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Limit the Number of Entries Scanned

This tutorial describes how to create indexes to limit the number of index entries scanned for queries that includes a $text expression and equality conditions.

A collection inventory contains the following documents:

{ _id: 1, dept: "tech", description: "lime green computer" }
{ _id: 2, dept: "tech", description: "wireless red mouse" }
{ _id: 3, dept: "kitchen", description: "green placemat" }
{ _id: 4, dept: "kitchen", description: "red peeler" }
{ _id: 5, dept: "food", description: "green apple" }
{ _id: 6, dept: "food", description: "red potato" }

Consider the common use case that performs text searches by individual departments, such as:

db.inventory.find( { dept: "kitchen", $text: { $search: "green" } } )

To limit the text search to scan only those documents within a specific dept, create a compound index that first specifies an ascending/descending index key on the field dept and then a text index key on the field description:

dept: 1,
description: "text"

Then, the text search within a particular department will limit the scan of indexed documents. For example, the following query scans only those documents with dept equal to kitchen:

db.inventory.find( { dept: "kitchen", $text: { $search: "green" } } )
  • A compound text index cannot include any other special index types, such as multi-key or geospatial index fields.
  • If the compound text index includes keys preceding the text index key, to perform a $text search, the query predicate must include equality match conditions on the preceding keys.
  • When creating a compound text index, all text index keys must be listed adjacently in the index specification document.
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