N1QL is a modern query processing engine developed for Couchbase's distributed architecture, designed to provide SQL capabilities for document-based data models like JSON. The blog discusses the query planning phase in N1QL, detailing how queries are executed on Couchbase's clustered database system. N1QL queries are submitted to query nodes, which analyze and create optimal execution plans using metadata and indices, with operations performed in parallel for efficiency. The query execution process includes keyscan, primary scan, and index scan access methods, with nested loop access for joins. The blog highlights the importance of access path selection, join methods, and order in query planning, offering examples of how different access methods and indices influence performance. The planner decides on access paths, join order, and types, executing some operations in serial and others in parallel. Examples demonstrate the use of keyspace indices and join operations to efficiently retrieve and manipulate data, emphasizing the significance of understanding document keys and data modeling in constructing effective N1QL queries.