Thanks for doing this! I have a few comments, nothing major I don't think.

I'm generally good on this. Just minor things. This is a classic example of something that'd be nice to exercise against real store dirs from the previous version.

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pkg/migration/migrations/public_schema_migration.go, line 72 at r8 (raw file):

	for _, dbID := range databaseIDs {
		fmt.Println("dbID:", dbID)

detritus

pkg/migration/migrations/public_schema_migration.go, line 86 at r8 (raw file):

	return d.CollectionFactory.Txn(ctx, d.InternalExecutor, d.DB, func(
		ctx context.Context, txn *kv.Txn, descriptors *descs.Collection,
	) error {

nit: buy yourself an extra layer of indentation by making the body of this a function all on its own that maybe takes just the codec, settings, txn, collection, dbID? Generally good practice to scope down your deps when it's easy.

pkg/migration/migrations/public_schema_migration.go, line 129 at r8 (raw file):

		if desc.DatabaseDesc().Schemas == nil {
			dbDesc.Schemas = map[string]descpb.DatabaseDescriptor_SchemaInfo{

while you're here, thoughts on just adding a method to the dbdesc.Mutable which will construct the map if it doesn't exist and then add the entry? Seems bad that we have to do this.

pkg/migration/migrations/public_schema_migration.go, line 190 at r8 (raw file):

	}
	for _, modified := range modifiedDescs {
		err := descriptors.WriteDescToBatch(

I'm not sure how real of a concern this is, but it seems to me like we could end up with a large batch here. It's unlikely to be larger than the maximum size of 16 MiB because, we'll generally folks don't have hundreds of thousands of descriptors in their database, but, it's still a concern that would suck to run into. Thoughts on passing in the *Txn and then also having this return a batch, and then, when the batch reaches some size, send it and create a new one. You can test this by lowering the KV max command size to something quite small and then making some descriptors with some biggish strings. This is definitely an edge case concern.

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pkg/migration/migrations/public_schema_migration.go, line 136 at r5 (raw file):

Done

pkg/migration/migrations/public_schema_migration.go, line 165 at r5 (raw file):

Done

pkg/migration/migrations/public_schema_migration.go, line 72 at r8 (raw file):

Done

pkg/migration/migrations/public_schema_migration.go, line 86 at r8 (raw file):

Done.

pkg/migration/migrations/public_schema_migration.go, line 129 at r8 (raw file):

Did this in a separate PR since we have other places where we do this
#73942

pkg/migration/migrations/public_schema_migration.go, line 190 at r8 (raw file):

Addressed all but this one so far, trying to figure out exactly how to do this.

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pkg/migration/migrations/public_schema_migration.go, line 190 at r8 (raw file):

Added batching PTAL.

Just one minor comment. Looking forward to stamping this. Good work!

Reviewed 210 of 213 files at r5, 4 of 4 files at r6, 1 of 3 files at r9, 4 of 4 files at r10, all commit messages.
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pkg/migration/migrations/migrations.go, line 62 at r10 (raw file):

	),
	migration.NewTenantMigration(
		"seed system.span_configurations with configs for existing for existing tenants",

s/for existing for existing tenants/for existing tenants/

pkg/migration/migrations/public_schema_migration.go, line 101 at r10 (raw file):

	if !found {
		return errors.Newf("expected to find database with id %d", dbID)
	}

Piggybacking on Andrew's comment: you don't need to check found if you pass the Required flag to the descriptors API call above. Use tree.DatabaseLookupFlags{Required: true} and if it's not found, err will be set appropriately, meaning you can ignore found. This will also bypass leasing and all that, so it's what you want to do here.

pkg/migration/migrations/public_schema_migration.go, line 175 at r10 (raw file):

	batch := txn.NewBatch()
	for _, desc := range allDescriptors {
		b := desc.NewBuilder()

You'll want to move this further down, below the if-continue block which will eliminate most descriptors from further consideration. Under the hood, creating a descriptor builder does a protoutil.Clone, which isn't cheap. We've ran into perf issues from this kind of thing before, which is why I'm being careful.

Updated, also made a minor change such that we don't run the test that creates 500 tables under race.

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pkg/migration/migrations/migrations.go, line 62 at r10 (raw file):

Done.

pkg/migration/migrations/public_schema_migration.go, line 101 at r10 (raw file):

Done.

pkg/migration/migrations/public_schema_migration.go, line 175 at r10 (raw file):

Thanks good call

Reviewed 4 of 4 files at r11, all commit messages.
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