Couchbase Flex Index is a new feature that allows N1QL queries to leverage FTS indexes without modifying the query syntax, providing flexibility and versatility for search capabilities. It enables applications to access both exact search services from a single API using standard N1QL predicates, simplifying application development and delegating more of the search processing to the back-end services. Flex Index can solve many challenges found in applications that provide searches, such as handling varied predicate sets, logical operators, hierarchical document elements, and combining SQL aggregation and JOIN with FTS capabilities. It also allows developers to use N1QL predicate syntax over FTS syntax, making it easier to integrate search capabilities into existing applications. The Flex Index feature can be used retrospectively on existing applications by adding the `use_fts` parameter to query API calls, and its benefits include reduced index size compared to GSI indexes, especially when array elements are involved. Query performance optimization is available only with GSI, but Flex Index provides pagination and JOIN capabilities, making it a powerful tool for building search-enabled applications.