# 🔍 What Actually Happens When PostgreSQL Executes a SELECT Query?

<p><em>B-tree internals, column order and planner statistics in Postgres 19, checked against MongoDB's ESR rule.</em></p> <p>Most "add an index" advice stops at <code>CREATE

<h2> I Thought My Index Would Fix Everything </h2> <p>I added the index.</p> <p>Ran the query.</p> <p>And… nothing changed.</p> <p>Same slow response. Same frustration.</p>

<p>PostgreSQL index design is often misunderstood.</p> <p>Many developers think that creating a good index simply means:</p> <blockquote> <p>“Create an index containing the

<p>If your app has a slow endpoint and you're staring at application logs trying to guess which query is the culprit, stop. Postgres already tracked every query it ran, how long

<p><strong>TL;DR:</strong> Postgres has <code>pg_stat_user_indexes</code>. It tells you how many times each index was scanned. It does <em>not</em> tell you whether the slow query

<p>At 2:14 PM on a Tuesday in Q3 2024, our production PostgreSQL 16 cluster hit a p99 query latency of 2.8 seconds for our core order lookup endpoint, triggering 12 customer

<p>PROFESSIONAL DJANGO ENGINEERING SERIES #6</p> <blockquote> <p>If you have never read a query execution plan, you are flying blind on database performance. Here is how to read