Diagnose and improve SQL query performance. Use when a task involves slow queries, EXPLAIN plan analysis, index tuning, join optimization, N+1 detection, pagination efficiency, or query regression checks. Trigger phrases: slow query, explain plan, missing index, full table scan, N+1, query timeout, lock contention. Negative trigger: schema-only migrations with no query or access-pattern change.
Diagnose and improve SQL query performance. Use when a task involves slow queries, EXPLAIN plan analysis, index tuning, join optimization, N+1 detection, pagination efficiency, or query regression checks. Trigger phrases: slow query, explain plan, missing index, full table scan, N+1, query timeout, lock contention. Negative trigger: schema-only migrations with no query or access-pattern change.
Establish a real baseline. Identify the query under review, its caller, and the tables/views involved, then anchor the analysis to representative data volume and workload. Do not reason from an empty local database or a toy fixture and call the result a performance review.
Inspect the plan, not the SQL alone. Obtain EXPLAIN ANALYZE (plus buffers/verbose output when available) and compare estimated vs actual row counts. Large estimate skew often matters as much as the raw cost because it points to stale statistics, low-selectivity predicates, or a plan choice the optimizer is making on bad assumptions.
Review predicate, join, and sort shape together. Verify filters, join keys, and ORDER BY clauses line up with actual indexes. Check composite-index ordering, low-selectivity leading columns, function-wrapped predicates, implicit casts, and join types that inflate work or defeat index use.
Audit access-pattern pathologies. Look for N+1 loops, repeated round-trips, unbounded result sets, SELECT * on wide rows, redundant ORM eager loading, and OFFSET pagination on hot mutable tables. A slow endpoint is often many cheap queries plus network chatter rather than one obviously bad statement.
Model the write-side and concurrency cost of the fix. Adding an index, changing pagination, or rewriting a join is not free. Review write amplification, index-build lock behavior, temp-file sort spill, memory pressure, and whether the proposed optimization changes the observable ordering or contract of the endpoint.
Propose the smallest safe improvement and prove it. Prefer the minimal change that addresses the real bottleneck: predicate rewrite, better projection, batch load, cursor pagination, or an index with clear justification. Require before/after plan or latency evidence, and note rollback or migration impact when the fix changes schema or API behavior.
Reference Guide
Topic
Reference
Load When
Performance checklist
references/checklist.md
Any query performance review or optimization task
Failure Patterns
Adding DISTINCT to hide duplicate rows caused by a bad join rather than fixing the join shape.
Adding indexes before confirming selectivity, predicate shape, and write-path cost.
Trusting EXPLAIN output from empty or toy datasets and calling the conclusion production-ready.
Replacing one measurable SQL bottleneck with many application-level round-trips or N+1 fetches.
Shipping cursor pagination without a stable, unique sort order that preserves user-visible semantics.
Wrapping indexed columns in functions or casts that silently defeat index use.
Constraints
Do not approve optimization claims without before/after evidence or at least plan-level comparison on representative data.
Do not add indexes without considering write-path cost, table size, and existing index overlap.
Do not rewrite queries in application code without verifying the new plan against realistic data volumes.
Do not silently convert lazy-loaded associations to eager-load without assessing memory and payload impact.
Do not change pagination semantics or ordering guarantees silently while chasing speed.
Do not move filtering or sorting into application code unless the new cost profile is measured and bounded.
Treat pagination strategy changes and lock-hint additions as migration-level risks requiring rollback safety.
