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Stage 3 - Correct and Fast

You can now query and design. This stage is what makes an application production-ready: queries that stay fast as data grows, and writes that stay correct when many of them happen at once. These two topics - performance and transactions - are exactly what self-taught learners most often miss, and together they are the early-intermediate milestone.

Learning objectives

By the end of this stage you will be able to:

  • Read a query plan and tell SCAN (full table read) from SEARCH ... USING INDEX.
  • Judge selectivity to predict whether an index will actually help.
  • Design the right index - composite, covering, or partial - for a given query.
  • Explain the read-speed-versus-write-cost trade-off of every index you add.
  • Name the isolation anomalies - dirty, non-repeatable, and phantom reads - and the levels that prevent them.
  • Handle concurrency in production with deadlock retry loops and savepoints.

Map of this stage​

The lessons in this stage​

  1. Indexes and query plans - read a query plan first (live SCAN-vs-SEARCH demo) and judge selectivity, then design indexes - composite, covering, and partial - plus the N+1 / SELECT * execution mental model.
  2. Transactions and isolation - the concurrency anomalies (dirty, non-repeatable, phantom reads), the four isolation levels, how locking and MVCC enforce them, and the deadlocks, retry logic, and savepoints production code needs.
  3. Stage 3 review - cumulative challenges and a quiz that put plans, index design, and concurrency together, plus a cheatsheet to keep.
Status

All three Stage 3 lessons are ready. Transactions were introduced in Stage 1; here we go deeper on concurrency.