PostgreSQL Query Rewriting Techniques

The previous articles in this series covered performance problems you fix by adding indexes, restructuring joins, or tuning memory. This one is about the queries where the plan is "fine" — every node is doing something reasonable — but the query itself is asking the wrong question, producing unnecessarily large intermediate results or forcing the planner down a path that a different SQL shape would avoid.

These rewrites don't change what the query returns. They change how PostgreSQL goes about computing it. Learn to recognise the patterns and most of them are mechanical — if the original form matches X, rewrite to Y — and the performance improvement is often an order of magnitude or more with no downside.

This article is the seventh in the Complete Guide to PostgreSQL SQL Query Analysis & Optimization series.…