Handle correct nullability with low card in HashJoin

src/Columns/ColumnLowCardinality.h

@@ -188,6 +188,7 @@ public:
      */
     bool nestedIsNullable() const { return isColumnNullable(*dictionary.getColumnUnique().getNestedColumn()); }
     void nestedToNullable() { dictionary.getColumnUnique().nestedToNullable(); }
+    void nestedRemoveNullable() { dictionary.getColumnUnique().nestedRemoveNullable(); }
 
     const IColumnUnique & getDictionary() const { return dictionary.getColumnUnique(); }
     const ColumnPtr & getDictionaryPtr() const { return dictionary.getColumnUniquePtr(); }

src/Columns/ColumnUnique.h

@@ -51,6 +51,7 @@ public:
     const ColumnPtr & getNestedNotNullableColumn() const override { return column_holder; }
     bool nestedColumnIsNullable() const override { return is_nullable; }
     void nestedToNullable() override;
+    void nestedRemoveNullable() override;
 
     size_t uniqueInsert(const Field & x) override;
     size_t uniqueInsertFrom(const IColumn & src, size_t n) override;
@@ -271,6 +272,14 @@ void ColumnUnique<ColumnType>::nestedToNullable()
     createNullMask();
 }
 
+template <typename ColumnType>
+void ColumnUnique<ColumnType>::nestedRemoveNullable()
+{
+    is_nullable = false;
+    nested_null_mask = nullptr;
+    nested_column_nullable = nullptr;
+}
+
 template <typename ColumnType>
 const ColumnPtr & ColumnUnique<ColumnType>::getNestedColumn() const
 {

src/Columns/IColumnUnique.h

@@ -24,6 +24,7 @@ public:
 
     virtual bool nestedColumnIsNullable() const = 0;
     virtual void nestedToNullable() = 0;
+    virtual void nestedRemoveNullable() = 0;
 
     /// Returns array with StringRefHash calculated for each row of getNestedNotNullableColumn() column.
     /// Returns nullptr if nested column doesn't contain strings. Otherwise calculates hash (if it wasn't).

src/Interpreters/HashJoin.cpp

@@ -160,7 +160,7 @@ static ColumnWithTypeAndName correctNullability(ColumnWithTypeAndName && column,
 {
     if (nullable)
     {
-        JoinCommon::convertColumnToNullable(column, false);
+        JoinCommon::convertColumnToNullable(column);
         if (column.type->isNullable() && !negative_null_map.empty())
         {
             MutableColumnPtr mutable_column = IColumn::mutate(std::move(column.column));
@@ -1085,7 +1085,8 @@ void HashJoin::joinBlockImpl(
             ColumnWithTypeAndName right_col(col.column, col.type, right_key.name);
             if (right_col.type->lowCardinality() != right_key.type->lowCardinality())
                 JoinCommon::changeLowCardinalityInplace(right_col);
-            block.insert(correctNullability(std::move(right_col), is_nullable));
+            right_col = correctNullability(std::move(right_col), is_nullable);
+            block.insert(right_col);
         }
     }
     else if (has_required_right_keys)
@@ -1114,7 +1115,8 @@ void HashJoin::joinBlockImpl(
             ColumnWithTypeAndName right_col(thin_column, col.type, right_key.name);
             if (right_col.type->lowCardinality() != right_key.type->lowCardinality())
                 JoinCommon::changeLowCardinalityInplace(right_col);
-            block.insert(correctNullability(std::move(right_col), is_nullable, null_map_filter));
+            right_col = correctNullability(std::move(right_col), is_nullable, null_map_filter);
+            block.insert(right_col);
 
             if constexpr (need_replication)
                 right_keys_to_replicate.push_back(block.getPositionByName(right_key.name));

src/Interpreters/join_common.cpp

@@ -122,6 +122,20 @@ void convertColumnsToNullable(Block & block, size_t starting_pos)
 /// @warning It assumes that every NULL has default value in nested column (or it does not matter)
 void removeColumnNullability(ColumnWithTypeAndName & column)
 {
+    if (column.type->lowCardinality())
+    {
+        /// LowCardinality(Nullable(T)) case
+        ColumnLowCardinality * col_as_lc = assert_cast<ColumnLowCardinality *>(column.column->assumeMutable().get());
+        if (col_as_lc->nestedIsNullable())
+        {
+            col_as_lc->nestedRemoveNullable();
+            const auto & dict_type = typeid_cast<const DataTypeLowCardinality *>(column.type.get())->getDictionaryType();
+            column.type = std::make_shared<DataTypeLowCardinality>(removeNullable(dict_type));
+        }
+
+        return;
+    }
+
     if (!column.type->isNullable())
         return;