Interview Guides
Join choice affects both correctness and performance in analytical and operational queries. In a high-concurrency PostgreSQL system such as one serving Revolut app and ledger-facing analytics, using the wrong join can either hide missing relationships or return more rows than intended.
Explain the difference between an INNER JOIN and a LEFT JOIN. You should describe how each join type determines which rows appear in the result, how unmatched rows are represented, and how NULL values affect downstream filtering and aggregation. Then discuss whether high concurrency changes the logical behavior of these joins, and what practical performance considerations still matter in a busy database.
Keep your answer focused on SQL semantics first, then add a short discussion of PostgreSQL execution behavior under load. The interviewer is looking for a clear explanation, not deep optimizer internals.
