The document discusses testing the limits of MySQL by creating an extremely long SQL query with over 4,000 "UNION ALL" selections of the value "b". This exceeded the maximum length of SQL queries that MySQL servers can execute, which is defined by the max_allowed_packet system variable and has a maximum value of 1GB. Creating such a long meaningless query demonstrated that MySQL has limitations on
![SQL 脳から見た Ruby](https://cdn-ak-scissors.b.st-hatena.com/image/square/4f72da7df936463d850bfd2e9b84eb0b66219c97/height=288;version=1;width=512/https%3A%2F%2Fcdn.slidesharecdn.com%2Fss_thumbnails%2Frubyseenfromsqlishbrain1468-151108075250-lva1-app6892-thumbnail.jpg%3Fwidth%3D640%26height%3D640%26fit%3Dbounds)