How does the existence of a compound index affect queries?

The presence of a compound index significantly enhances query performance by allowing queries to utilize multiple fields effectively. A compound index consists of multiple fields indexed together, which means that when a query specifies values for those fields, it can leverage the index to quickly locate the relevant documents without needing to scan the entire collection.

By utilizing multiple fields, the compound index can improve the efficiency of complex queries that filter data based on several criteria simultaneously. When queries make use of the fields defined in a compound index, MongoDB can optimize the search process, reducing the number of documents it needs to examine and improving response times.

While the order of the fields in the index does impact how queries can utilize the index, the key advantage conveyed here is the ability to handle queries that search on multiple fields, which is a primary feature of compound indexes. This capability allows for more optimized and efficient query execution compared to using single-field indexes or no index at all.