Database Internals

Use When

• Deep database internals: B-tree storage, WAL/REDO logging, MVCC, buffer pool mechanics, transaction isolation, and distributed database concepts. Use when making database design decisions that require understanding how MySQL works internally —...
• 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 database-internals 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	Storage engine and access path note	Markdown doc covering B-tree depth, buffer pool sizing, and MVCC implications for the workload	docs/data/internals-note-orders.md

References

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

Mental models from Database Internals (Alex Petrov, O'Reilly 2019) translated into practical design rules. Each section ends with a So what block — the actionable implication for real MySQL/SaaS work.

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1. B-Tree Structure — The Clustered Index IS the Table

InnoDB organises every table as a B+-Tree index-organised table (IOT). The primary key is not a pointer to a row — it is the row. Leaf nodes of the clustered index hold the full row data.

Root
 └─ Internal nodes (separator keys only)
     └─ Leaf nodes (PK + all column data)

Every secondary index stores the primary key value, not a physical row pointer. A secondary index lookup therefore does two B-tree traversals:

1. Traverse the secondary index B-tree → find the PK value.
2. Traverse the clustered index B-tree → find the row.

A covering index contains all columns the query needs, so step 2 is skipped.

-- Two traversals: secondary index + clustered index
SELECT name FROM orders WHERE status = 'paid';

-- One traversal: status+name covering index eliminates the second lookup
SELECT name FROM orders WHERE status = 'paid';  -- with INDEX(status, name)

UUID vs AUTO_INCREMENT PK impact:

• AUTO_INCREMENT inserts always append to the rightmost leaf. B-tree splits are predictable, pages stay ~70–90% full. PostgreSQL calls this the "fastpath".
• Random UUID inserts hit random leaf positions, causing splits throughout the tree, leaving pages ~50% full and generating 2–3× more write I/O. InnoDB page fragmentation accumulates rapidly.

So what: Always use INT UNSIGNED AUTO_INCREMENT or BIGINT AUTO_INCREMENT as PK. If UUIDs are required for business reasons, use UUIDv7 (time-ordered) or store the UUID as a BINARY(16) with a separate auto-increment surrogate PK for the clustered index.

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Additional Guidance

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

Use that deep dive for:

• 2. Page Structure — The 16 KB Unit of All I/O
• 3. Write-Ahead Log (WAL / Redo Log)
• 4. Buffer Pool Mechanics — The Memory Manager
• 5. MVCC — Consistent Reads Without Locking
• 6. Transaction Isolation Levels — What Each Actually Does
• 7. Lock Types and Interactions
• 8. LSM Trees vs B-Trees — The Core Tradeoff
• 9. Distributed Systems Concepts Applied to MySQL
• 10. Index Structures Beyond B-Trees
• 11. Write Amplification — Why SSDs Matter
• 12. MySQL vs PostgreSQL Internals — Key Architectural Differences
• 13. Design Rules Derived from Internals
• Additional deep-dive sections continue in the reference file.