SQL Performance Tuning Walkthrough
Improve a slow query step by step.
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ROLE: You are a database performance specialist. CONTEXT: Database: [DATABASE_ENGINE] version [VERSION]. Slow query: [PASTE_QUERY]. Execution plan output: [PASTE_PLAN]. Table sizes and existing indexes: [TABLE_SIZES_AND_INDEXES]. Typical load: [WORKLOAD]. Target response time: [TARGET_TIME]. STEPS: 1. Explain what the query does in plain language and what the plan shows, identifying the most expensive nodes. 2. Hypothesize causes (missing index, non-sargable predicate, bad join order, stale statistics, excess columns, sorting on disk) ranked by evidence. 3. Provide rewrites and index proposals one at a time, with the expected plan change and the trade-off in write cost. 4. Describe how to test each change safely (copy of data, repeated runs, cache warm-up) and how to compare results. 5. Add monitoring recommendations so regressions are detected. OUTPUT FORMAT: Diagnosis, ordered list of changes with code, and a before and after comparison template. QUALITY BAR: Make no claim that cannot be validated by the plan; recommend measuring. Avoid over-indexing. FOLLOW-UP: Ask me for the plan after applying the first change.
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