Question

我正在使用 mongo-java-driver 3.0.2 。

我有一个使用MongoCollection.aggregate(List<Bson> pipeline)排序和限制的方法：

private static MongoIterable<Document> selectTop(int n) {
    BasicDBObject sortFields = new BasicDBObject("score", -1);
    BasicDBObject sort = new BasicDBObject("$sort", sortFields);

    BasicDBObject limit = new BasicDBObject("$limit", n);

    List<BasicDBObject> pipeline = new ArrayList<>();
    pipeline.add(sort);
    pipeline.add(limit);

    return playersCollection.aggregate(pipeline);
}

当n很大时，它会失败：

com.mongodb.MongoCommandException: Command failed with error 16820: 'exception: Sort exceeded memory limit of 104857600 bytes, but did not opt in to external sorting. Aborting operation. Pass allowDiskUse:true to opt in.'

我发现 MongoDB shell 提供了一种方法db.collection.aggregate(pipeline, options)（link），其中options可以包含allowDiskUse字段

我无法在 Java API 中找到与此相同的内容。虽然有AggregationOptions课程，但MongoCollection课程并未提供aggregate(List<Bson> pipeline, AggregationOptions options)课程。

Answer 1

这仍适用于3.0.3驱动程序：

    MongoClient client = new MongoClient(new ServerAddress("127.0.0.1", 27017));

    DB test = client.getDB("test");

    DBCollection sample = test.getCollection("sample");

    List<DBObject> aggregationQuery = Arrays.<DBObject>asList(
            new BasicDBObject("$sort",new BasicDBObject("score",-1)),
            new BasicDBObject("$limit",1)
    );

    System.out.println(aggregationQuery);

    Cursor aggregateOutput = sample.aggregate(
            aggregationQuery,
            AggregationOptions.builder()
                    .allowDiskUse(true)
                    .build()
    );

    while ( aggregateOutput.hasNext() ) {
        DBObject doc = aggregateOutput.next();
        System.out.println(doc);
    }

当然你也可以使用更新的课程：

    MongoClient client = new MongoClient(new ServerAddress("192.168.2.4", 27017));

    MongoDatabase db = client.getDatabase("test");

    MongoCollection<Document> collection = db.getCollection("sample");

    AggregateIterable<Document> result = collection.aggregate(Arrays.asList(
            new BasicDBObject("$sort", new BasicDBObject("score", -1)),
            new BasicDBObject("$limit", 1)
    )).allowDiskUse(true);

    MongoCursor<Document> cursor = result.iterator();

    while (cursor.hasNext()) {
        Document doc = cursor.next();
        System.out.println(doc);
    }

因此MongoCollection上的.aggregate()会返回一个AggregateIterable类实例，该实例具有.allowDiskuse()方法以及其他设置聚合选项的方法。

MongoDB Java驱动程序3.x：如何将allowDiskUse = true传递给aggregate（）方法？

1 个答案: