在mongodb分片集群下，直接用count统计会不准确,用聚合统计则可以

但是在java或mongodb客户端（非命令行）调用mongodb，使用聚合统计时，统计的结果和count同样不准确，请问大神们，我的代码如下，请大神指点，找不到原因！

@Test
public void testCount() throws Exception {
    DynamicSqlParameter dsp = new DynamicSqlParameter();
    long sT = System.currentTimeMillis();
    MongoDatasource mongoDatasource = MongoDatasource.getInstance(mongoService.getDatasource());
    DBCollection dbCollection = mongoDatasource.getDB().getCollection("dayFlow");
    List arrayList = new ArrayList<>();
    DBObject dbObject1 = new BasicDBObject();
    dbObject1.put("usedDayFlow", 2);
    DBObject dbObject2 = new BasicDBObject();
    dbObject2.put("_id", null);
    dbObject2.put("count", new BasicDBObject("$sum", 1));
    arrayList.add(new BasicDBObject("$match", dbObject1));
    arrayList.add(new BasicDBObject("$group", dbObject2));
    System.out.println(JSON.serialize(arrayList));
    AggregationOutput size = dbCollection.aggregate(arrayList);
    System.out.println(size.results());
    System.out.println("运行时间：" + ((System.currentTimeMillis() - sT) /1000) + "s");
}

执行结果：

[ { "$match" : { "usedDayFlow" : 2}} , { "$group" : { "_id" : null , "count" : { "$sum" : 1}}}]

[{ "_id" : null , "count" : 1002223}]

该统计结果比实际数据量要多一些，请教大神，对于分片集群的聚合统计要如何操作？