Case Insensitive Indexes不区分大小写的索引

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Case insensitive indexes support queries that perform string comparisons without regard for case.不区分大小写的索引支持执行字符串比较而不考虑大小写的查询。

You can create a case insensitive index with db.collection.createIndex() by specifying the collation parameter as an option. 通过将collation参数指定为选项，可以使用db.collection.createIndex()创建不区分大小写的索引。For example:例如：

db.collection.createIndex( { "key" : 1 },
                           { collation: {
                               locale : <locale>,
                               strength : <strength>
                             }
                           } )

To specify a collation for a case sensitive index, include:要指定区分大小写索引的排序规则，请包括：

For additional collation fields, see Collation.有关其他排序规则字段，请参阅排序规则。

Behavior行为

Using a case insensitive index does not affect the results of a query, but it can increase performance; see Indexes for a detailed discussion of the costs and benefits of indexes.使用不区分大小写的索引不会影响查询结果，但可以提高性能；有关索引的成本和收益的详细讨论，请参阅索引。

To use an index that specifies a collation, query and sort operations must specify the same collation as the index. 要使用指定排序规则的索引，查询和排序操作必须指定与索引相同的排序规则。If a collection has defined a collation, all queries and indexes inherit that collation unless they explicitly specify a different collation.如果集合定义了排序规则，则所有查询和索引将继承该排序规则，除非它们显式指定不同的排序规则。

Examples示例

Create a Case Insensitive Index创建不区分大小写的索引

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). 要在没有默认排序规则的集合上使用不区分大小写的索引，请创建具有排序规则的索引，并将strength参数设置为1或2（有关强度参数的详细说明，请参阅排序规则）。You must specify the same collation at the query level in order to use the index-level collation.为了使用索引级排序规则，必须在查询级指定相同的排序规则。

The following example creates a collection with no default collation, then adds an index on the type field with a case insensitive collation.下面的示例创建了一个没有默认排序规则的集合，然后使用不区分大小写的排序规则在type字段上添加索引。

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

To use the index, queries must specify the same collation.要使用索引，查询必须指定相同的排序规则。

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具有默认排序规则的集合上不区分大小写的索引

When you create a collection with a default collation, all the indexes you create subsequently inherit that collation unless you specify a different collation. 创建具有默认排序规则的集合时，除非指定其他排序规则，否则随后创建的所有索引都将继承该排序规则。All queries which do not specify a different collation also inherit the default collation.所有未指定不同排序规则的查询也会继承默认排序规则。

The following example creates a collection called names with a default collation, then creates an index on the first_name field.以下示例使用默认排序规则创建名为names的集合，然后在first_name字段上创建索引。

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:插入一小部分名称：

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.默认情况下，此集合上的查询使用指定的排序规则，如果可能，还使用索引。

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. 上述操作使用集合的默认排序规则并查找所有三个文档。It uses the index on the first_name field for better performance.它使用first_name字段上的索引以获得更好的性能。

It is still possible to perform case sensitive searches on this collection by specifying a different collation in the query:通过在查询中指定不同的排序规则，仍然可以对此集合执行区分大小写的搜索：

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. 上述操作只找到一个文档，因为它使用的strength没有指定强度值。It does not use the collection's default collation or the index.它不使用集合的默认排序规则或索引。

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