Create Queries that Ensure Selectivity

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Selectivity is the ability of a query to narrow results using the index. Effective indexes are more selective and allow MongoDB to use the index for a larger portion of the work associated with fulfilling the query.

To ensure selectivity, write queries that limit the number of possible documents with the indexed field. Write queries that are appropriately selective relative to your indexed data.

Example

Suppose you have a field called status where the possible values are new and processed. If you add an index on status you've created a low-selectivity index. The index will be of little help in locating records.

A better strategy, depending on your queries, would be to create a compound index that includes the low-selectivity field and another field. For example, you could create a compound index on status and created_at.

Another option, again depending on your use case, might be to use separate collections, one for each status.

Example

Consider an index { a : 1 } (i.e. an index on the key a sorted in ascending order) on a collection where a has three values evenly distributed across the collection:

{ _id: ObjectId(), a: 1, b: "ab" }
{ _id: ObjectId(), a: 1, b: "cd" }
{ _id: ObjectId(), a: 1, b: "ef" }
{ _id: ObjectId(), a: 2, b: "jk" }
{ _id: ObjectId(), a: 2, b: "lm" }
{ _id: ObjectId(), a: 2, b: "no" }
{ _id: ObjectId(), a: 3, b: "pq" }
{ _id: ObjectId(), a: 3, b: "rs" }
{ _id: ObjectId(), a: 3, b: "tv" }

If you query for { a: 2, b: "no" } MongoDB must scan 3 documents in the collection to return the one matching result. Similarly, a query for { a: { $gt: 1}, b: "tv" } must scan 6 documents, also to return one result.

Consider the same index on a collection where a has nine values evenly distributed across the collection:

{ _id: ObjectId(), a: 1, b: "ab" }
{ _id: ObjectId(), a: 2, b: "cd" }
{ _id: ObjectId(), a: 3, b: "ef" }
{ _id: ObjectId(), a: 4, b: "jk" }
{ _id: ObjectId(), a: 5, b: "lm" }
{ _id: ObjectId(), a: 6, b: "no" }
{ _id: ObjectId(), a: 7, b: "pq" }
{ _id: ObjectId(), a: 8, b: "rs" }
{ _id: ObjectId(), a: 9, b: "tv" }

If you query for { a: 2, b: "cd" }, MongoDB must scan only one document to fulfill the query. The index and query are more selective because the values of a are evenly distributed and the query can select a specific document using the index.

However, although the index on a is more selective, a query such as { a: { $gt: 5 }, b: "tv" } would still need to scan 4 documents.

If overall selectivity is low, and if MongoDB must read a number of documents to return results, then some queries may perform faster without indexes. To determine performance, see Measure Index Use.

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