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Reading execution plans, statistics and cardinality estimation, and diagnosing real regressions.
3 questions in Databases & SQL.
Rank by the total time a statement shape consumes across a measured window rather than by the slowest single execution, then check what that time was spent waiting on before touching the SQL. The cheap statement running ten thousand times a minute usually outranks the ten-second report.
Read the plan tree innermost-node-first, remember that EXPLAIN only estimates while EXPLAIN ANALYZE executes, multiply each node's actual rows by its loops, and treat a large gap between estimated and actual rows as the primary finding.
When the same query is fast in psql but slow from the application, first separate database time from connection wait, network transfer and ORM hydration. Then compare the real application plan, bind parameters, session settings, fetch size and result volume.