MySQL Query Performance — Expert Reference

Use When

• Expert MySQL 8 query performance tuning: EXPLAIN analysis, index design, optimizer hints, slow query diagnosis, and profiling. Use when optimizing slow queries, designing indexes, analyzing EXPLAIN output, or diagnosing MySQL performance problems.
• The task needs reusable judgment, domain constraints, or a proven workflow rather than ad hoc advice.

Do Not Use When

• The task is unrelated to mysql-query-performance or would be better handled by a more specific companion skill.
• The request only needs a trivial answer and none of this skill's constraints or references materially help.

Required Inputs

• Gather relevant project context, constraints, and the concrete problem to solve; load references only as needed.
• Confirm the desired deliverable: design, code, review, migration plan, audit, or documentation.

Workflow

• Read this SKILL.md first, then load only the referenced deep-dive files that are necessary for the task.
• Apply the ordered guidance, checklists, and decision rules in this skill instead of cherry-picking isolated snippets.
• Produce the deliverable with assumptions, risks, and follow-up work made explicit when they matter.

Quality Standards

• Keep outputs execution-oriented, concise, and aligned with the repository's baseline engineering standards.
• Preserve compatibility with existing project conventions unless the skill explicitly requires a stronger standard.
• Prefer deterministic, reviewable steps over vague advice or tool-specific magic.

Anti-Patterns

• Treating examples as copy-paste truth without checking fit, constraints, or failure modes.
• Loading every reference file by default instead of using progressive disclosure.

Outputs

• A concrete result that fits the task: implementation guidance, review findings, architecture decisions, templates, or generated artifacts.
• Clear assumptions, tradeoffs, or unresolved gaps when the task cannot be completed from available context alone.
• References used, companion skills, or follow-up actions when they materially improve execution.

Evidence Produced

Category	Artifact	Format	Example
Performance	Slow-query diagnosis and tuning report	Markdown doc covering EXPLAIN, index, and tuning changes	docs/data/mysql-tuning-2026-04-16.md

References

• Use the references/ directory for deep detail after reading the core workflow below.

Sources: MySQL 8 Query Performance Tuning (Krogh, Apress 2020) + Efficient MySQL Performance (Nichter, O'Reilly 2022)

1. EXPLAIN ANALYZE vs EXPLAIN FORMAT=JSON

EXPLAIN ANALYZE (MySQL 8.0.18+) executes the query and returns actual measured statistics alongside estimates. This is the single most important diagnostic upgrade over plain EXPLAIN.

-- Standard EXPLAIN: estimates only, query NOT executed
EXPLAIN FORMAT=JSON SELECT * FROM orders WHERE customer_id = 42;

-- EXPLAIN ANALYZE: executes query, returns actual vs estimated
EXPLAIN ANALYZE SELECT * FROM orders WHERE customer_id = 42;

Reading EXPLAIN ANALYZE output — key fields:

-> Index lookup on orders using idx_customer (customer_id=42)
   (cost=18.50 rows=41)
   (actual time=0.134..2.847 rows=38 loops=1)

• cost=18.50 rows=41 — optimizer's estimate before execution
• actual time=0.134..2.847 — real elapsed ms (first row .. last row)
• rows=38 loops=1 — actual rows returned, loop count for nested joins
• When rows (estimated) diverges greatly from actual rows, index statistics are stale or a histogram is needed
• loops=N on an inner table means the entire sub-operation ran N times — multiply actual time by loops for total cost

EXPLAIN FORMAT=JSON shows the cost model breakdown without executing:

EXPLAIN FORMAT=JSON SELECT o.id, c.name
FROM orders o JOIN customers c ON o.customer_id = c.id
WHERE o.status = 'pending'\G

Look for "cost_info" → "read_cost" + "eval_cost" to understand why the optimizer chose a plan. The "chosen" boolean inside "considered_execution_plans" reveals rejected alternatives.

EXPLAIN FOR CONNECTION — get the live plan of a running query without re-running it (critical when index stats may have changed): EXPLAIN FOR CONNECTION 42;

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Additional Guidance

Extended guidance for mysql-query-performance was moved to references/skill-deep-dive.md to keep this entrypoint compact and fast to load.

Use that deep dive for:

• 2. Index Selection — Why MySQL Ignores Your Index
• 3. Covering Index Strategy and ICP
• 4. Index Merge — Usually a Warning Sign
• 5. Invisible Index Trick
• 6. Histogram Statistics
• 7. Optimizer Hints (MySQL 8.0)
• 8. Derived Table Materialization
• 9. Slow Query Log Analysis
• 10. Essential Performance Schema Queries
• 11. InnoDB Buffer Pool
• 12. Sort Optimisation (filesort vs Index Sort)
• 13. JOIN Algorithms
• Additional deep-dive sections continue in the reference file.