Every PostgreSQL performance conversation eventually lands on a question that sounds trivial: what does this EXPLAIN mean? The output is almost readable. There are node names in English, numbers that look familiar, and enough structure that you can guess at the intent. But if you're guessing, you're going to miss the signal that actually matters — and the difference between a plan that returns in 0.3 ms and one that returns in 400 ms is often one line of EXPLAIN output that looks like boilerplate. This article is a systematic walk through how to read an EXPLAIN plan on PostgreSQL 17, using real output captured from a live database. By the end you should be able to look at a plan, identify what each node is doing and why, spot the three places where things usually go wrong, and articulate in one sentence why the query is slow — or whether it's actually fine and something else is wrong. This is part of the Complete Guide to PostgreSQL SQL Query Analysis & Optimization series.…