Storage Engines
Rule: the database does not read rows — it reads fixed-size pages. Almost every "slow query"
Replication & Sharding
Rule: a replica is a different point in time, and a shard key is a decision you cannot undo.
Session Consistency: Read-Your-Writes, Monotonic Reads & Causal Order
Rule: "eventually consistent" is not a promise you can hand a user. Each read path has to
Connection Pools & Latency
Rule: a database does not scale with connection count, and a 10 ms query does not make a 10 ms
Locks, Deadlocks & Long Transactions
Rule: a transaction holds its locks until it commits, so the only reliable way to reduce
Indexes & Query Plans
Rule: an index that exists is not an index that is used. A query is fast in development
Schema Migrations Under Load
Rule: a migration's danger is how long it holds its lock, not how long it runs. A lock request