Question: What is MongoDB clustered index?

Answer

MongoDB, a NoSQL database, uses a variety of indexing strategies to optimize query performance. Unlike traditional relational databases, MongoDB does not use the term 'clustered index' in the same way. However, understanding how MongoDB handles data storage and indexing can clarify the concept.

In relational databases, a clustered index determines the physical order of data within a table. Each table can have only one clustered index, and it significantly impacts data retrieval speed because it dictates how data is stored on disk.

MongoDB stores data in BSON documents, which are grouped into collections. It does not store these documents on disk in a sorted order based on a particular field or index. Instead, MongoDB uses a B-tree-based indexing mechanism to improve query performance. When you create an index on a collection, MongoDB uses that index to quickly locate documents but does not reorder the physical documents on the disk based on that index.

The closest concept to a 'clustered index' in MongoDB might be the _id field that each document has. This field is automatically indexed, and while it doesn't dictate the physical storage order of documents (since MongoDB's storage model differs from that of relational databases), it is unique for each document and commonly used as a primary identifier.

If your concern is about optimizing read performance by ensuring data is stored close together on disk, MongoDB provides a feature called 'prefetching' to load data into memory ahead of time, and the WiredTiger storage engine (the default as of MongoDB 3.2 and later) efficiently manages data storage and access patterns.

For example, to create an index on a username field in a users collection, you would use the following command:

db.users.createIndex({username: 1});

This creates a single field ascending index on the username field, which helps MongoDB quickly find documents based on username. However, it doesn't reorder documents on disk like a clustered index in SQL databases would.

In summary, while MongoDB does not use clustered indexes as SQL databases do, its indexing capabilities and storage engine optimizations ensure efficient data retrieval without the need for physically reordering documents on disk.