Jon Chery
496303471d
---ci--- project: atelier phase: 7 milestone: v0.1 status: complete phase_role: final milestone_complete: true requirements: covered: [ATELIER-01, ATELIER-02, ATELIER-03, ATELIER-04, ATELIER-05, ATELIER-06, ATELIER-07, ATELIER-08, ATELIER-09, ATELIER-10, ATELIER-11, ATELIER-12, ATELIER-13, ATELIER-14, ATELIER-15, ATELIER-16, ATELIER-17, ATELIER-18, ATELIER-19, ATELIER-20, ATELIER-21, ATELIER-22, ATELIER-23, ATELIER-24, ATELIER-25, ATELIER-26, ATELIER-27, ATELIER-28, ATELIER-29, ATELIER-30, ATELIER-31, ATELIER-32, ATELIER-33, ATELIER-34, ATELIER-35] partial: [] ship: milestone: v0.1 type: NFR tag: v0.0.7 merge: milestone/v0.1-atelier -> main release: https://git.cloudinit.dev/cloudinit-bot/atelier/releases/tag/v0.0.7 ---/ci--- Milestone v0.1 — Initial Framework (NFR, complete). 8 core principles (C1-C8), 11 domains, 110 domain principles, 27 derived docs, 4 good + 3 bad examples, 4 language docs, full matrix, 3 review docs. All 35 requirements covered. 7 patches (v0.0.0 pre-execution through v0.0.7 final). v0.0.7 IS the v0.1.0 milestone release.
49 lines
2.2 KiB
Markdown
49 lines
2.2 KiB
Markdown
# Indexing — Derived Rules
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> Derives from `domains/data/first-principles.md` P5 (Indexing with Intent), P10 (Performance Awareness).
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## Index for Queries, Not Tables (P5)
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- An index serves a query. No query, no index.
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- The query plan is the spec. `EXPLAIN` is the test. An index that is not used is dead weight.
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- Index the columns you filter on (`WHERE`), join on (`JOIN`), and sort on (`ORDER BY`).
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## Composite Indexes (P5, P10)
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- Order matters: `INDEX(a, b)` serves `WHERE a = ? AND b = ?` and `WHERE a = ?`, but NOT `WHERE b = ?`.
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- Put the most selective column first. Or the column used in every query. Depends on the workload.
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- An index on every column is not a strategy. It is write amplification.
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## Unique Indexes (P3 Invariants in Schema)
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- A uniqueness constraint is a unique index. Use it for invariants: `email`, `username`.
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- Unique indexes enforce; application checks defend. Both belong.
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- A partial unique index: `UNIQUE(email) WHERE deleted_at IS NULL` — allows soft-deleted duplicates.
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## Covering Indexes (P10)
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- An index that covers all columns of a query is an "index-only scan" — no table lookup.
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- PostgreSQL: `INCLUDE` clause. MySQL: all columns in the index.
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- Use for hot queries. Don't cover everything; index size matters.
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## Don't Over-Index (P10, core C8 Economy)
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- Every index costs a write. The write budget is the index count.
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- Indexes take disk and memory. A 1GB index on a 500MB table is a smell.
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- Remove unused indexes. `pg_stat_user_indexes` shows usage. An unused index is debt.
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## Migration and Indexes (P4 Migration Safety)
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- Adding an index on a large table is expensive. Do it concurrently (`CREATE INDEX CONCURRENTLY`).
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- An index migration that locks the table blocks writes. Plan for it.
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- Build the index, then deploy the query that uses it. Not the reverse.
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## What Violates Indexing Discipline
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| Violation | Principle |
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|-----------|-----------|
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| Index on every column | P10, C8 Economy |
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| No index on a foreign key | P10 (join performance) |
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| `WHERE b = ?` with only `INDEX(a, b)` | P5 (wrong order) |
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| Index created without checking the query plan | P5 (no intent) |
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| `CREATE INDEX` (non-concurrent) on a 10M-row table in prod | P4 Migration Safety | |