Query Optimization & Performance covers: Essential PostgreSQL, Learn PostgreSQL. Master the art and science of making databases fast. Indexing strategies, query plan analysis, connection pooling, partitioning, and the PostgreSQL-specific performance toolkit. This is the most directly career-impactful module in the entire curriculum.
N+1 queries, counter caches, database-level constraints vs model validations, advisory locks, SELECT FOR UPDATE — these are the daily performance decisions of a Rails engineer. After this module, you understand why includes fixes N+1 (it generates a second query with WHERE id IN (...) or a LEFT OUTER JOIN), why counter_cache exists (because COUNT(*) in PostgreSQL must scan all visible tuples due to MVCC), why find_each exists (because loading 1M records into memory kills your process), and why strong_migrations rejects add_index without algorithm: :concurrently (because CREATE INDEX takes an exclusive lock on the table).
This course unlocks once you've finished its prerequisite. Open prerequisite →
This module has the highest interview ROI in the entire databases curriculum. "How would you optimize this slow query?" is the number one database question in Rails interviews. It…
"The query is slow" is the most common production complaint a Rails engineer hears, and most fixes are applied the way you'd fix a vending machine — hit it with an index and see i…
An index on (userid) is the index everyone adds. A partial index on WHERE state = 'pending' that is one-fiftieth the size, a covering index that answers the query without touching…
Work through each item before the checkpoint.
Nine deliberately slow queries over a 150,000-row dataset — N+1s, missing indexes, non-sargable predicates, leading wildcards, deep OFFSET pagination. Your job is to cure each one…
5 lessons. Read in order; spiral back when you need to. By the end you'll have used the core ideas twice — once on the abstract, once on something you'll meet at work next week.