[$250] [Due for payment 2025-07-14] Use indexed object structure for OnyxCache

[mountiny]

Coming from here

Background

react-native-onyx is using its internal cache mechanism to get value from cache while first connection to Onyx is being made through useOnyx so the cached value can be returned immediately.

Problem

When useOnyx requests collection data from cache while the first connection to Onyx is being made, the implementation loops through all cached keys to find matching items, which causes response times to degrade linearly with cache size, which prevents large accounts from achieving acceptable performance in multiple flows of the app.

Solution

Change the shape of the Onyx cache to store collections as indexed objects instead of flat key-value pairs. When requesting collection data, the useOnyx should directly access the pre-organized collection object instead of iterating through thousands of keys. This transforms collection access from O(n) complexity to O(1), delivering instant retrieval regardless of cache size.
Some numbers from account with 42k cached Onyx keys:

• iOS Simulator: time spent on getting values from cache on app start: 1.4s → 23ms
• iOS Simulator: time spent on getting values from cache on opening Expense chat: 1s → 18ms
• Android emulator: app launch 5s → 3s
  Thanks to that useOnyx times go down as well - on opening Expense chat total time dropped from ~650ms to ~170ms

Issue Owner

Current Issue Owner: @

Upwork Automation - Do Not Edit

• Upwork Job URL: https://www.upwork.com/jobs/~021942727528896232627
• Upwork Job ID: 1942727528896232627
• Last Price Increase: 2025-07-15

[melvin-bot]

Triggered auto assignment to @jliexpensify (Bug), see https://stackoverflow.com/c/expensify/questions/14418 for more details. Please add this bug to a GH project, as outlined in the SO.

[melvin-bot]

@TMisiukiewicz Eep! 4 days overdue now. Issues have feelings too...

[TMisiukiewicz]

Onyx PR in review

[allgandalf]

Please assign me here, I reviewed the related pR