MySQL

Sharding adds an order of magnitude of complexity. Exhaust the cheaper options first — split the DB onto its own host, add a cache, add read replicas, scale the box up — and only shard when those run out.

How to create a least-privilege MySQL account for analytics or support — and the specific grants, host restrictions, and cleanup steps that keep it least-privilege over time.

ALGORITHM=INPLACE, LOCK=NONE lets DDL run while reads and writes continue. Works for many but not all alterations — knowing which is the difference between zero downtime and a stuck production.

For matching a column against several values, IN beats OR by orders of magnitude (binary search vs linear), and BETWEEN beats IN when the values form a contiguous range.

How to read the EXPLAIN plan column by column — type, key, rows, filtered, Extra — what the values mean, what to fix, and how the driving table cascades through JOINs.

JOIN multiplies rows then dedupes; EXISTS short-circuits at the first match. For pure filtering, EXISTS is the semantically correct choice — no DISTINCT, no optimizer roulette.

The logical order MySQL runs your SELECT — FROM/JOIN, WHERE, GROUP BY, HAVING, SELECT, ORDER BY/LIMIT — and the optimization hook each stage gives you.