MongoDB v7 Upgrade – Extreme Slowdown with collation: { locale: 'en' }

After upgrading to MongoDB v7, we’ve noticed a drastic performance regression with our membership query. The execution time went from milliseconds to over 20 minutes.

Here’s what we found:

  • With collation: { locale: 'en' }:
    • Query execution time: 20+ minutes
    • docsExamined: 2.47 billion (way more than the 435.6k documents in the target collection)
  • Without collation: { locale: 'en' }:
    • Query execution time: ~700ms
    • docsExamined: 1270 (acceptable range)

This suggests that applying collation in MongoDB v7 is causing an unexpectedly large scan. Is this a known issue with v7? Could this be related to changes in collation indexing or query planning?

Would appreciate any insights or suggestions on how to mitigate this issue. Thanks!