mongodb内存不足怎么解决?[mongodb教程]

python

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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