MongoDB Cursor with Examples

Question: What is cursor in MongoDB?
Cursor is a pointer to the result set of a query.
When you run a query for documents we retrieve a pointer (cursor) to the results instead of all result return.


Question: Give example of Cursor?

var myCursor=db.users.find({"age":30});
while (myCursor.hasNext()) {
   print(tojson(myCursor.next()));
}

Question: How to get the data type of variable?

var data=10;
typeof data;

var data='10';
typeof data;

Output

number
string

Question: What is use of forEach with Cursor?
Iterates the cursor to each document.
Example 1

db.users.find({"age":30}).forEach(printjson);

Example 2

db.users.find({"age":30}).forEach(function(data){
    print(data.name);
});

Output

name1
name2
name3

Question: How to use limit with Query?

 
db.users.find({"age":30}).limit(5).pretty();

Now, cursor will return maximum 5 records.


Question: How to get results in Array instead of JSON?

 
db.users.find({"age":30}).toArray()

Question: How to get number of results from Query?

 
db.users.find({"age":30}).size(); //2

Question: How to sort records by name in Ascending order?

 
 db.users.find({"age":30}).sort({"name":1}).forEach(function(data){print(data.name);});

Question: How to sort records by name in Descending order?

 
 db.users.find({"age":30}).sort({"name":-1}).forEach(function(data){print(data.name);});

Question: How to skip 4th record from query?

 
 db.users.find({"age":30}).skip(4).forEach(function(data){print(data.name);});

Question: How to search records with indexing?

 
 db.users.find({"age":30}).hint({"age":1}).pretty();  

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What is sharding in MongoDB?

Question: What is sharding in MongoDB?
Sharding is a method for distributing data across multiple machines and its MongoDB's approach to meeting the demands of data growth. With increase of the size of the data, a single machine may not be sufficient to store the data OR read and write data. to fix this problem there are two different methods.
Vertical Scaling: Increase the capacity of single server and increase the power.

• Powerful CPU
• Increase RAM
• Increase storage

Horizontal Scaling: dividing the system dataset and load over multiple servers.
We can use multiple server for storing and read/data data.
Using multiple server is lower cost than single machine with high-end hardware but increase the complexity in infrastructure and maintenance for the deployment.


Question: What are MongoDB Query Capabilities?

• Sorting on one/multiple fields with ascending/descending order.
• Projection (Retrieving only specified fields from each document in the cursor, not the entire document).
• Cursor skip() and limit() to easily implement pagination.
• explain() will return the detail on query will help to optimize the query.
• aggregate() function used for grouping the records. db.users.aggregate([{$match:{gender:"male"}}, { $group: { _id: null, count: { $sum: 1 } } }]);
• $match and $unbind function.

Question: How to database details?

db.stats();

Output

{
  "db": "bank",
  "collections": 4,
  "objects": 12,
  "avgObjSize": 245.33333333333334,
  "dataSize": 2944,
  "storageSize": 28672,
  "numExtents": 4,
  "indexes": 2,
  "indexSize": 16352,
  "fileSize": 67108864,
  "nsSizeMB": 16,
  "extentFreeList": {
    "num": 0,
    "totalSize": 0
  },
  "dataFileVersion": {
    "major": 4,
    "minor": 22
  },
  "ok": 1
}

Question: How to collection details?

db.users.stats();

Output

{
  "ns": "bank.users",
  "count": 5,
  "size": 2480,
  "avgObjSize": 496,
  "numExtents": 1,
  "storageSize": 8192,
  "lastExtentSize": 8192,
  "paddingFactor": 1,
  "paddingFactorNote": "paddingFactor is unused and unmaintained in 3.0. It remains hard coded to 1.0 for compatibility only.",
  "userFlags": 1,
  "capped": false,
  "nindexes": 1,
  "totalIndexSize": 8176,
  "indexSizes": {
    "_id_": 8176
  },
  "ok": 1
}

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