Implement arraySlice for arrays with aggregate function states.

dbms/src/Columns/ColumnAggregateFunction.cpp

@@ -157,6 +157,21 @@ void ColumnAggregateFunction::ensureOwnership()
 }
 
 
+bool ColumnAggregateFunction::structureEquals(const IColumn & to) const
+{
+    const auto * to_concrete = typeid_cast<const ColumnAggregateFunction *>(&to);
+    if (!to_concrete)
+        return false;
+
+    /// AggregateFunctions must be the same.
+
+    const IAggregateFunction & func_this = *func;
+    const IAggregateFunction & func_to = *to_concrete->func;
+
+    return typeid(func_this) == typeid(func_to);
+}
+
+
 void ColumnAggregateFunction::insertRangeFrom(const IColumn & from, size_t start, size_t length)
 {
     const ColumnAggregateFunction & from_concrete = assert_cast<const ColumnAggregateFunction &>(from);

dbms/src/Columns/ColumnAggregateFunction.h

@@ -204,6 +204,8 @@ class ColumnAggregateFunction final : public COWHelper<IColumn, ColumnAggregateF
     }
 
     void getExtremes(Field & min, Field & max) const override;
+
+    bool structureEquals(const IColumn &) const override;
 };
 
 

dbms/tests/queries/0_stateless/01088_array_slice_of_aggregate_functions.reference

@@ -0,0 +1 @@
+['\0
\0\0\0\0']

dbms/tests/queries/0_stateless/01088_array_slice_of_aggregate_functions.sql

@@ -0,0 +1 @@
+select arraySlice(groupArray(x),1,1) as y from (select uniqState(number) as x from numbers(10) group by number);