While there is no limit to the number of stages used in the query, it is worth noting that the order of the stages matters and there are optimizations that can help your pipeline perform better. Each stage uses namesake operators to complete transformation so that we can achieve our goal. Implementing a pipeline helps us to break down queries into easier stages. These three stages collectively can be called an aggregation pipeline. The output from the $match stage is fed into $group and then the output from the $group stage into $sort. $match, $group, and $sort are various stages in a pipeline. In the above example, input refers to one or more documents. Input -> $match -> $group -> $sort -> output In a pipeline, successive operations are informed by the previous result. The aggregation pipeline refers to a specific flow of operations that processes, transforms, and returns results. What is the Aggregation Pipeline in MongoDB? Moreover, starting from MongoDB version 4.4, it can also define custom aggregation expressions with $accumulator and $function. Pipeline operators need not produce one output document for every input document, but can also generate new documents or filter out documents. It is designed specifically to improve performance and usability for aggregation. The aggregation pipeline is often preferred and the recommended way of doing aggregations in MongoDB. But their biggest downside is that they lack the flexibility and capabilities of the aggregation pipeline and map-reduce. Single-purpose aggregation provides simple access to common aggregation processes like counting the number of documents within a single collection and/or returning unique documents. This method fails to provide a simple interface, requiring you to implement JavaScript within MongoDB, and thus suffers from performance overhead. Map-reduce uses custom JavaScript functions to perform the map and reduce operations. There are three ways to perform aggregation in MongoDB: Fear not: The aggregation pipeline, a multi-stage pipeline that transforms the documents into aggregated results, is here to help. When you start doing advanced queries in MongoDB, you realize that the basic find() command won’t give you the flexibility and robustness that you need.
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