Databases

Tables, keys, and the rules that keep data sane.

SELECT everything you need to know.

From full scan to seek: how engines find rows fast.

ACID, isolation, and what can go wrong when two queries run at once.

Read replicas, sharding, pools, and caches — when one server is not enough.

EXPLAIN, plan reading, and the rewrites that beat the planner.

Picking by access pattern, not by hype.

Expand-contract, online schema-change, and blue/green for the schema itself.

Embeddings, HNSW, and when to add a vector store vs cram into Postgres.