In query optimization, what role do compound indexes play?

Compound indexes are a powerful aspect of query optimization in MongoDB. They are designed to enhance the performance of queries that filter or sort on multiple fields simultaneously. When a compound index is created on a combination of fields, MongoDB can use it to quickly locate documents that match a specific query that includes those fields, resulting in faster retrieval times.

For example, if you have a collection of documents with fields like "category," "price," and "date," a compound index on these fields allows for efficient querying when you need to filter by category and sort by price. This significantly improves the performance compared to using single-field indexes, as those would require separate lookups and potentially more resource-intensive operations to combine the results.

In contrast, while compound indexes can limit the ability to use single-field indexes optimally, this is not their primary role. They do not need to be mandatory for all queries, and they are not limited to functioning exclusively with text searches; they can enhance performance across various types of queries involving multiple fields. Therefore, their main advantage lies in improving the performance of multi-field queries.