timeseriesBucketsMayHaveMixedSchemaData is a new option which can be provided to the collMod command for a time-series collection. This option can only be set to true (cannot use false). Setting it to true sets an internal flag on the time-series collection indicating that it may contain buckets holding mixed-schema data. Mixed schema time-series buckets were only possible to be created on older versions such as v5.0.

For time-series collections, some tools interface directly with the underlying buckets collection. If such a tool is used to restore old time-series data (inserted on v5.0 and containing mixed-schema buckets) into a new time-series collection, now inserting those old buckets may fail with a error code CannotInsertTimeseriesBucketsWithMixedSchema. If this error is received, use the new collMod option mentioned above. Then once the flag is set using collMod, the mixed-schema buckets can be successfully inserted.

Description of Linked Ticket

All newly created time-series collections have timeseriesBucketsMayHaveMixedSchemaData set to false. This flag never gets changed as time-series inserts through the view can never result in mixed-schema buckets. However, one can still insert mixed-schema buckets through the system.buckets collection (direct write) and we wouldn't update the flag to true. mongodump/mongorestore does this. We should check whether buckets inserted via direct writes contain mixed-schema data and set the flag appropriately.