**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.
{
    
    StorageInMemoryMetadata new_metadata = getInMemoryMetadata();
    StorageInMemoryMetadata old_metadata = getInMemoryMetadata();
    const auto & settings = local_context->getSettingsRef();
    if (!settings.allow_non_metadata_alters)
    {
        auto mutation_commands = commands.getMutationCommands(new_metadata, settings.materialize_ttl_after_modify, getContext());
        if (!mutation_commands.empty())
            throw Exception(ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN, "The following alter commands: '{}' will modify data on disk, but setting `allow_non_metadata_alters` is disabled", queryToString(mutation_commands.ast()));
    }
    commands.apply(new_metadata, getContext());
    
    NameSet columns_alter_type_forbidden;
    
    
    NameSet columns_alter_type_metadata_only;
    
    NameSet columns_alter_type_check_safe_for_partition;
    if (old_metadata.hasPartitionKey())
    {
        
        auto partition_key_expr = old_metadata.getPartitionKey().expression;
        for (const auto & action : partition_key_expr->getActions())
        {
            for (const auto * child : action.node->children)
                columns_alter_type_forbidden.insert(child->result_name);