How Well a Query Optimizer Handles Subqueries?

At the beginning of December, at the UKOUG Tech17 conference in Birmingham (GB), I presented a comparison of the query optimizers of MySQL 8.0.3 and PostgreSQL 10.1. One of the things I talked about is their ability to handle subqueries. I summarized my findings with the following sentence:

Simple sub-queries that are not correctly optimized were observed.

It goes without saying that such a sentence leaves a lot of questions open. After all, it is just a summary. The aim of this post is to show you which subqueries I tested, and to compare my expectations with the execution plans generated by the query optimizers. In addition, since I’m not limited in time and scope as during a 50-minute presentation, I also discuss how the Oracle Database 12.2 query optimizer handles the same queries.