聚合查询处理来自多个索引条目的数据以返回单个汇总值。
Cloud Firestore 支持count()
聚合查询。 count()
允许您确定集合或查询中的文档数。服务器计算计数,并仅将结果(一个整数)传输回您的应用程序,与执行完整查询相比,节省了计费文档读取和传输的字节数。
聚合查询依赖于您的查询已使用的现有索引配置,并根据扫描的索引条目数按比例缩放。这意味着中小型数据集的聚合在 20-40 毫秒内执行,尽管延迟会随着计数的项目数量的增加而增加。
使用count()
聚合
参考我们在获取数据中设置的示例数据。
以下count()
聚合返回cities
集合中的城市总数。
Web version 9
const coll = collection(db, "cities"); const snapshot = await getCountFromServer(coll); console.log('count: ', snapshot.data().count);
迅速
let collection = db.collection("cities") let countQuery = collection.count do { let snapshot = try await countQuery.getAggregation(source: .server) print(snapshot.count) } catch { print(error); }
目标-C
FIRCollectionReference* collection = [db collectionWithPath:@"cities"]; [collection.count aggregationWithSource:FIRAggregateSourceServer completion:^(FIRAggregateQuerySnapshot *snapshot, NSError *error) { if (error == nil) { NSLog(@"Cities count: %@", snapshot.count); } else { NSLog(@"Error fetching count: %@", error); } } ];
Kotlin+KTX
val collection = db.collection("cities") val countQuery = collection.count() countQuery.get(AggregateSource.SERVER).addOnCompleteListener { task -> if (task.isSuccessful) { val snapshot = task.result Log.d(TAG, "Count: ${snapshot.count}") } else { Log.d(TAG, "Count failed: ", task.getException()) } }
Java
CollectionReference collection = db.collection("cities"); AggregateQuery countQuery = collection.count(); countQuery.get(AggregateSource.SERVER).addOnCompleteListener(task -> { if (task.isSuccessful()) { AggregateQuerySnapshot snapshot = task.getResult(); Log.d(TAG, "Count: " + snapshot.getCount()); } else { Log.d(TAG, "Count failed: ", task.getException()); } });
爪哇
CollectionReference collection = db.collection("cities"); AggregateQuerySnapshot snapshot = collection.count().get().get(); System.out.println("Count: " + snapshot.getCount());
节点.js
const collectionRef = db.collection('cities'); const snapshot = await collectionRef.count().get(); console.log(snapshot.data().count);
count()
聚合考虑了查询中的任何过滤器和任何limit
子句。例如,以下聚合返回state
等于CA
的城市数。
Web version 9
const coll = collection(db, "cities"); const query_ = query(coll, where('state', '==', 'CA')); const snapshot = await getCountFromServer(query_); console.log('count: ', snapshot.data().count);
迅速
let collection = db.collection("cities") let query = collection.whereField("state", isEqualTo: "CA") let countQuery = query.count do { let snapshot = try await countQuery.getAggregation(source: .server) print(snapshot.count) } catch { print(error); }
目标-C
FIRCollectionReference* collection = [db collectionWithPath:@"cities"]; FIRQuery* query = [collection queryWhereField:@"state" isEqualTo:@"CA"]; [query.count aggregationWithSource:FIRAggregateSourceServer completion:^(FIRAggregateQuerySnapshot *snapshot, NSError *error) { if (error == nil) { NSLog(@"Cities count: %@", snapshot.count); } else { NSLog(@"Error fetching count: %@", error); } } ];
Kotlin+KTX
val collection = db.collection("cities") val query = collection.whereEqualTo("state", "CA") val countQuery = query.count() countQuery.get(AggregateSource.SERVER).addOnCompleteListener { task -> if (task.isSuccessful) { val snapshot = task.result Log.d(TAG, "Count: ${snapshot.count}") } else { Log.d(TAG, "Count failed: ", task.getException()) } }
Java
CollectionReference collection = db.collection("cities"); Query query = collection.whereEqualTo("state", "CA"); AggregateQuery countQuery = query.count(); countQuery.get(AggregateSource.SERVER).addOnCompleteListener(task -> { if (task.isSuccessful()) { AggregateQuerySnapshot snapshot = task.getResult(); Log.d(TAG, "Count: " + snapshot.getCount()); } else { Log.d(TAG, "Count failed: ", task.getException()); } });
爪哇
CollectionReference collection = db.collection("cities"); Query query = collection.whereEqualTo("state", "CA"); AggregateQuerySnapshot snapshot = query.count().get().get(); System.out.println("Count: " + snapshot.getCount());
节点.js
const collectionRef = db.collection('cities'); const query = collectionRef.where('state', '==', 'CA'); const snapshot = await query.count().get(); console.log(snapshot.data().count);
Cloud Firestore 安全规则对count()
聚合查询的作用与对返回文档的普通查询的作用相同。换句话说,当且仅当您的规则允许客户端执行某些集合或集合组查询时,客户端还可以对这些查询执行count()
聚合。详细了解Cloud Firestore 安全规则如何与查询交互。
限制
请注意count()
聚合查询的以下限制:
count()
聚合查询目前仅通过直接服务器响应支持。查询仅由 Cloud Firestore 后端提供,跳过本地缓存和任何缓冲更新。此行为与在Cloud Firestore 事务中执行的操作相同。您目前不能将count()
查询与实时侦听器和离线查询一起使用。如果
count()
聚合无法在 60 秒内解决,它会返回DEADLINE_EXCEEDED
错误。性能取决于您的索引配置和数据集的大小。如果无法在 60 秒期限内完成操作,可能的解决方法是对大型数据集使用计数器。
count()
聚合从索引条目中读取并仅计算索引字段。将
OrderBy
子句添加到查询会将计数限制为存在排序属性的实体。
价钱
count()
的定价取决于查询匹配的索引条目数。您需要为大量匹配条目支付少量阅读费用。
查看更详细的定价信息。