Databases
Transactions and Isolation Levels: ACID in Practice
What ACID guarantees actually mean, what isolation levels exist and what anomalies each one allows, and how to choose the right level for your use case.
Databases
Indexes: How They Work, When They Help, When They Hurt
An index is a tradeoff - faster reads at the cost of slower writes and more storage. Knowing how B-trees work and how the planner uses them tells you when that tradeoff is worth making.
Databases
How Relational Databases Work Under the Hood
What actually happens between your SQL query and the rows that come back - storage pages, the query planner, and why understanding this makes you a better developer.