why are composite indexes more effective to describe global cities?

Note: The primary key of a table is automatically indexed.

Adding records by using an append query or by appending imported records is also likely to be slower if the destination table contains indexes. When you enter data in a table that contains one or more indexed fields, Access must update the indexes each time a record is added or changed. Indexes can speed up searches and queries, but they can slow down performance when you add or update data. You'll probably want to index fields that you search frequently, fields that you sort, and fields that you join to fields in other tables in multiple table queries.

You can create indexes that are based on a single field or on multiple fields. In this way, using an index can be considerably faster than scanning through all of the records to find the data. After Access obtains the location from the index, it can then retrieve the data by moving directly to the correct location. An index stores the location of records based on the field or fields that you choose to index. You can use an index to help Access find and sort records faster. Performance of a web database depends on the performance of several factors such as the SharePoint server that is hosting the web database. Note: You cannot use the methods described by this article to create an index for a table in a web database.

Why are composite indexes more effective to describe global cities?