Limit the row list allocation based on the row count

[franz1981]

Profiling https://github.com/topicusonderwijs/tribe-krd-quarkus startup it reports a huge amount of allocations due to this:

and zooming in (the orange ones are "out of TLAB allocations":

Configuration of H2 is mem and the ORM policy is drop and create, but still, with this PR it would save a huge amount of allocations (it's a quite large model, actually).

[franz1981]

I've forgot to mention @katzyn
re

Configuration of H2 is mem

In the reproducer linked, the H2 version is 1.4.197 that, despite being configured as mem isn't using the in-memory branch i.e. rebuildIndexBuffered, but rebuildIndexBlockMerge, see:

h2database/h2/src/main/org/h2/mvstore/db/MVTable.java

Lines 554 to 560 in 739c747

	if (session.getDatabase().getMvStore() == null ||
	index instanceof MVSpatialIndex) {
	// in-memory
	rebuildIndexBuffered(session, index);
	} else {
	rebuildIndexBlockMerge(session, index);
	}

It seems suspicious to me, but this patch still apply here, I think.
Let me know if I should fill a separate issue for it

[grandinj]

Thank you for your contribution!

Please send a licence statement as described here
https://h2database.com/html/build.html#providing_patches
to our mailing list / Google group:
https://groups.google.com/g/h2-database

[grandinj]

In the reproducer linked, the H2 version is 1.4.197 that, despite being configured as mem isn't using the in-memory branch i.e. rebuildIndexBuffered, but rebuildIndexBlockMerge, see:

h2database/h2/src/main/org/h2/mvstore/db/MVTable.java

Lines 554 to 560 in 739c747

	if (session.getDatabase().getMvStore() == null ||
	index instanceof MVSpatialIndex) {
	// in-memory
	rebuildIndexBuffered(session, index);
	} else {
	rebuildIndexBlockMerge(session, index);
	}

It seems suspicious to me, but this patch still apply here, I think. Let me know if I should fill a separate issue for it

Does using rebuildIndexBuffered for your example make a difference?

[franz1981]

thanks @grandinj to reach out! I'll send the licence statement ASAP!

re

Does using rebuildIndexBuffered for your example make a difference?

I'm sure it will because it contains

h2database/h2/src/main/org/h2/mvstore/db/MVTable.java

Line 459 in 65395d1

int bufferSize = (int) Math.min(total, database.getMaxMemoryRows());

that's the key optimization to save huge arraylists to be created when no rows are found

Sole problem is that even by setting a mem store 1.4.197 isn't not able to hit that code path (that looks like a different issue to me eh!)

[grandinj]

On current GIT master, that line of code reads

 if (!session.getDatabase().isPersistent() || index instanceof MVSpatialIndex) {

so it should work better for your case.

[franz1981]

@grandinj waiting subscription to the google group to be accepted and ready to send the first msg :)

[franz1981]

@grandinj email sent; given that this is not a huge patch I didn't add the full statement, let me know if I should, instead.

[grandinj]

Awesome, thanks!

[franz1981]

many thanks for the quick review!!! @grandinj