The Postgres EXPLAIN command is invaluable when trying to understand query performance. SQL is a declarative language, and the Postgres query planner will decide the most efficient way to execute a query. However, plan selection is based on statistics, configuration settings, and heuristics—not a crystal ball. Sometimes there's a substantial gap between what the planner thinks is most efficient and reality. In those situations, EXPLAIN can help Postgres users understand the planner's "reaso...
