrary in MapD'.format(how)
+        )
+
+    if where is not None:
+        arg = where.ifelse(arg, ibis.NA)
+
+    return 'SAMPLE({})'.format(translator.translate(arg))
+
+
 # MATH
 
 class NumericTruncate(ops.NumericBinaryOp):
@@ -707,6 +721,7 @@ class ByteLength(ops.StringLength):
 _agg_ops = {
     ops.HLLCardinality: approx_count_distinct,
     ops.DistinctColumn: unary_prefix_op('distinct'),
+    ops.Arbitrary: _arbitrary
 }
 
 # GENERAL
@@ -734,7 +749,6 @@ class ByteLength(ops.StringLength):
     ops.CumulativeAny,
     ops.CumulativeAll,
     ops.IdenticalTo,
-    ops.Arbitrary,
     ops.RowNumber,
     ops.DenseRank,
     ops.MinRank,
@@ -746,7 +760,6 @@ class ByteLength(ops.StringLength):
     ops.Lead,
     ops.NTile,
     ops.GroupConcat,
-    ops.Arbitrary,
     ops.NullIf,
     ops.NullIfZero,
     ops.NullLiteral,

ibis/mapd/tests/conftest.py

@@ -37,6 +37,11 @@ def batting(con):
     return con.table('batting')
 
 
+@pytest.fixture(scope='module')
+def df_alltypes(alltypes):
+    return alltypes.execute()
+
+
 @pytest.fixture
 def translate():
     """

ibis/mapd/tests/test_operations.py

@@ -134,3 +134,17 @@ def test_literal_geospatial():
         "SELECT 'MULTIPOLYGON(((0 0, 0 0), (0 0, 0 0)), "
         "((0 0, 0 0), (0 0, 0 0)))' AS tmp"
     )
+
+
+@pytest.mark.parametrize(('result_fn', 'expected_fn'), [
+    param(
+        lambda t: t.double_col.arbitrary(),
+        lambda t: t.double_col.iloc[-1],
+        id='double_col_arbitrary_none'
+    ),
+])
+def test_arbitrary_none(alltypes, df_alltypes, result_fn, expected_fn):
+    expr = result_fn(alltypes)
+    result = expr.execute()
+    expected = expected_fn(df_alltypes)
+    np.testing.assert_allclose(result, expected)

ibis/pandas/execution/generic.py

@@ -461,10 +461,14 @@ def execute_count_distinct_series_groupby(
 
 @execute_node.register(ops.Arbitrary, SeriesGroupBy, type(None))
 def execute_arbitrary_series_groupby(op, data, _, aggcontext=None, **kwargs):
-    if op.how not in {'first', 'last'}:
+    how = op.how
+    if how is None:
+        how = 'first'
+
+    if how not in {'first', 'last'}:
         raise com.OperationNotDefinedError(
-            'Arbitrary {!r} is not supported'.format(op.how))
-    return aggcontext.agg(data, op.how)
+            'Arbitrary {!r} is not supported'.format(how))
+    return aggcontext.agg(data, how)
 
 
 def _filtered_reduction(mask, method, data):