Case-Insensitive Indexes

Case-insensitive indexes support queries that perform string comparisons without regard for case. Case insensitivity is derived from collation.

Important

Case-insensitive indexes do not improve performance for $regex queries, as the $regex operator is not collation-aware and therefore cannot take advantage of such indexes.

Command Syntax

You can create a case-insensitive index with db.collection.createIndex() by specifying the collation option:

db.collection.createIndex(
   {
      <field>: <sortOrder>
   },
   {
      collation:
         {
            locale : <locale>,
            strength : < 1 | 2 >
         }
   }
)

To specify a collation for a case-insensitive index, include the following fields in the collation object:

Field	Description
`locale`	Specifies language rules. For a list of available locales, see Supported Languages and Locales.
`strength`	Determines comparison rules. A `strength` value of 1 or 2 indicates case-insensitive collation.

For additional collation fields, see Collation.

Behavior

To use an index that specifies a collation, query and sort operations must specify the same collation as the index. If a collection has defined a collation, all queries and indexes inherit that collation unless they explicitly specify a different collation.

Examples

Create a Case-Insensitive Index

To use a case-insensitive index on a collection with no default collation, create an index with a collation and set the strength parameter to 1 or 2 (see Collation for a detailed description of the strength parameter). You must specify the same collation at the query level in order to use the index-level collation.

The following example creates a collection with no default collation, then adds an index on the type field with a case-insensitive collation.

db.createCollection("fruit")

db.fruit.createIndex(
   { type: 1 },
   { collation: { locale: 'en', strength: 2 } }
)

To use the index, queries must specify the same collation.

db.fruit.insertMany( [
   { type: "apple" },
   { type: "Apple" },
   { type: "APPLE" }
] )

db.fruit.find( { type: "apple" } ) // does not use index, finds one result

db.fruit.find( { type: "apple" } ).collation( { locale: 'en', strength: 2 } )
// uses the index, finds three results

db.fruit.find( { type: "apple" } ).collation( { locale: 'en', strength: 1 } )
// does not use the index, finds three results

Case-Insensitive Indexes on Collections with a Default Collation

When you create a collection with a default collation, all the indexes you create subsequently inherit that collation unless you specify a different collation. All queries which do not specify a different collation also inherit the default collation.

The following example creates a collection called names with a default collation, then creates an index on the first_name field.

db.createCollection("names", { collation: { locale: 'en_US', strength: 2 } } )

db.names.createIndex( { first_name: 1 } ) // inherits the default collation

Insert a small collection of names:

db.names.insertMany( [
   { first_name: "Betsy" },
   { first_name: "BETSY"},
   { first_name: "betsy"}
] )

Queries on this collection use the specified collation by default, and if possible use the index as well.

db.names.find( { first_name: "betsy" } )
// inherits the default collation: { collation: { locale: 'en_US', strength: 2 } }
// finds three results

The above operation uses the collection's default collation and finds all three documents. It uses the index on the first_name field for better performance.

It is still possible to perform case sensitive searches on this collection by specifying a different collation in the query:

db.names.find( { first_name: "betsy" } ).collation( { locale: 'en_US' } )
// does not use the collection's default collation, finds one result

The above operation finds only one document, because it uses a collation with no strength value specified. It does not use the collection's default collation or the index.

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