The Data Layer

Index Scan vs Seq Scan: Rows Examined

Count rows examined for both plans and say which one the numbers favour.

Composite Indexes & the Leftmost Prefix

Say which predicate sets a composite index can serve, and which it only filters.

The Cost Model Behind EXPLAIN

Price an index scan against a seq scan and predict the planner's choice.

Cursor Pagination vs OFFSET

Replace OFFSET with a keyset cursor and quantify what it saves.

Isolation Levels & the Anomalies They Permit

Name which anomaly each isolation level permits, and prove it with a trace.

Transactions, Rollback & Prepared Statements

Implement commit and rollback, and say what a prepared statement actually reuses.

Connection Pools & Where Latency Goes

Read queue wait out of a pool simulation and size a pool for a reason.