Background:

When $out targets an existing viewless timeseries collection, the current implementation strips fixedBucketing from the snapshot options before creating the temporary collection. The primary reason is FCV downgrades: during a downgrade the collection transitions from viewless to viewful (legacy), and viewful timeseries don't accept the fixedBucketing parameter. Without stripping, attempting to create a viewful temp collection with fixedBucketing in the options would fail with InvalidOptions. By stripping it, the temp collection is created cleanly and the defaulting logic applies.

A side effect is that $out always produces a collection with fixedBucketing: true (the default for new viewless collections), even if the target collection previously had fixedBucketing: false. This is a silent semantic change.

Decision needed:

Once featureFlagFixedBucketingCatalog is removed (SERVER-127534) and FCV < 9.0 is no longer
supported, the FCV-downgrade justification for stripping goes away. At that point, decide whether:

1. Keep stripping: $out always produces fixedBucketing: true. Simple, consistent.
2. Preserve target value: Plumb the target's fixedBucketing value through temp collection
   creation, avoiding the silent change from false to true.

Affects: src/mongo/db/exec/agg/out_stage.cpp (createTemporaryCollection)

See also: SERVER-126823, SERVER-127534

Github conversation:

• https://github.com/10gen/mongo/pull/55098#discussion_r3372223396
• https://github.com/10gen/mongo/pull/55098#discussion_r3380607882