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What are the three methods of aggregation in MongoDB?
MongoDB provides three ways to perform aggregation: the aggregation pipeline, the map-reduce function, and single purpose aggregation methods. MongoDB’s aggregation framework is modeled on the concept of data processing pipelines. Documents enter a multi-stage pipeline that transforms the documents into an aggregated result.
How to start an aggregation pipeline in MongoDB?
Getting an aggregation pipeline started is a simple affair: simply call the aggregate function on any collection. Let’s start by using our customers entity from the previous article on validations: Our customers collection contains any number of customers, each with a similar data format.
How to optimize query performance in MongoDB?
When you need only a subset of fields from documents, you can achieve better performance by returning only the fields you need: For example, if in your query to the posts collection, you need only the timestamp, title, author, and abstract fields, you would issue the following command:
When to use find ( ) command in MongoDB?
When you start with MongoDB, you will use the find () command for querying data and it will probably be sufficient, but as soon as you start doing anything more advanced than data retrieval, you will need to know more about the MongoDB aggregation pipeline.
When to use the$ out operator in MongoDB?
The $out stage must be the last stage in the pipeline. The $out operator lets the aggregation framework return result sets of any size. The $out stage has the following syntax: Starting in MongoDB 4.4, $out can take a document to specify the output database as well as the output collection: The output database name.
When does aggregate return an empty cursor in MongoDB?
A cursor to the documents produced by the final stage of the aggregation pipeline operation, or if you include the explain option, the document that provides details on the processing of the aggregation operation. If the pipeline includes the $out operator, aggregate () returns an empty cursor.
How to indicate the default batch size in MongoDB?
To indicate a cursor with the default batch size, specify cursor: {}. To indicate a cursor with a non-default batch size, use cursor: { batchSize: }. For more information about the aggregation pipeline Aggregation Pipeline, Aggregation Reference, and Aggregation Pipeline Limits.