Partial indexes only include entries for documents that satisfy a specified filter expression. Documents that do not satisfy this expression are skipped by the index.

SBE engine cannot utilise partial indexes during a $lookup operation. This is because SBE cannot find documents when they are not present in the index. Classic engine avoids this issue by re-planning the lookup for each value of the localfield. Consequently, customers switching to SBE may experience a performance regression with $lookup queries that previously used partial indexes under the Classic engine. To address this, we should prevent pushing down a $lookup if the Classic engine would have been able to leverage a partial index.