Introduction to MongoDB collection.dataSize() Method

dataSize() is a method in MongoDB used to return the size of all documents in a collection or index in bytes. The method can be used to quickly understand the space occupied by a collection or index.

Syntax

The syntax of the dataSize() method is as follows:

db.collection.dataSize()
db.collection.dataSize({ query })

Here, db.collection refers to the name of the collection, and {query} is an optional parameter used to specify filter criteria.

Use cases

Using the dataSize() method can help us quickly understand the size of all documents in a collection or index. The method may be particularly useful in the following situations:

  • When we need to understand the size of a collection or index to perform space management.
  • When we need to compare the sizes of two collections or indexes, we can use this method to make comparisons.
  • When we need to understand the size of a query result set, we can use this method to obtain information about the size of the result set.

Examples

Assuming we have a collection named users containing the following documents:

{ "_id" : 1, "name" : "Alice", "age" : 25 }
{ "_id" : 2, "name" : "Bob", "age" : 30 }
{ "_id" : 3, "name" : "Charlie", "age" : 35 }

We can use the dataSize() method to obtain the size of the entire collection as follows:

> db.users.dataSize()
48

The above command returns the size of the entire collection in bytes. In the example above, the size of each document is 16 bytes (12 bytes for the ID, 5 bytes for the name, and 4 bytes for the age), so the size of the entire collection is 48 bytes.

We can also use query criteria to obtain the size of a collection of documents that meet certain conditions, as shown below:

> db.users.dataSize({age:{$gt:30}})
32

The above command returns the size of the collection of documents where the age field is greater than 30 in bytes. In the example above, there are two documents that meet the criteria, each document has a size of 16 bytes, so the size of the collection is 32 bytes.

Conclusion

The dataSize() method is a method in MongoDB used to obtain the size of all documents in a collection or index. It can help us quickly understand the space occupied by a collection or index. The size of the collection of documents that meet certain conditions can be obtained by passing query criteria, and it is important to note that the unit of size returned is bytes.