Partial Indexes部分索引

Query Coverage查询覆盖率

MongoDB will not use the partial index for a query or sort operation if using the index results in an incomplete result set.如果使用索引导致不完整的结果集，MongoDB将不会在查询或排序操作中使用部分索引。

To use the partial index, a query must contain the filter expression (or a modified filter expression that specifies a subset of the filter expression) as part of its query condition.要使用部分索引，查询必须包含筛选表达式（或指定筛选表达式子集的修改后的筛选表达式）作为其查询条件的一部分。

For example, given the following index:例如，给定以下索引：

db.restaurants.createIndex(
   { cuisine: 1 },
   { partialFilterExpression: { rating: { $gt: 5 } } }
)

The following query can use the index since the query predicate includes the condition rating: { $gte: 8 } that matches a subset of documents matched by the index filter expression rating: { $gt: 5 }:以下查询可以使用索引，因为查询谓词包括条件rating: { $gte: 8 }，该条件评级与索引筛选器表达式rating: { $gt: 5 }匹配的文档子集相匹配：

db.restaurants.find( { cuisine: "Italian", rating: { $gte: 8 } } )

However, the following query cannot use the partial index on the cuisine field because using the index results in an incomplete result set. 但是，以下查询无法在cuisine字段上使用部分索引，因为使用索引会导致结果集不完整。Specifically, the query predicate includes the condition rating: { $lt: 8 } while the index has the filter rating: { $gt: 5 }. 具体而言，查询谓词包括条件rating: { $lt: 8 }，而索引具有筛选器rating: { $gt: 5 }。That is, the query { cuisine: "Italian", rating: { $lt: 8 } } matches more documents (e.g. an Italian restaurant with a rating equal to 1) than are indexed.也就是说，查询{ cuisine: "Italian", rating: { $lt: 8 } }匹配的文档（例如，评级等于1的意大利餐厅）比索引的文档多。

db.restaurants.find( { cuisine: "Italian", rating: { $lt: 8 } } )

Similarly, the following query cannot use the partial index because the query predicate does not include the filter expression and using the index would return an incomplete result set.类似地，以下查询不能使用部分索引，因为查询谓词不包括筛选器表达式，并且使用索引将返回不完整的结果集。

db.restaurants.find( { cuisine: "Italian" } )

Comparison with the sparse Index与稀疏索引的比较

Partial indexes offer a more expressive mechanism than Sparse Indexes indexes to specify which documents are indexed.与稀疏索引相比，部分索引提供了一种更具表现力的机制，用于指定索引哪些文档。

Sparse indexes select documents to index solely based on the existence of the indexed field, or for compound indexes, the existence of the indexed fields.稀疏索引仅根据索引字段的存在来选择要索引的文档，对于复合索引，则根据索引字段是否存在来选择文档。

Partial indexes determine the index entries based on the specified filter. 部分索引根据指定的筛选器确定索引项。The filter can include fields other than the index keys and can specify conditions other than just an existence check. 筛选器可以包括索引键以外的字段，并且可以指定除存在性检查以外的条件。For example, a partial index can implement the same behavior as a sparse index:例如，部分索引可以实现与稀疏索引相同的行为：

db.contacts.createIndex(
   { name: 1 },
   { partialFilterExpression: { name: { $exists: true } } }
)

This partial index supports the same queries as a sparse index on the name field.此部分索引支持与name字段上的稀疏索引相同的查询。

However, a partial index can also specify filter expressions on fields other than the index key. 但是，部分索引也可以在索引键以外的字段上指定筛选器表达式。For example, the following operation creates a partial index, where the index is on the name field but the filter expression is on the email field:例如，以下操作创建了一个部分索引，其中索引位于name字段上，而筛选器表达式位于email字段上：

db.contacts.createIndex(
   { name: 1 },
   { partialFilterExpression: { email: { $exists: true } } }
)

For the query optimizer to choose this partial index, the query predicate must include a condition on the name field as well as a non-null match on the email field.为了让查询优化器选择此部分索引，查询谓词必须在name字段中包含一个条件，在email字段中也必须包含一个非空匹配项。

For example, the following query can use the index because it includes both a condition on the name field and a non-null match on the email field:例如，以下查询可以使用索引，因为它在name字段中包含条件，在email字段中包含非空匹配：

db.contacts.find( { name: "xyz", email: { $regex: /\.org$/ } } )

However, the following query cannot use the index because it includes a null match on the email field, which is not permitted by the filter expression { email: { $exists: true } }:但是，以下查询无法使用索引，因为它在email字段中包含空匹配项，这是筛选器表达式{ email: { $exists: true } }不允许的：

db.contacts.find( { name: "xyz", email: { $exists: false } } )

Create a Partial Index On A Collection在集合上创建部分索引

Consider a collection restaurants containing documents that resemble the following考虑包含以下类似文档的restaurants集合：

{
   "_id" : ObjectId("5641f6a7522545bc535b5dc9"),
   "address" : {
      "building" : "1007",
      "coord" : [
         -73.856077,
         40.848447
      ],
      "street" : "Morris Park Ave",
      "zipcode" : "10462"
   },
   "borough" : "Bronx",
   "cuisine" : "Bakery",
   "rating" : { "date" : ISODate("2014-03-03T00:00:00Z"),
                "grade" : "A",
                "score" : 2
              },
   "name" : "Morris Park Bake Shop",
   "restaurant_id" : "30075445"
}

You could add a partial index on the borough and cuisine fields choosing only to index documents where the rating.grade field is A:您可以在borough和cuisine字段上添加部分索引，只选择索引rating.grade字段为：

db.restaurants.createIndex(
   { borough: 1, cuisine: 1 },
   { partialFilterExpression: { 'rating.grade': { $eq: "A" } } }
)

Then, the following query on the restaurants collection uses the partial index to return the restaurants in the Bronx with rating.grade equal to A:然后，对restaurants集合的以下查询使用部分索引返回布朗克斯区rating.grade等于A的餐厅：

db.restaurants.find( { borough: "Bronx", 'rating.grade': "A" } )

However, the following query cannot use the partial index because the query expression does not include the rating.grade field:但是，以下查询无法使用部分索引，因为查询表达式不包括rating.grade字段：

db.restaurants.find( { borough: "Bronx", cuisine: "Bakery" } )

Partial Index with Unique Constraint具有唯一约束的部分索引

Partial indexes only index the documents in a collection that meet a specified filter expression. 部分索引仅索引集合中满足指定筛选表达式的文档。If you specify both the partialFilterExpression and a unique constraint, the unique constraint only applies to the documents that meet the filter expression. 如果同时指定partialFilterExpression和唯一约束，则唯一约束仅适用于满足筛选表达式的文档。A partial index with a unique constraint does not prevent the insertion of documents that do not meet the unique constraint if the documents do not meet the filter criteria.如果文档不符合筛选条件，则具有唯一约束的部分索引不会阻止插入不符合唯一约束的文档。

For example, a collection users contains the following documents:例如，集合users包含以下文档：

{ "_id" : ObjectId("56424f1efa0358a27fa1f99a"), "username" : "david", "age" : 29 }
{ "_id" : ObjectId("56424f37fa0358a27fa1f99b"), "username" : "amanda", "age" : 35 }
{ "_id" : ObjectId("56424fe2fa0358a27fa1f99c"), "username" : "rajiv", "age" : 57 }

The following operation creates an index that specifies a unique constraint on the username field and a partial filter expression age: { $gte: 21 }.

db.users.createIndex(
   { username: 1 },
   { unique: true, partialFilterExpression: { age: { $gte: 21 } } }
)

The index prevents the insertion of the following documents since documents already exist with the specified usernames and the age fields are greater than 21:索引阻止插入以下文档，因为已存在具有指定用户名且age字段大于21的文档：

db.users.insertMany( [
   { username: "david", age: 27 },
   { username: "amanda", age: 25 },
   { username: "rajiv", age: 32 }
] )

However, the following documents with duplicate usernames are allowed since the unique constraint only applies to documents with age greater than or equal to 21.但是，由于唯一约束仅适用于age大于或等于21岁的文档，因此允许以下具有重复用户名的文档。

db.users.insertMany( [
   { username: "david", age: 20 },
   { username: "amanda" },
   { username: "rajiv", age: null }
] )