How would you debug production issues related to Query Execution Plans?

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Quick interview answer

Debug Query Execution Plans by confirming blast radius, checking lag/error metrics, grabbing profiles/traces, then mitigating before deep root-cause. Use Query Execution Plans as the core idea. Example shape: Sudden seq scan on million-row table traced to outdated stats after bulk load..

Detailed answer

Production debug playbook for Query Execution Plans: 1) Stabilize: rate-limit, shed load, or roll back if users are hurting. 2) Orient: dashboards for latency, errors, saturation tied to Query Execution Plans. 3) Evidence: traces, profiles, logs with correlation IDs. 4) Hypothesize + prove with a safe experiment. 5) Fix + add a regression test/alert so it cannot silently return. Watch for: Tuning without measuring; ignoring rows mismatch (estimate 10, actual 1M). Parent context: Use Query Execution Plans as the core idea. Example shape: Sudden seq scan on million-row table traced to outdated stats after bulk load..

Full explanation

Interviewers score structured incident thinking: mitigate → measure → root cause → prevent. Optimizer chooses join order, access paths (seek vs scan), and algorithms (hash vs merge join). EXPLAIN/EXPLAIN ANALYZE shows estimated vs actual rows — key for tuning.

Follow-up questions

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Parent context — Query Execution Plans

Use Query Execution Plans as the core idea. Example shape: Sudden seq scan on million-row table traced to outdated stats after bulk load..

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