Index Intersection

Index Prefix Intersection

With index intersection, MongoDB can use an intersection of either the entire index or the index prefix. An index prefix is a subset of a compound index, consisting of one or more keys starting from the beginning of the index.

Consider a collection orders with the following indexes:

{ qty: 1 }
{ status: 1, ord_date: -1 }

To fulfill the following query which specifies a condition on both the qty field and the status field, MongoDB can use the intersection of the two indexes:

db.orders.find( { qty: { $gt: 10 } , status: "A" } )

Index Intersection and Compound Indexes

Index intersection does not eliminate the need for creating compound indexes. However, because both the list order (i.e. the order in which the keys are listed in the index) and the sort order (i.e. ascending or descending), matter in compound indexes, a compound index may not support a query condition that does not include the index prefix keys or that specifies a different sort order.

For example, if a collection orders has the following compound index, with the status field listed before the ord_date field:

{ status: 1, ord_date: -1 }

The compound index can support the following queries:

db.orders.find( { status: { $in: ["A", "P" ] } } )
db.orders.find(
   {
     ord_date: { $gt: new Date("2014-02-01") },
     status: {$in:[ "P", "A" ] }
   }
)

But not the following two queries:

db.orders.find( { ord_date: { $gt: new Date("2014-02-01") } } )
db.orders.find( { } ).sort( { ord_date: 1 } )

However, if the collection has two separate indexes:

{ status: 1 }
{ ord_date: -1 }

The two indexes can, either individually or through index intersection, support all four aforementioned queries.

The choice between creating compound indexes that support your queries or relying on index intersection depends on the specifics of your system.

Index Intersection and Sort

Index intersection does not apply when the sort() operation requires an index completely separate from the query predicate.

For example, the orders collection has the following indexes:

{ qty: 1 }
{ status: 1, ord_date: -1 }
{ status: 1 }
{ ord_date: -1 }

MongoDB cannot use index intersection for the following query with sort:

db.orders.find( { qty: { $gt: 10 } } ).sort( { status: 1 } )

That is, MongoDB does not use the { qty: 1 } index for the query, and the separate { status: 1 } or the { status: 1, ord_date: -1 } index for the sort.

However, MongoDB can use index intersection for the following query with sort since the index { status: 1, ord_date: -1 } can fulfill part of the query predicate.

db.orders.find( { qty: { $gt: 10 } , status: "A" } ).sort( { ord_date: -1 } )

Index Intersection and Atlas Search Indexes

If you use MongoDB Atlas, you can create an Atlas Search index to fulfill queries when the query optimizer selects a plan that involves index intersection. Unlike compound indexes, fields in Atlas Search indexes can be defined in any order. As a result, the Atlas Seach index can support queries that:

• Specify a different sort order than the index sort order.

• Contain fields that are not part of the index prefix.

For a tutorial on using Atlas Search indexes when index intersection is required, see Flexible Querying with Atlas Search.