Sparse Indexes稀疏索引

Sparse Index and Incomplete Results稀疏索引与不完全结果

If a sparse index would result in an incomplete result set for queries and sort operations, MongoDB will not use that index unless a hint() explicitly specifies the index.如果稀疏索引会导致查询和排序操作的结果集不完整，MongoDB将不会使用该索引，除非hint()显式指定索引。

For example, the query { x: { $exists: false } } will not use a sparse index on the x field unless explicitly hinted. 例如，除非明确提示，否则查询{ x: { $exists: false } }不会在x字段上使用稀疏索引。See Sparse Index On A Collection Cannot Return Complete Results for an example that details the behavior.有关详细行为的示例，请参阅集合上的稀疏索引无法返回完整结果。

If you include a hint() that specifies a sparse index when you perform a count() of all documents in a collection (i.e. with an empty query predicate), the sparse index is used even if the sparse index results in an incorrect count.如果在对集合中的所有文档执行count()时包含指定稀疏索引的hint()（即使用空查询谓词），则即使稀疏索引导致计数不正确，也会使用稀疏索引。

db.collection.insertOne( { _id: 1, y: 1 } );
db.collection.createIndex( { x: 1 }, { sparse: true } );

db.collection.find().hint( { x: 1 } ).count();

To obtain the correct count, do not hint() with a sparse index when performing a count of all documents in a collection.为了获得正确的计数，在对集合中的所有文档执行计数时，不要使用稀疏索引hint()。

db.collection.find().count();

db.collection.createIndex( { y: 1 } );
db.collection.find().hint( { y: 1 } ).count();

Indexes that are Sparse by Default默认情况下稀疏的索引

The following index types are always sparse:以下索引类型始终是稀疏的：

• 2d
• 2dsphere (version 2)
• Text
• Wildcard

Sparse Compound Indexes稀疏复合索引

Compound indexes can contain different types of sparse indexes. 复合索引可以包含不同类型的稀疏索引。The combination of index types determines how the compound index matches documents.索引类型的组合决定了复合索引与文档的匹配方式。

This table summarizes the behavior of a compound index that contains different types of sparse indexes:此表总结了包含不同类型稀疏索引的复合索引的行为：

Sparse and Unique Properties稀疏性和唯一性

An index that is both sparse and unique prevents a collection from having documents with duplicate values for a field but allows multiple documents that omit the key.稀疏且唯一的索引可以防止集合中某个字段的值重复，但允许多个文档省略键。

Create a Sparse Index On A Collection在集合上创建稀疏索引

Consider a collection scores that contains the following documents:考虑包含以下文档的scores集合：

{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }

The collection has a sparse index on the field score:该集合对字段score有一个稀疏索引：

db.scores.createIndex( { score: 1 } , { sparse: true } )

Then, the following query on the scores collection uses the sparse index to return the documents that have the score field less than ($lt) 90:然后，对scores集合的以下查询使用稀疏索引返回score字段小于（$lt）90的文档：

db.scores.find( { score: { $lt: 90 } } )

Because the document for the userid "newbie" does not contain the score field and thus does not meet the query criteria, the query can use the sparse index to return the results:由于用户ID"newbie"的文档不包含score字段，因此不符合查询条件，因此查询可以使用稀疏索引返回结果：

{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }

Sparse Index On A Collection Cannot Return Complete Results集合上的稀疏索引无法返回完整结果

Consider a collection scores that contains the following documents:考虑包含以下文档的scores集合：

{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }

The collection has a sparse index on the field score:该集合对字段score有一个稀疏索引：

db.scores.createIndex( { score: 1 } , { sparse: true } )

Because the document for the userid "newbie" does not contain the score field, the sparse index does not contain an entry for that document.因为userid: "newbie"的文档不包含score字段，所以稀疏索引不包含该文档的条目。

Consider the following query to return all documents in the scores collection, sorted by the score field:考虑以下查询以返回scores集合中的所有文档，按score字段排序：

db.scores.find().sort( { score: -1 } )

Even though the sort is by the indexed field, MongoDB will not select the sparse index to fulfill the query in order to return complete results:即使按索引字段排序，MongoDB也不会选择稀疏索引来完成查询，以返回完整的结果：

{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }

To use the sparse index, explicitly specify the index with hint():要使用稀疏索引，请使用hint()显式指定索引：

db.scores.find().sort( { score: -1 } ).hint( { score: 1 } )

The use of the index results in the return of only those documents with the score field:使用索引只会返回那些带有score字段的文档：

{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }

Sparse Index with Unique Constraint具有唯一约束的稀疏索引

Consider a collection scores that contains the following documents:考虑包含以下文档的集合scores：

{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }

You could create an index with a unique constraint and sparse filter on the score field using the following operation:您可以使用以下操作在score字段上创建具有唯一约束和稀疏筛选器的索引：

db.scores.createIndex( { score: 1 } , { sparse: true, unique: true } )

This index would permit the insertion of documents that had unique values for the score field or did not include a score field. 该索引将允许插入score字段具有唯一值或不包括score字段的文档。As such, given the existing documents in the scores collection, the index permits the following insert operations:因此，考虑到scores集合中的现有文档，索引允许以下插入操作：

db.scores.insertMany( [
   { "userid": "newbie", "score": 43 },
   { "userid": "abby", "score": 34 },
   { "userid": "nina" }
] )

However, the index would not permit the addition of the following documents since documents already exists with score value of 82 and 90:然而，该索引不允许添加以下文件，因为已经存在score值为82和90的文件：

db.scores.insertMany( [
   { "userid": "newbie", "score": 82 },
   { "userid": "abby", "score": 90 }
] )

Sparse and Non-Sparse Unique Indexes稀疏和非稀疏唯一索引

Starting in MongoDB 5.0, unique sparse and unique non-sparse indexes with the same key pattern can exist on a single collection.从MongoDB 5.0开始，具有相同键模式的唯一稀疏索引和唯一非稀疏索引可以存在于单个集合中。

db.scoreHistory.createIndex( { score : 1 }, { name: "unique_index", unique: true } )
db.scoreHistory.createIndex( { score : 1 }, { name: "unique_sparse_index", unique: true, sparse: true } )

db.scoreHistory.createIndex( { score : 1 }, { name: "sparse_index", sparse: true } )
db.scoreHistory.createIndex( { score : 1 }, { name: "basic_index" } )