The Index Advisor for N1QL Statement (ADVISE statement) in Couchbase Server 6.6 is designed to provide global secondary index recommendations for each keyspace involved in a query, aiming to improve query performance by recommending better indexes than the existing ones. The advisor makes best efforts to recommend covering indexes with higher pushdown properties, such as GROUP BY/aggregate, ORDER BY, and limit/offset, which can further optimize query performance. The verification framework ensures the correctness and robustness of the index recommendations, while the support for cost-based query optimizer will be available in a future release. Additionally, the advisor takes into account the complexity of nested objects in ARRAY indexes and provides innovative features such as virtual index verification to guarantee the correctness of covering index recommendations.