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。