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Create Indexes to Ensure Query Selectivity

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Selectivity is the ability of a query to narrow results using indexes. Effective queries are more selective and allow MongoDB to use indexes 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 or fields. Write queries that are appropriately selective relative to your indexed data.

Consider a collection of documents that have the following form:

{
status: "processed",
product_type: "electronics"
}

In this example, the status of 99% of documents in the collection is processed. If you add an index on status and query for documents with the status of processed, the index has low selectivity with this query. However, if you want to query for documents that do not have the status of processed, this index has high selectivity because the query only reads 1% of the index.

Consider a collection of documents where the status field has three values distributed across the collection:

[
{ _id: ObjectId(), status: "processed", product_type: "electronics" },
{ _id: ObjectId(), status: "processed", product_type: "grocery" },
{ _id: ObjectId(), status: "processed", product_type: "household" },
{ _id: ObjectId(), status: "pending", product_type: "electronics" },
{ _id: ObjectId(), status: "pending", product_type: "grocery" },
{ _id: ObjectId(), status: "pending", product_type: "household" },
{ _id: ObjectId(), status: "new", product_type: "electronics" },
{ _id: ObjectId(), status: "new", product_type: "grocery" },
{ _id: ObjectId(), status: "new", product_type: "household" }
]

If you add an index on status and query for { "status": "pending", "product_type": "electronics" }, MongoDB must read three index keys to return the one matching result. Similarly, a query for { "status": { $in: ["processed", "pending"] }, "product_type" : "electronics" } must read six documents to return the two matching documents.

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

[
{ _id: ObjectId(), status: 1, product_type: "electronics" },
{ _id: ObjectId(), status: 2, product_type: "grocery" },
{ _id: ObjectId(), status: 3, product_type: "household"},
{ _id: ObjectId(), status: 4, product_type: "electronics" },
{ _id: ObjectId(), status: 5, product_type: "grocery"},
{ _id: ObjectId(), status: 6, product_type: "household"},
{ _id: ObjectId(), status: 7, product_type: "electronics" },
{ _id: ObjectId(), status: 8, product_type: "grocery" },
{ _id: ObjectId(), status: 9, product_type: "household" }
]

If you query for { "status": 2, "product_type": "grocery" }, MongoDB only reads one document to fulfill the query. The index and query are more selective because there is only one matching document and the query can select that specific document using the index.

Although this example's query on status equality is more selective, a query such as { "status": { $gt: 5 }, "product_type": "grocery" } would still need to read four documents. However, if you create a compound index on product_type and status, a query for { "status": { $gt: 5 }, "product_type": "grocery" } would only need to read two documents.

To improve selectivity, you can create a compound index that narrows the documents that queries read. For example, if you want to improve selectivity for queries on status and product_type, you could create a compound index on those two fields.

If MongoDB reads a high number of documents to return results, some queries may perform faster without indexes. To determine performance, see Measure Index Use.

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