Reading a query plan

EXPLAIN QUERY PLAN prints the strategy: which tables are scanned, which indexes are used and in what order tables are joined. Postgres and MySQL use EXPLAIN / EXPLAIN ANALYZE for the same thing.

The rewrites that pay off most: stop wrapping filtered columns in functions, filter before joining rather than after, select only the columns you need, and replace a correlated subquery that runs per row with a single join or window function.

Measure before and after on realistic data volumes. On tiny tables a scan is often the fastest plan and every optimisation looks pointless.