**How to reproduce**
Clickhouse version 21.2-21.3

```
SELECT x
FROM
(
    SELECT 1 AS x
    UNION DISTINCT
    SELECT 1
)
LIMIT 1

Query id: b82bbb8d-dea9-4f1c-9a2f-5b0e9b6c2422


0 rows in set. Elapsed: 0.002 sec.

Received exception from server (version 21.2.3):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: Bad cast from type DB::ASTSelectWithUnionQuery to DB::ASTSelectQuery.
```


settings.max_block_size,
            limit,
            settings.max_bytes_before_remerge_sort,
            settings.remerge_sort_lowered_memory_bytes_ratio,
            settings.max_bytes_before_external_sort,
            context->getTemporaryVolume(),
            settings.min_free_disk_space_for_temporary_data);
    merge_sorting_step->setStepDescription("Merge sorted blocks for ORDER BY");
    query_plan.addStep(std::move(merge_sorting_step));
    
    executeMergeSorted(query_plan, output_order_descr, limit, "for ORDER BY");
}
void InterpreterSelectQuery::executeMergeSorted(QueryPlan & query_plan, const std::string & description)
{
    auto & query = getSelectQuery();
    SortDescription order_descr = getSortDescription(query, *context);
    UInt64 limit = getLimitForSorting(query, *context);
    executeMergeSorted(query_plan, order_descr, limit, description);
}
void InterpreterSelectQuery::executeMergeSorted(QueryPlan & query_plan, const SortDescription & sort_description, UInt64 limit, const std::string & description)
{
    const Settings & settings = context->getSettingsRef();
    auto merging_sorted = std::make_unique<MergingSortedStep>(
            query_plan.getCurrentDataStream(),
            sort_description,
            settings.max_block_size, limit);
    merging_sorted->setStepDescription("Merge sorted streams " + description);
    query_plan.addStep(std::move(merging_sorted));
}
void InterpreterSelectQuery::executeProjection(QueryPlan & query_plan, const ActionsDAGPtr & expression)