Query Plan

The Query Planner is the "brain" of the database. It is responsible for taking a declarative SQL query string (which describes what data to retrieve) and converting it into a highly optimized, imperative plan (which describes how to retrieve that data).

This process is one of the most complex and important aspects of a relational database system.

Core Concepts

• AST (Abstract Syntax Tree): The raw SQL text is first parsed into a tree structure that represents the syntax of the query.

• Logical Plan: The AST is then converted into a logical plan. This is a tree of relational algebra operators (e.g., Filter, Projection, Join) that describes the logical steps required to fulfill the query, independent of any specific algorithm or data layout.

• Physical Plan: The logical plan is then converted into a physical plan. This plan consists of concrete operators (or "iterators") that implement specific algorithms (e.g., NestedLoopJoin, SeqScan). This is the plan that is actually executed.

This section contains:

• The Lifecycle of a Query: A deep dive into the journey from SQL string to executable physical plan.