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Indexes

Index Intersection

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  • Index Prefix Intersection
  • Index Intersection and Compound Indexes
  • Index Intersection and Sort

Important

This page documents cases where the query optimizer may be able to use index intersection.

In practice, the query optimizer rarely selects plans that use index intersection.

Hash-based index intersection is disabled by default and sort-based index intersection is disfavored in plan selection. The optimizer behaves in this fashion in order to prevent bad plan selection.

Schema designs should not rely on index intersection. Instead, compound indexes should be used.

Future improvements to the query optimizer may allow the system to better identify cases where an index intersection plan would be beneficial.

MongoDB can use the intersection of multiple indexes to fulfill queries. In general, each index intersection involves two indexes; however, MongoDB can employ multiple/nested index intersections to resolve a query.

To illustrate index intersection, consider a collection orders that has the following indexes:

{ qty: 1 }
{ item: 1 }

MongoDB can use the intersection of the two indexes to support the following query:

db.orders.find( { item: "abc123", qty: { $gt: 15 } } )

To determine if MongoDB used index intersection, run explain(); the results of explain() will include either an AND_SORTED stage or an AND_HASH stage.

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.

Tip

See also:

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 because the sort() requires an index separate from the query predicate:

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

That is, MongoDB cannot use either of these index intersections:

  • { qty: 1 } and { status: 1 }

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

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 } )

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