**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.
const auto & deps_mv = name_deps[command.column_name];
                if (!deps_mv.empty())
                {
                    throw Exception(
                        "Trying to ALTER DROP column " + backQuoteIfNeed(command.column_name) + " which is referenced by materialized view "
                            + toString(deps_mv),
                        ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
                }
            }
            dropped_columns.emplace(command.column_name);
        }
        else if (command.isRequireMutationStage(getInMemoryMetadata()))
        {
            
            
            if (columns_alter_type_forbidden.count(command.column_name))
                throw Exception("ALTER of key column " + backQuoteIfNeed(command.column_name) + " is forbidden",
                    ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
            if (command.type == AlterCommand::MODIFY_COLUMN)
            {
                if (columns_alter_type_check_safe_for_partition.count(command.column_name))
                {
                    auto it = old_types.find(command.column_name);
                    assert(it != old_types.end());
                    if (!isSafeForKeyConversion(it->second, command.data_type.get()))
                        throw Exception("ALTER of partition key column " + backQuoteIfNeed(command.column_name) + " from type "
                                + it->second->getName() + " to type " + command.data_type->getName()
                                + " is not safe because it can change the representation of partition key",
                            ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
                }