I need to find out the duplicate data content in my 40 Millions records, then I can make the unique index to my name field.
> db.collecton.aggregate([
... { $group : {_id : "$field_name", total : { $sum : 1 } } },
... { $match : { total : { $gte : 2 } } },
... { $sort : {total : -1} },
... { $limit : 5 }],
... { allowDiskUse: true}
... );
{ "_id" : "data001", "total" : 2 }
{ "_id" : "data004231", "total" : 2 }
{ "_id" : "data00751", "total" : 2 }
{ "_id" : "data0021", "total" : 2 }
{ "_id" : "data001543", "total" : 2 }
>
{ allowDiskUse: true} is optional if your data is not huge.
{ $limit : 5 }, you can set display more data.