Method executeForNullableThenElse

src/Functions/if.cpp:817–879

    }

    ColumnPtr executeForNullableThenElse(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t input_rows_count) const
    {
        const ColumnWithTypeAndName & arg_cond = arguments[0];
        const ColumnWithTypeAndName & arg_then = arguments[1];
        const ColumnWithTypeAndName & arg_else = arguments[2];

        const auto * then_is_nullable = checkAndGetColumn<ColumnNullable>(*arg_then.column);
        const auto * else_is_nullable = checkAndGetColumn<ColumnNullable>(*arg_else.column);

        if (!then_is_nullable && !else_is_nullable)
            return nullptr;

        /** Calculate null mask of result and nested column separately.
          */
        ColumnPtr result_null_mask;

        {
            ColumnsWithTypeAndName temporary_columns(
            {
                arg_cond,
                {
                    then_is_nullable
                        ? then_is_nullable->getNullMapColumnPtr()
                        : DataTypeUInt8().createColumnConstWithDefaultValue(input_rows_count),
                    std::make_shared<DataTypeUInt8>(),
                    ""
                },
                {
                    else_is_nullable
                        ? else_is_nullable->getNullMapColumnPtr()
                        : DataTypeUInt8().createColumnConstWithDefaultValue(input_rows_count),
                    std::make_shared<DataTypeUInt8>(),
                    ""
                }
            });

            result_null_mask = executeImpl(temporary_columns, std::make_shared<DataTypeUInt8>(), input_rows_count);
        }

        ColumnPtr result_nested_column;

        {
            ColumnsWithTypeAndName temporary_columns(
            {
                arg_cond,
                {
                    recursiveGetNestedColumnWithoutNullable(arg_then.column),
                    removeNullable(arg_then.type),
                    ""
                },
                {
                    recursiveGetNestedColumnWithoutNullable(arg_else.column),
                    removeNullable(arg_else.type),
                    ""
                }
            });

            result_nested_column = executeImpl(temporary_columns, removeNullable(result_type), temporary_columns.front().column->size());