Suppose a client sends SELECT * FROM users WHERE id = 1 . The path that single line travels before coming back as a result row is longer than you might expect. Inside the PostgreSQL backend, that SQL goes through a five-stage pipeline. Backend entry and dispatch . The backend receives the message from the client and decides which processing path it should follow. Parser and Analyzer . The SQL text is parsed, and the catalog is consulted to give it meaning. Rewriter . The RULE system expands views, injects RLS policies, and otherwise transforms the query tree. Planner . Using a cost model, it explores possible execution paths and picks the best one. Executor . It walks the chosen plan, pulling tuples up and sending them to the client. Chapter 1 of this book unfolds this pipeline one chapter at a time. The chapter you're reading now covers stage 1: the entry point. It has three sections.…