Database indexes are used to improve the performance of database queries by creating a data structure that facilitates faster lookup, insertion, and deletion operations. The main purpose of a database index is to enable efficient retrieval of specific data from a large database. Database indexes can be created on any field in a table, but not all fields should be indexed equally, as some may have varying levels of selectivity, or the frequency with which they are used in queries. The type of database also plays a role in indexing, with relational databases often using B-tree indexes and NoSQL databases sometimes using hash indexes. InfluxDB's unique approach to indexing, where data inserted as a tag is indexed while data inserted as a field is not, can provide significant performance advantages.