I have a compound index and an index on a single field A. If in a find query, the compound index was chosen as the winning plan, and the results are sorted by the field A, will field A's index be used to sort it?

CodePudding user response：

Unfortunately, no.

MongoDB cannot use different indexes for sorting and document selection.

See https://www.mongodb.com/docs/manual/tutorial/sort-results-with-indexes/#use-indexes-to-sort-query-results

Note that it can use a compound index for sorting, i.e. if the compound index were on {a:1, otherfield:1}, that index can be used for selection by multiple fields, and sorting by a.