How good are query optimizers, really? Leis et al. asked this exact question in 2015. Then, they asked it again 10 years later . Despite an enormous body of research spanning a decade since their original exploration, they found that query optimizers continue to leave much to be desired. I was surprised when I first learned about this. A Postgres database should know everything about the stuff that lives in its tables, no? How hard can it be? As it turns out: enormously hard. In fact, one particular task a query optimizer needs to do, join ordering, is known to be NP-hard . So query optimizers are hard. What’s not as hard is verifying whether a query plan an optimizer picks is good or not. Put simply, a good query optimizer produces plans that run fast, and a bad one produces slow plans. Language models are particularly good at learning how to do tasks with easily verifiable outputs.…