Add assets index for app and folder queries

[NoahHahm]

Fixes #1344

What

Adds an index { _ai: 1, pi: 1, mt: -1, id: 1 } to States_Assets2.

Why

The assets query filters by app and parent folder and sorts by mt desc, id asc. The existing index starts with mt, so MongoDB walks the index in mt order across the whole collection and filters afterwards. With a large collection this makes root folder queries and their total count very slow, and they hit assets.timeoutQuery (408).

The new index starts with the equality fields (_ai, pi) and then follows the sort order, so the query and the count only read the requested folder.

[NoahHahm]

hi. @SebastianStehle
I have resolved the issue using this index for now.
If there is a better alternative, feel free to close the PR.

[SebastianStehle]

I think we have to check out the indexes and see if they match with the queries. Just having an additional index is not necessarily the best idea

[NoahHahm]

I think we have to check out the indexes and see if they match with the queries. Just having an additional index is not necessarily the best idea

I agree.
If there is room for improvement without adding an index, I think that is a good approach.

[SebastianStehle]

I have pushed a fix.