**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.
{
            if (!out_disable_reason->empty())
            {
                *out_disable_reason += ". ";
            }
            *out_disable_reason += "Cannot select parts for optimization";
        }
        return {};
    }
    
    if (isTTLMergeType(future_part->merge_type))
        getContext()->getMergeList().bookMergeWithTTL();
    merging_tagger = std::make_unique<CurrentlyMergingPartsTagger>(future_part, MergeTreeDataMergerMutator::estimateNeededDiskSpace(future_part->parts), *this, metadata_snapshot, false);
    return std::make_shared<MergeMutateSelectedEntry>(future_part, std::move(merging_tagger), MutationCommands::create());
}
bool StorageMergeTree::merge(
    bool aggressive,
    const String & partition_id,
    bool final,
    bool deduplicate,
    const Names & deduplicate_by_columns,
    String * out_disable_reason,
    bool optimize_skip_merged_partitions)
{
    auto table_lock_holder = lockForShare(RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
    auto metadata_snapshot = getInMemoryMetadataPtr();
    SelectPartsDecision select_decision;
    std::shared_ptr<MergeMutateSelectedEntry> merge_mutate_entry;
    {
        std::unique_lock lock(currently_processing_in_background_mutex);