Important Nodes of the Query Plan Tree in PostgreSQL

The query plan is a tree structure that outlines the steps taken by a database to execute an SQL query. It consists of three classes of nodes - scan nodes, join nodes, and auxiliary nodes. Scan nodes are used for reading data from tables, while join nodes combine rows from two or more tables. Auxiliary nodes perform various operations such as sorting, aggregation, and limiting the number of returned rows. The planner selects the appropriate type of node based on the query, data size, and availability of indices.