Database indexing, particularly in systems like Couchbase, is a critical technique for optimizing query performance and ensuring efficient data retrieval. Asynchronous indexing, such as Global Secondary Indexes (GSI), allows indexes to be managed without disrupting ongoing operations, enhancing system isolation and scalability. Best practices include running index services on dedicated nodes to improve performance and understanding the differences between Memory-Optimized Indexes (MOI) and Standard GSI to select the appropriate index type based on use-case requirements. It's advised to use index replicas for load balancing and redundancy, and to leverage different types of indexes, such as covered, array, and partial indexes, to tailor database performance to specific query needs. Avoiding primary indexes in production, using EXPLAIN plans to validate index usage, and choosing the right consistency options are key strategies for maintaining performance and data integrity. Monitoring index catch-ups and employing deferred builds can further optimize indexing processes, while the USE KEYS directive provides an alternative to indexing by directly querying documents using keys.