[REVIEW] Fast path when possible for non numeric aggregation

dask_sql/physical/rel/logical/aggregate.py

@@ -7,6 +7,11 @@
 import dask.dataframe as dd
 import pandas as pd
 
+try:
+    import dask_cudf
+except ImportError:
+    dask_cudf = None
+
 from dask_sql.datacontainer import ColumnContainer, DataContainer
 from dask_sql.physical.rel.base import BaseRelPlugin
 from dask_sql.physical.rex.core.call import IsNullOperation
@@ -48,18 +53,42 @@ class AggregationSpecification:
     """
     Most of the aggregations in SQL are already
     implemented 1:1 in dask and can just be called via their name
-    (e.g. AVG is the mean). However sometimes those already
-    implemented functions only work well for numerical
-    functions. This small container class therefore
-    can have an additional aggregation function, which is
-    valid for non-numerical types.
+    (e.g. AVG is the mean). However sometimes those
+    implemented functions only work well for some datatypes.
+    This small container class therefore
+    can have an custom aggregation function, which is
+    valid for not supported dtypes.
     """
 
-    def __init__(self, numerical_aggregation, non_numerical_aggregation=None):
-        self.numerical_aggregation = numerical_aggregation
-        self.non_numerical_aggregation = (
-            non_numerical_aggregation or numerical_aggregation
-        )
+    def __init__(self, built_in_aggregation, custom_aggregation=None):
+        self.built_in_aggregation = built_in_aggregation
+        self.custom_aggregation = custom_aggregation or built_in_aggregation
+
+    def get_supported_aggregation(self, series):
+        built_in_aggregation = self.built_in_aggregation
+
+        # built-in aggregations work well for numeric types
+        if pd.api.types.is_numeric_dtype(series.dtype):
+            return built_in_aggregation
+
+        # Todo: Add Categorical when support comes to dask-sql
+        if built_in_aggregation in ["min", "max"]:
+            if pd.api.types.is_datetime64_any_dtype(series.dtype):
+                return built_in_aggregation
+
+            if pd.api.types.is_string_dtype(series.dtype):
+                # If dask_cudf strings dtype, return built-in aggregation
+                if dask_cudf is not None and isinstance(series, dask_cudf.Series):
+                    return built_in_aggregation
+
+                # With pandas StringDtype built-in aggregations work
+                # while with pandas ObjectDtype and Nulls built-in aggregations fail
+                if isinstance(series, dd.Series) and isinstance(
+                    series.dtype, pd.StringDtype
+                ):
+                    return built_in_aggregation
+
+        return self.custom_aggregation
 
 
 class LogicalAggregatePlugin(BaseRelPlugin):
@@ -303,13 +332,9 @@ def _collect_aggregations(
                         f"Aggregation function {aggregation_name} not implemented (yet)."
                     )
             if isinstance(aggregation_function, AggregationSpecification):
-                dtype = df[input_col].dtype
-                if pd.api.types.is_numeric_dtype(dtype):
-                    aggregation_function = aggregation_function.numerical_aggregation
-                else:
-                    aggregation_function = (
-                        aggregation_function.non_numerical_aggregation
-                    )
+                aggregation_function = aggregation_function.get_supported_aggregation(
+                    df[input_col]
+                )
 
             # Finally, extract the output column name
             output_col = str(agg_call.getValue())

tests/integration/test_compatibility.py

@@ -19,6 +19,14 @@
 from dask_sql import Context
 
 
+def cast_datetime_to_string(df):
+    cols = df.select_dtypes(include=["datetime64[ns]"]).columns
+    # Casting to object first as
+    # directly converting to string looses second precision
+    df[cols] = df[cols].astype("object").astype("string")
+    return df
+
+
 def eq_sqlite(sql, **dfs):
     c = Context()
     engine = sqlite3.connect(":memory:")
@@ -30,6 +38,10 @@ def eq_sqlite(sql, **dfs):
     dask_result = c.sql(sql).compute().reset_index(drop=True)
     sqlite_result = pd.read_sql(sql, engine).reset_index(drop=True)
 
+    # casting to object to ensure equality with sql-lite
+    # which returns object dtype for datetime inputs
+    dask_result = cast_datetime_to_string(dask_result)
+
     # Make sure SQL and Dask use the same "NULL" value
     dask_result = dask_result.fillna(np.NaN)
     sqlite_result = sqlite_result.fillna(np.NaN)
@@ -54,6 +66,11 @@ def make_rand_df(size: int, **kwargs):
             r = [f"ssssss{x}" for x in range(10)]
             c = np.random.randint(10, size=size)
             s = np.array([r[x] for x in c])
+        elif dt is pd.StringDtype:
+            r = [f"ssssss{x}" for x in range(10)]
+            c = np.random.randint(10, size=size)
+            s = np.array([r[x] for x in c])
+            s = pd.array(s, dtype="string")
         elif dt is datetime:
             rt = [datetime(2020, 1, 1) + timedelta(days=x) for x in range(10)]
             c = np.random.randint(10, size=size)
@@ -337,7 +354,14 @@ def test_agg_sum_avg():
 
 def test_agg_min_max_no_group_by():
     a = make_rand_df(
-        100, a=(int, 50), b=(str, 50), c=(int, 30), d=(str, 40), e=(float, 40)
+        100,
+        a=(int, 50),
+        b=(str, 50),
+        c=(int, 30),
+        d=(str, 40),
+        e=(float, 40),
+        f=(pd.StringDtype, 40),
+        g=(datetime, 40),
     )
     eq_sqlite(
         """
@@ -352,6 +376,10 @@ def test_agg_min_max_no_group_by():
             MAX(d) AS max_d,
             MIN(e) AS min_e,
             MAX(e) AS max_e,
+            MIN(f) as min_f,
+            MAX(f) as max_f,
+            MIN(g) as min_g,
+            MAX(g) as max_g,
             MIN(a+e) AS mix_1,
             MIN(a)+MIN(e) AS mix_2
         FROM a
@@ -362,7 +390,14 @@ def test_agg_min_max_no_group_by():
 
 def test_agg_min_max():
     a = make_rand_df(
-        100, a=(int, 50), b=(str, 50), c=(int, 30), d=(str, 40), e=(float, 40)
+        100,
+        a=(int, 50),
+        b=(str, 50),
+        c=(int, 30),
+        d=(str, 40),
+        e=(float, 40),
+        f=(pd.StringDtype, 40),
+        g=(datetime, 40),
     )
     eq_sqlite(
         """
@@ -374,6 +409,10 @@ def test_agg_min_max():
             MAX(d) AS max_d,
             MIN(e) AS min_e,
             MAX(e) AS max_e,
+            MIN(f) AS min_f,
+            MAX(f) AS max_f,
+            MIN(g) AS min_g,
+            MAX(g) AS max_g,
             MIN(a+e) AS mix_1,
             MIN(a)+MIN(e) AS mix_2
         FROM a GROUP BY a, b