OLIVER When a UNION or UNION ALL query in PostgreSQL returns rows with unexpected NULL values in columns that clearly have data in the underlying tables, the cause is almost always a mismatch between the columns being combined — not a bug in PostgreSQL itself, which handles UNION very literally. The Problem You combine results from … Read more
Having an index on a column doesn’t guarantee PostgreSQL will actually use it, or that using it will make your query fast. When a query stays slow despite an index existing on the exact column being filtered or joined on, the planner usually has a good — if non-obvious — reason for its decision. The … Read more
The “too many connections” error in PostgreSQL means the server has hit its configured connection limit and is refusing new clients. It’s rarely caused by genuinely needing more simultaneous users than the default allows — far more often, it’s a sign that connections aren’t being closed properly somewhere in your application. The Problem New connection … Read more
A “connection refused” error between two containers on the same Docker network almost always means the PostgreSQL container isn’t reachable yet, isn’t listening where you expect, or the requesting container is looking in the wrong place entirely. Unlike a “connection refused” from a completely wrong host, this version is deceptively close to working, which makes … Read more
Resetting a PostgreSQL password inside a Docker container is a bit different from a normal installation, because you’re working through docker exec rather than a local shell, and the common instinct to just delete and recreate the container can wipe out your data if you’re not careful about volumes. The Problem You’ve lost or forgotten … Read more
If you change a file in your project but docker build keeps producing an image with the old content, Docker isn’t actually ignoring your change — it’s reusing a cached layer that it (incorrectly, from your point of view) still considers valid. This is one of the most common sources of “why isn’t my fix … Read more
When a containerized application needs to reach a PostgreSQL server running directly on your host machine (not inside another container), connection failures are almost always caused by networking assumptions that don’t hold true across the container boundary — not by PostgreSQL itself being broken. The Problem Your app runs fine locally, connecting to PostgreSQL on … Read more
Getting a “permission denied” error when a container tries to read or write to a mounted volume is one of the most common friction points when running Docker on Linux. It almost always comes down to a mismatch between the user ID running inside the container and the ownership of the files on the host, … Read more
If your Docker container starts and then exits immediately without printing any error, it usually means the main process inside the container finished running or crashed silently. This is one of the most common issues developers hit when containerizing an application for the first time, and it can be confusing precisely because there’s often nothing … Read more
When containers in a Docker Compose setup can’t reach each other by name — for example, your app container failing to connect to a database container using its service name — the cause is almost always related to how Compose networks and DNS resolution work, not a broken Docker installation. This is one of the … Read more