sql query optimization is the single skill that separates the engineer who writes a query from the one who ships it: a 30-second SELECT that returns the right rows is still a production incident, and the discipline of reading an explain plan , picking the right index types ( b-tree index , hash, partial, covering), recognising which of the three join algorithms ( nested loop , hash join , merge join) the planner will choose, and then rewriting SARGable predicates is what turns 30 seconds into 300 milliseconds. The senior round is rarely "do you know JOIN " — it is "show me the plan, find the bottleneck node, and tell me the one change that will move the needle".…