Apply time window to memtables after compaction

[evenyag]

What type of enhancement is this?

Tech debt reduction

What does the enhancement do?

If users don't specify time window explicitly, the compactor will infer a time window and apply it to the version. But the region can only set that window to the memtable after reopening the region.

greptimedb/src/mito2/src/region/opener.rs

Lines 211 to 218 in 7d8b256

	let part_duration = options.compaction.time_window();
	// Initial memtable id is 0.
	let mutable = Arc::new(TimePartitions::new(
	metadata.clone(),
	memtable_builder.clone(),
	0,
	part_duration,
	));

We should set the part duration to TimePartitions according to the time window.

Implementation challenges

Setting the part duration for TimePartitions has several challenges:

• The TimePartitions struct doesn't support updating the part_duration now.
• If a TimePartitions struct holds partitions with the old time window, then we should not override its part duration.
• If we alter compaction time window, should we also alter the part duration?

Instead of altering the part duration, maybe we can create a new TimePartitions with the new time window. It's possible to set this in MemtableVersion::freeze_mutable.

greptimedb/src/mito2/src/memtable/version.rs

Lines 73 to 82 in 7d8b256

	if self.mutable.is_empty() {
	// No need to freeze the mutable memtable.
	return Ok(None);
	}
	// Marks the mutable memtable as immutable so it can free the memory usage from our
	// soft limit.
	self.mutable.freeze()?;
	// Fork the memtable.
	let mutable = Arc::new(self.mutable.fork(metadata));