**How to reproduce**
Clickhouse server 20.13.1.5273
```
CREATE TABLE default.test
(
`key` UInt32,
`ts` DateTime CODEC(DoubleDelta, LZ4),
`db_time` DateTime DEFAULT now() COMMENT 'spout-ignore' CODEC(DoubleDelta, LZ4)
)
ENGINE = MergeTree
PARTITION BY toStartOfTenMinutes(db_time)
ORDER BY (key, ts)
TTL db_time + toIntervalHour(3)
SETTINGS index_granularity = 8192, merge_with_ttl_timeout = 3600, min_rows_for_compact_part = 1000000, min_bytes_for_compact_part = 200000000, in_memory_parts_enable_wal = 0;


INSERT INTO test(key, ts) SELECT number % 1000, now() + intDiv(number,1000) FROM numbers(500);

SELECT * FROM test;
500 rows in set. Elapsed: 0.003 sec.

DETACH TABLE test;

ATTACH TABLE test;

SELECT * FROM test;

0 rows in set. Elapsed: 0.002 sec.
```

**Expected behavior**
Clickhouse would flush parts on disk.
{
    MergeTreeData * src_data = dynamic_cast<MergeTreeData *>(&source_table);
    if (!src_data)
        throw Exception("Table " + source_table.getStorageID().getNameForLogs() +
                        " supports attachPartitionFrom only for MergeTree family of table engines."
                        " Got " + source_table.getName(), ErrorCodes::NOT_IMPLEMENTED);
    if (my_snapshot->getColumns().getAllPhysical().sizeOfDifference(src_snapshot->getColumns().getAllPhysical()))
        throw Exception("Tables have different structure", ErrorCodes::INCOMPATIBLE_COLUMNS);
    auto query_to_string = [] (const ASTPtr & ast)
    {
        return ast ? queryToString(ast) : "";
    };
    if (query_to_string(my_snapshot->getSortingKeyAST()) != query_to_string(src_snapshot->getSortingKeyAST()))
        throw Exception("Tables have different ordering", ErrorCodes::BAD_ARGUMENTS);
    if (query_to_string(my_snapshot->getPartitionKeyAST()) != query_to_string(src_snapshot->getPartitionKeyAST()))
        throw Exception("Tables have different partition key", ErrorCodes::BAD_ARGUMENTS);
    if (format_version != src_data->format_version)
        throw Exception("Tables have different format_version", ErrorCodes::BAD_ARGUMENTS);
    return *src_data;
}
MergeTreeData & MergeTreeData::checkStructureAndGetMergeTreeData(