The Estimate That Said One: Query Optimizers, Cost Models and Cardinality Estimation

Deep End Era. Engineering with Sankalp · Oct 28, 2026 · 59m

At ten past six on a Sunday, data platform lead Ingrid Valtonen finds a weekly seven-table report that ran in forty seconds stuck for over an hour. Nothing in the SQL changed; one nightly back-fill did. In episode one hundred and twenty one we go inside the query optimizer. We start with the declarative bargain and the taxi driver who picks the route, then System R's 1979 recipe: a cost formula, catalogue statistics, dynamic programming over join orders, interesting orders and left-deep trees. We size the search space, meet PostgreSQL's genetic optimizer and its collapse limits, and the three…