PostgreSQL
The Database Migration That Locked the Table for Nine Minutes
Adding a column should be instant. Sometimes it is not, and the difference is worth knowing before you run it in production.
Databases
Finding the N+1 Queries You Do Not Know You Have
The code reads beautifully and issues four thousand queries. Here is how to detect them automatically instead of discovering them in production.
PostgreSQL
FATAL: sorry, too many clients already
Connection pooling explained through the failure it prevents, and why raising max_connections is usually the wrong fix.