planifest DB Query Optimization
Overview
Use this skill to improve query performance based on execution evidence, not intuition.
Scope Boundaries
- Hot-path latency or database CPU/IO usage is query-bound.
- Query plans are unstable across parameter distributions.
- Workload changes expose previously acceptable query anti-patterns.
Core Judgments
- Dominant bottleneck: scan cost, join explosion, sort spill, lock wait, network round trips.
- Rewrite scope: query shape, index changes, schema adjustment, or materialization.
- Plan stability and parameter-sensitivity risk.
- Correctness risk from aggressive rewrite or approximation.
Practitioner Heuristics
- Start from actual execution plans and runtime metrics.
- Optimize the highest-impact query families, not one-off outliers.
- Sargability and predicate selectivity usually dominate early wins.
- Keep optimization readable; opaque SQL hacks create long-term maintenance debt.
Workflow
- Identify high-impact queries by frequency and user/business impact.
- Capture plan/runtime evidence under representative parameters.
- Propose rewrites and access-path changes with expected effects.
- Compare candidates for latency gain versus complexity and risk.
- Roll selected change and monitor plan stability and resource usage.
- Record conditions that should trigger re-optimization.
Common Failure Modes
- Tuning for small dev datasets misleads production behavior.
- Index-only fixes mask poor query shape.
- Query changes improve p50 while degrading tail latency.
Failure Conditions
- Stop when no representative workload evidence is available.
- Stop when optimization changes correctness semantics.
- Escalate when required performance target is unattainable without model changes.