How many array fields can be included in a single index?

In MongoDB, a single index can include only one array field at a time. When you index an array, MongoDB creates an index entry for each element of the array independently. This means that if you have an array field in a document, the index will reflect all the elements of that array, but you cannot include multiple array fields within a single index definition.

This limitation exists because having multiple array fields in the same index would complicate the indexing structure and lead to ambiguity in how the index should function when querying. Each indexed element from an array can still be optimized for efficient search, but it must be done in isolation within a single index entry per array field. Therefore, when designing indexes for documents that contain arrays, it is important to consider whether to create separate indexes for different array fields as needed.