In MongoDB 8.0, timeseries collections are implemented as views over system.buckets collections. The reIndex command handler rejects calls on a timeseries namespace with CommandNotSupportedOnView — "can't re-index a view" — via the view check in CmdReIndex::run. This is the correct behavior.

In MongoDB 9.0, timeseries collections are no longer views (viewless). As a result, the view check no longer fires, and reIndex silently succeeds on a standalone instance targeting a timeseries namespace. This is unintended: reIndex has no meaningful semantics on timeseries collections and should be explicitly forbidden.

Root cause

CmdReIndex::run guards against views via a CommandNotSupportedOnView uassert. With viewless timeseries (FCV >= 9.0), the collection is no longer a view, so the guard is silently bypassed.

Proposed fix

Add an explicit check in CmdReIndex::run that returns CommandNotSupported when the target namespace is a timeseries collection, regardless of FCV.

Testing

There are currently no jstests covering reIndex behavior on timeseries collections. A new test should verify:

• Standalone, FCV >= 9.0: reIndex on a timeseries namespace fails with CommandNotSupported
• Standalone, FCV < 9.0: reIndex on a timeseries namespace fails with CommandNotSupportedOnView (existing behavior, unchanged)
• Replica set: fails with IllegalOperation (unchanged)
• Sharded cluster: CommandNotFound from mongos (unchanged)