mongodb每一个文档默认只有16M。聚合的结果是一个BSON文档,当超过16M大小时,就会报内存不够错误。
exceeded memory limit for $group.but didn't allow external sort.
可以采用打开使用磁盘来解决大小问题。例如
db.flowlog.aggregate([{$group:{_id:"$_id"}}], {allowDiskUse: true})
java代码片段
AggregationOptions options = new AggregationOptions.Builder().allowDiskUse(true).build(); Aggregation agg = Aggregation.newAggregation().withOptions(options);
但是如果结果集超过了16M,那么依然会报错误。
采用一个下面的聚合方法
Aggregation agg = Aggregation.newAggregation( Aggregation.group(field1 , field2 , field3) .sum(field4).as("sampleField1") .sum(field5).as("sampleField2"), Aggregation.project(field4, field5), new AggregationOperation() { @Override public DBObject toDBObject(AggregationOperationContext context) { return new BasicDBObject("$out", "test"); } }).withOptions(options); mongo.aggregate(agg, sourceCollection, Test.class);
如果要在聚合的时候增加一个常量,可采用以下形式
Aggregation agg = Aggregation.newAggregation( Aggregation.group( , OnofflineUserHistoryField.MAC , StalogField.UTC_CODE) .sum(OnofflineUserHistoryField.WIFI_UP_DOWN).as(OnofflineUserHistoryField.WIFI_UP_DOWN) .sum(OnofflineUserHistoryField.ACTIVE_TIME).as(OnofflineUserHistoryField.ACTIVE_TIME), Aggregation.project("mac","buildingId","utcCode",OnofflineUserHistoryField.ACTIVE_TIME, OnofflineUserHistoryField.WIFI_UP_DOWN).and( new AggregationExpression() { @Override public DBObject toDbObject(AggregationOperationContext context) { return new BasicDBObject( "$cond", new Object[]{ new BasicDBObject( "$eq", new Object[]{ "$tenantId", 0} ), 20161114, 20161114 }); } }).as("day").andExclude("_id"), 或者 and(new AggregationExpression() { @Override public DBObject toDbObject(AggregationOperationContext context) { return new BasicDBObject("$add", new Object[] { 20141114 }); } }).as("day").andExclude("_id"), new AggregationOperation() { @Override public DBObject toDBObject(AggregationOperationContext context) { return new BasicDBObject("$out", "dayStaInfoTmp"); } }).withOptions(options);
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