Say we have {$or:[

{a:1}

,

{b:2,c:3}

]}. On the initial query most of the result may come from the

{a:1}

clause, and only a few results from the

{b:2,c:3}

clause. The selection of index

{b:1}

or

{c:1}

will be based on less data than it would be otherwise, and arguably shouldn't be recorded for future operations.