We can force MongoDB to use an index by applying the hint() parameter:

> db.books.createIndex( { isbn: 1 }, { background: true } )
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 8,
"numIndexesAfter" : 9,
"ok" : 1
}

The output from createIndex notifies us that the index was created ("ok" : 1), no collection was created automatically as part of index creation ("createdCollectionAutomatically" : false), the number of indexes before this index creation was 8, and now there are 9 indexes in total for this collection.

If we now try to search a book by isbn, we can use the explain() command to see the winningPlan subdocument where we can find which query plan was used:

> db.books.find({isbn: "1001"}).explain()
…
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"isbn" : 1,
"name" : 1
},
"indexName" : "isbn_1_name_1",
...

This means that an index with isbn:1 and name:1 was used instead of our newly-created index. We can also view our index in the rejectedPlans subdocument of the output:

…
"rejectedPlans" : [
{
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"isbn" : 1
},
"indexName" : "isbn_1",
...

This is in fact right as MongoDB is trying to reuse an index that is more specific than a more generic one.

We may have stumbled though in cases where our isbn_1 index is performing better than the isbn_1_name_1 one.

We can force MongoDB to use our newly created index as follows:

> db.books.find({isbn: "1001"}).hint("international_standard_book_number_index")
.explain()
{
...
                "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "keyPattern" : {
                                        "isbn" : 1
                                },
...

Now the winningPlan subdocument contains our index, isbn_1, and there are no rejectedPlans elements: it's an empty array in the result set.

We cannot use hint() with the special type of text indexes.