Databases

Database concepts for backend engineers: storage internals, transaction semantics, and operational patterns.

Indexes	B-tree, hash, GIN/GiST, partial, composite — when each wins
Transactions	ACID guarantees, isolation levels, phantom reads, MVCC
Connection pooling	Sizing the pool, pgBouncer, connection overhead
Query planning	EXPLAIN ANALYZE, index scan vs seq scan, statistics
Migrations	Rolling migrations, backward compatibility, zero-downtime