A brief analysis of group grouping in MongoDB

This article mainly introduces the implementation method and example of the group grouping in MongoDB, it is very simple and practical, the need of small partners can be consulted.

Group aggregation is somewhat complex. The key that the group is based on is selected, and MongoDB then divides the collection into groups based on the selected key value. You can then generate a result document by aggregating the documents within each group.
As with databases, group is often used for statistics. MongoDB Group also has many restrictions, such as: The return result set can not exceed 16M, the group operation will not handle more than 10,000 unique keys, as if the index is not available [not very sure].

First, the group needs a few parameters.

1.key: The field used to group the document. And Keyf both must have a
2.KEYF: You can accept a JavaScript function. The field used to dynamically determine the grouped document. And key both must have a
Initialization of variables used in 3.initial:reduce
4.reduce: The reduce function is executed. The function needs to return a value.
5.cond: The condition to perform the filter.
6.finallize: At reduce execution completion, the result set returns the function that was last executed on the result set. Optional.

Second, an example is described below:

Insert test data first:for (var i=1; i<20; i++) {var num=i%6;db.test.insert ({_id:i,name: "User_" +i,age:num}); 1. General Group Query db.test.group ({                        key:{age:true},                        initial:{num:0},                         $reduce: Function (Doc,prev) {                                prev.num++                       }                      });  db.runcommand ({group:{ns: "Test", key : {age:true},initial:{num:0}, $redUce:function (Doc,prev) {prev.num++}}});   2. Filter and then groupDb.test.group ({key:{age:true},initial:{num:0}, $reduce: function (doc,prev) {prev.num++},condition:{age:{$gt: 2}}) ; Db.runcommand ({group:{ns: "Test", key:{age:true},initial:{num:0}, $reduce: function (Doc,prev) {prev.num++}, condition:{age:{$GT: 2}}}); 3, the ordinary $where query:Db.test.find ({$where: function () {return this.age>2;}}); Group Federated $where QueryDb.test.group ({key:{age:true},initial:{num:0}, $reduce: function (doc,prev) {prev.num++},condition:{$where: function () {return this.age>2;}}); 4. Grouping using function return values //Note that $KEYF the specified function must return an objectDb.test.group ({$keyf: function (DOC) {return {age:doc.age};},initial:{num:0}, $reduce: function (doc,prev) {prev.num++ }}); Db.runcommand ({group:{ns: "Test", $keyf: function (DOC) {return {age:doc.age};},initial:{num:0}, $reduce: function (doc , prev) {prev.num++}}); 5. Using finalizersDb.test.group ({$keyf: function (DOC) {return {age:doc.age};},initial:{num:0}, $reduce: function (doc,prev) {prev.num++ },finalize:function (DOC) {Doc.count=doc.num;delete doc.num;}}); Db.runcommand ({group:{ns: "Test", $keyf: function (DOC) {return {age:doc.age};},initial:{num:0}, $reduce: function (doc , prev) {prev.num++},finalize:function (doc) {Doc.count=doc.num;delete doc.num;}});

About MapReduce

Insert test Data First
for (Var i=1;i<21;i++)
{
Db.test.insert ({_id:i,name: ' mm ' +i});
}

For MapReduce
Db.runcommand (
{
MapReduce: ' Test ',
Map:function () {Emit (This.name.substr (0,3), this);},
Reduce:function (key,vals) {return vals[0];},//Note: Vals is an object instead of an array
Out: ' Wq '
});

Note:
1.mapreduce is grouped according to the first parameter of the emit function called in the map function.
2. Key and document collections are processed by the reduce function only if a key matches multiple documents based on the grouping key. For example:
Db.runcommand (
{
MapReduce: ' Test ',
Map:function () {Emit (This.name.substr (0,3), this);},
Reduce:function (key,vals) {return ' Wq ';},
Out: ' Wq '
});
after executing the mapreduce command, review the WQ table data:
Db.wq.find ()

{"_id": "MM1", "Value": "Wq"}
{"_id": "mm2", "Value": "Wq"}
{"_id": "Mm3", "value": {"_id": 3, "name": "Mm3"}}
{"_id": "MM4", "value": {"_id": 4, "name": "Mm4"}}
{"_id": "MM5", "value": {"_id": 5, "name": "MM5"}}
{"_id": "MM6", "value": {"_id": 6, "name": "Mm6"}}
{"_id": "MM7", "value": {"_id": 7, "name": "Mm7"}}
{"_id": "MM8", "value": {"_id": 8, "name": "MM8"}}
{"_id": "Mm9", "value": {"_id": 9, "name": "Mm9"}}

A brief analysis of group grouping in MongoDB