Learning what the optimizer does

Before even attempting to think about query performance, it makes sense to familiarize yourself with what the query optimizer does. Having a deeper understanding of what is going on under the hood makes a lot of sense because it helps you see what the database is really up to.

A practical example – How the query optimizer handles a sample query

To demonstrate how the optimizer works, I have compiled an example. It is something that I have used over the years for PostgreSQL training. Let's assume that there are three tables, as follows:

CREATE TABLE a (aid int, ...);         -- 100 million rows
CREATE TABLE b (bid int, ...);         -- 200 million rows 
CREATE TABLE c (cid int, ...);         -- 300 million rows 

Let's further assume that those tables contain millions, or maybe hundreds of millions, of rows. In addition to that, there are indexes:

CREATE INDEX idx_a ON a (aid); 
CREATE INDEX idx_b ON b (bid); 
CREATE INDEX idx_c ON c (cid...