Many developers memorize the SQL write order—SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY, LIMIT—but this doesn't reflect how PostgreSQL actually processes a query. Understanding the true execution order is critical for writing efficient queries and debugging performance issues, especially as databases scale.

PostgreSQL compiles your query into an execution plan starting with the FROM clause, reading table data into memory. It then applies WHERE filters before grouping. This order matters: filtering rows before grouping with WHERE is far more efficient than filtering groups afterward with HAVING, which processes already-aggregated data.

Using EXPLAIN ANALYZE reveals this plan, showing sequential scans, filters, and aggregations. For example, a query filtering 1,000 rows before grouping is faster than grouping all rows first. Recognizing this distinction helps developers optimize complex queries and manage memory usage effectively.