SQL Optimization — Comprehensive Reference Skill

SQL Optimization — Comprehensive Reference

This skill provides actionable checklists, patterns, and templates for transactional (OLTP) SQL optimization: measurement-first triage, EXPLAIN/plan interpretation, balanced indexing (avoiding over-indexing), performance monitoring, schema evolution, migrations, backup/recovery, high availability, and security.

Supported Platforms: PostgreSQL, MySQL, SQL Server, Oracle, SQLite

For OLAP/Analytics: See data-lake-platform (ClickHouse, DuckDB, Doris, StarRocks)

Quick Reference

| Task | Tool/Framework | Command | When to Use | |------|----------------|---------|-------------| | Query Performance Analysis | EXPLAIN ANALYZE | EXPLAIN (ANALYZE, BUFFERS) SELECT ... (PG) / EXPLAIN ANALYZE SELECT ... (MySQL) | Diagnose slow queries, identify missing indexes | | Find Slow Queries | pg_stat_statements / slow query log | SELECT * FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 10; | Identify performance bottlenecks in production | | Index Analysis | pg_stat_user_indexes / SHOW INDEX | SELECT * FROM pg_stat_user_indexes WHERE idx_scan = 0; | Find unused indexes, validate index coverage | | Schema Migration | Flyway / Liquibase | flyway migrate / liquibase update | Version-controlled database changes | | Backup & Recovery | pg_dump / mysqldump | pg_dump -Fc dbname > backup.dump | Point-in-time recovery, disaster recovery | | Replication Setup | Streaming / GTID | Configure postgresql.conf / my.cnf | High availability, read scaling | | Safe Tuning Loop | Measure -> Explain -> Change -> Verify | Use tuning worksheet template | Reduce latency/cost without regressions |

Decision Tree: Choosing the Right Approach

Query performance issue?
    ├─ Identify slow queries first?
    │   ├─ PostgreSQL -> pg_stat_statements (top queries by total_exec_time)
    │   └─ MySQL -> Performance Schema / slow query log
    │
    ├─ Analyze execution plan?
    │   ├─ PostgreSQL -> EXPLAIN (ANALYZE, BUFFERS, VERBOSE)
    │   ├─ MySQL -> EXPLAIN FORMAT=JSON or EXPLAIN ANALYZE
    │   └─ SQL Server -> SET STATISTICS IO ON; SET STATISTICS TIME ON;
    │
    ├─ Need indexing strategy?
    │   ├─ PostgreSQL -> B-tree (default), GIN (JSONB), GiST (spatial), partial indexes
    │   ├─ MySQL -> BTREE (default), FULLTEXT (text search), SPATIAL
    │   └─ Check: Table >10k rows AND selectivity <10% AND 10x+ speedup verified
    │
    ├─ Schema changes needed?
    │   ├─ New database -> template-schema-design.md
    │   ├─ Modify schema -> template-migration.md (Flyway/Liquibase)
    │   └─ Large tables (MySQL) -> gh-ost / pt-online-schema-change (avoid locks)
    │
    ├─ High availability setup?
    │   ├─ PostgreSQL -> Streaming replication (template-replication-ha.md)
    │   └─ MySQL -> GTID-based replication (template-replication-ha.md)
    │
    ├─ Backup/disaster recovery?
    │   └─ template-backup-restore.md (pg_dump, mysqldump, PITR)
    │
    └─ Analytics on large datasets (OLAP)?
        └─ See data-lake-platform (ClickHouse, DuckDB, Doris, StarRocks)

When to Use This Skill

Codex should invoke this skill when users ask for:

Query Optimization (Modern Approaches)

SQL query performance review and tuning
EXPLAIN/plan interpretation with optimization suggestions
Index creation strategies with balanced approach (avoiding over-indexing)
Troubleshooting slow queries using pg_stat_statements or Performance Schema
Identifying and remediating SQL anti-patterns with operational fixes
Query rewrite suggestions or migration from slow to fast patterns
Statistics maintenance and auto-analyze configuration

Database Operations

Schema design with normalization and performance trade-offs
Database migrations with version control (Liquibase, Flyway)
Backup and recovery strategies (point-in-time recovery, automated testing)
High availability and replication setup (streaming, GTID-based)
Database security auditing (access controls, encryption, SQL injection prevention)
Lock analysis and deadlock troubleshooting
Connection pooling (pgBouncer, Pgpool-II, ProxySQL)

Performance Tuning (Modern Standards)

Memory configuration (work_mem, shared_buffers, effective_cache_size)
Automated monitoring with pg_stat_statements and query pattern analysis
Index health monitoring (unused index detection, index bloat analysis)
Vacuum strategy and autovacuum tuning (PostgreSQL)
InnoDB buffer pool optimization (MySQL)
Partition pruning improvements (PostgreSQL 18+)

Resources (Best Practices Guides)

Find detailed operational patterns and quick references in:

SQL Best Practices: references/sql-best-practices.md
Query Tuning Patterns: references/query-tuning-patterns.md
Indexing Strategies: references/index-patterns.md
EXPLAIN/Analysis: references/explain-analysis.md
SQL Anti-Patterns: references/sql-antipatterns.md
External Sources: data/sources.json — vendor docs and reference links
Operational Standards: references/operational-patterns.md — Deep operational checklists, database-specific guidance, and template selection trees
Connection Pooling: references/connection-pooling-patterns.md — PgBouncer, RDS Proxy, pool sizing, connection leak troubleshooting
Partition Strategies: references/partition-strategies.md — Range/list/hash partitioning, pruning, maintenance, migration patterns
Monitoring & Alerting: references/monitoring-alerting-patterns.md — pg_stat_statements dashboards, alert thresholds, slow query pipelines

Each file includes:

Copy-paste ready checklists (e.g., "query review", "index design", "explain review")
Anti-patterns with operational fixes and alternatives
Query rewrite and indexing strategies with examples
Troubleshooting guides (step-by-step)

Templates (Copy-Paste Ready)

Templates are organized by database technology for precision and clarity:

Related Skills

Infrastructure & Operations:

../ops-devops-platform/SKILL.md — Infrastructure, backups, monitoring, and incident response
../qa-observability/SKILL.md — Performance monitoring, profiling, and metrics
../qa-debugging/SKILL.md — Production debugging patterns

Application Integration:

../software-backend/SKILL.md — API/database integration and application patterns
../software-architecture-design/SKILL.md — System design and data architecture
../dev-api-design/SKILL.md — REST API and database interaction patterns

Quality & Security:

../qa-resilience/SKILL.md — Resilience, circuit breakers, and failure handling
../software-security-appsec/SKILL.md — Database security, auth, SQL injection prevention
../qa-testing-strategy/SKILL.md — Database testing strategies

Data Engineering:

../ai-ml-data-science/SKILL.md — SQLMesh, dbt, data transformations
../ai-mlops/SKILL.md — Data pipelines, ETL, and warehouse loading (dlt)
../ai-ml-timeseries/SKILL.md — Time-series databases and forecasting

Navigation

Resources

Templates

Data

data/sources.json — Curated external references

Operational Deep Dives

See references/operational-patterns.md for:

End-to-end optimization checklists and anti-pattern fixes
Database-specific quick references (PostgreSQL, MySQL, SQL Server, Oracle, SQLite)
Slow query troubleshooting workflow and reliability drills
Template selection decision tree and platform migration notes

Do / Avoid

GOOD: Do

Measure baseline before any optimization
Change one variable at a time
Verify results match after query changes
Update statistics before concluding "needs index"
Test with production-like data volumes
Document all optimization decisions
Include performance tests in CI/CD

BAD: Avoid

Adding indexes without checking if they'll be used
Using SELECT * in production queries
Optimizing for test data (use representative volumes)
Ignoring write performance impact of indexes
Skipping EXPLAIN analysis before changes
Multiple simultaneous changes (can't attribute improvement)
N+1 query patterns in application code

Anti-Patterns Quick Reference

| Anti-Pattern | Problem | Fix | |--------------|---------|-----| | **SELECT *** | Reads unnecessary columns | Explicit column list | | N+1 queries | Multiplied round trips | JOIN or batch fetch | | Missing WHERE | Full table scan | Add predicates | | Function on indexed column | Can't use index | Move function to RHS | | Implicit type conversion | Index bypass | Match types explicitly | | LIKE '%prefix' | Leading wildcard = scan | Full-text search | | Unbounded result set | Memory explosion | Add LIMIT/pagination | | OR conditions | Index may not be used | UNION or rewrite |

See references/sql-antipatterns.md for detailed fixes.

OLTP vs OLAP Decision Tree

Is your query for...?
├─ Point lookups (by ID/key)?
│   └─ OLTP database (this skill)
│       - Ensure proper indexes
│       - Use connection pooling
│       - Optimize for low latency
│
├─ Aggregations over recent data (dashboard)?
│   └─ OLTP database (this skill)
│       - Consider materialized views
│       - Index common filter columns
│       - Watch for lock contention
│
├─ Full table scans or historical analysis?
│   └─ OLAP database (data-lake-platform)
│       - ClickHouse, DuckDB, Doris
│       - Columnar storage
│       - Partitioning by date
│
└─ Mixed workload (both)?
    └─ Separate OLTP and OLAP
        - OLTP for transactions
        - Replicate to OLAP for analytics
        - Avoid running analytics on primary

Optional: AI/Automation

Note: AI tools assist but require human validation of correctness.

EXPLAIN summarization — Identify bottlenecks from complex plans
Query rewrite suggestions — Must verify result equivalence
Index recommendations — Check selectivity and write impact first

Bounded Claims

AI cannot determine correct query results
Automated index suggestions may miss workload context
Human review required for production changes

Analytical Databases (OLAP)

For OLAP databases and data lake infrastructure, see data-lake-platform:

Query engines: ClickHouse, DuckDB, Apache Doris, StarRocks
Table formats: Apache Iceberg, Delta Lake, Apache Hudi
Transformation: SQLMesh, dbt (staging/marts layers)
Ingestion: dlt, Airbyte (connectors)
Streaming: Apache Kafka patterns

This skill focuses on transactional database optimization (PostgreSQL, MySQL, SQL Server, Oracle, SQLite). Use data-lake-platform for analytical workloads.

Related Skills

This skill focuses on query optimization within a single database. For related workflows:

SQL Transformation & Analytics Engineering: -> ai-ml-data-science skill

SQLMesh templates for building staging/intermediate/marts layers
Incremental models (FULL, INCREMENTAL_BY_TIME_RANGE, INCREMENTAL_BY_UNIQUE_KEY)
DAG management and model dependencies
Unit tests and audits for SQL transformations

Data Ingestion (Loading into Warehouses): -> ai-mlops skill

dlt templates for extracting from REST APIs, databases
Loading to Snowflake, BigQuery, Redshift, Postgres, DuckDB
Incremental loading patterns (timestamp, ID-based, merge/upsert)
Database replication (Postgres, MySQL, MongoDB -> warehouse)

Data Lake Infrastructure: -> data-lake-platform skill

ClickHouse, DuckDB, Doris, StarRocks query engines
Iceberg, Delta Lake, Hudi table formats
Kafka streaming, Dagster/Airflow orchestration

Use Case Decision:

Query is slow in production -> Use this skill (data-sql-optimization)
Building feature pipelines in SQL -> Use ai-ml-data-science (SQLMesh)
Loading data from APIs/DBs to warehouse -> Use ai-mlops (dlt)
Analytics on large datasets (OLAP) -> Use data-lake-platform

External Resources

See data/sources.json for 62+ curated resources including:

Core Documentation:

RDBMS Documentation: PostgreSQL, MySQL, SQL Server, Oracle, SQLite, DuckDB official docs
Query Optimization: Use The Index, Luke, SQL Performance Explained, vendor optimization guides
Schema Design: Database Refactoring (Fowler), normalization guides, data type selection

Modern Optimization (Current):

PostgreSQL: official release notes and "current" docs for planner/optimizer changes
MySQL: official reference manual sections for EXPLAIN, optimizer, and Performance Schema
SQL Server / Oracle: official docs for execution plans, indexing, and concurrency controls

Operations & Infrastructure:

HA & Replication: Streaming replication, GTID-based replication, failover automation
Migrations: Liquibase, Flyway version control and deployment patterns
Backup/Recovery: pgBackRest, Percona XtraBackup, point-in-time recovery
Monitoring: pg_stat_statements, Performance Schema, EXPLAIN visualizers (Dalibo, depesz)
Security: OWASP SQL Injection Prevention, Postgres hardening, encryption standards
Analytical Databases: DuckDB extensions, Parquet specification, columnar storage patterns

Use references/operational-patterns.md and the templates directory for detailed workflows, migration notes, and ready-to-run commands.

Fact-Checking

Use web search/web fetch to verify current external facts, versions, pricing, deadlines, regulations, or platform behavior before final answers.
Prefer primary sources; report source links and dates for volatile information.
If web access is unavailable, state the limitation and mark guidance as unverified.

Agent Skills: SQL Optimization — Comprehensive Reference

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