Backend & systems·Data modelling and storage engines
you run EXPLAIN and find out the database chose a sequential scan over the index you added last week.
Query planner
Also calledquery optimiser, execution plan, EXPLAIN
The component that turns your declarative query into a physical plan, choosing join order, access methods and algorithms based on table statistics. It is only as good as those statistics, which is why a plan can flip catastrophically after a bulk load and before the next ANALYZE. Reading plans is the single highest-leverage database skill; guessing at query performance without one is superstition.