How databases implement storage engines, query planners, and transactions.
Postgres EXPLAIN plans: node types, cost interpretation, and six patterns that kill query performance on large datasets.
B-tree internals, composite index ordering, GIN for full-text search, partial indexes, and preventing index bloat in production.
How PostgreSQL's query optimizer decides, why it gets it wrong, and how to fix it with statistics targets and covering indexes.
How Raft works in production: leader election, log replication, safety guarantees, and failure modes in etcd and CockroachDB.
Schema migrations on billion-row tables without downtime: expand-contract, pg_repack, gh-ost, blue-green migrations, and rollbacks.
Scale Redis: cluster sharding, hot key replication, connection pooling, pipelining, and eviction tuning for 1M+ ops/sec.