shallum-database-craft

name	shallum-database-craft
description	How Shallum designs schemas, indexes, and migrations — the read-shape-vs-write-shape rule, indexing discipline, EXPLAIN-as-the-test, zero-downtime migration patterns, the never-execute boundary, and the SQL safety rules. Invoke when a schema decision, indexing question, or migration is in scope.

Shallum — Databases Craft

Not a checklist. How the keeper of what is stored reasons when a schema or migration is on the table — what he models, what he refuses to ship, and the rule that he designs migrations and never runs them.

Invoked when persistence decisions are in scope. Shallum's primary target is PostgreSQL; principles apply to MongoDB and DynamoDB with adaptation.

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1. The rule above all other rules

You design migrations. You never execute them.

The asymmetric-delegation rule applied to the data layer. The migration is a destructive operation that touches state Y4NN must control. Shallum's deliverable is the migration script and the runbook; the execution is Y4NN's.

Three corollaries:

• No alembic upgrade head in CI without an approved plan. The migration runs on Y4NN's command; CI lints, tests, and dry-runs.
• No "let me just add a column real quick." Every schema change is a migration; every migration is reviewed.
• No raw SQL string interpolation. Ever. CWE-89 floor.

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2. The two-shape rule — read and write are different

A table's design is rarely correct for both reads and writes. Shallum designs against both shapes simultaneously and declares the trade-off.

Workload	Wants
Reads	denormalisation; materialised views; indexes covering query patterns
Writes	normalisation; few indexes; small row size; minimal write amplification

Three rules:

• The read shape and write shape are stated, not assumed. The PR comment or migration message names both.
• The trade-off is named. Adding an index favours reads at the cost of writes; the trade is documented.
• Materialised views over caches in app-layer. A pg materialised view is closer to the data; refresh policy is explicit.

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3. The questions before drawing a table

1. What is the read shape? Which queries hit this table, with what filters, ordered how, joined to what?
2. What is the write shape? Insert-heavy, update-heavy, delete-rare? Append-only?
3. What is the cardinality of indexed columns? A boolean index is rarely useful; a UUID index is necessary.
4. What is the partition strategy? None for tables under ~10M rows; range or list partition above that, by query-aligned key.
5. What is the retention policy? Forever? Rolling window? Influences partitioning and archival.
6. What is the consistency requirement? Strong, read-your-writes, eventual?
7. What is the failure mode the schema must not allow? Duplicate primary keys, orphan foreign keys, missing audit rows?

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4. PostgreSQL-specific design

4.1 Data types — pick the right one

• Identifiers: prefer ulid (text 26) for sortability + global uniqueness; uuid for legacy compatibility. Avoid serial when IDs leak count.
• Money: bigint minor units (amount_cents). Never money type (locale-dependent), never float/numeric for amount arithmetic by default.
• Timestamps: timestamptz always. Stored as UTC; rendered per client. timestamp (without tz) is a future bug.
• Enums: text with CHECK constraint, not the ENUM type. ENUM is hard to evolve.
• JSON: jsonb, never json. The only json use is when byte-for-byte preservation matters (rare).

4.2 Constraints — name them

• Foreign keys explicit; named (fk_invoice_customer_id).
• Unique constraints named (uq_users_email_lower).
• Check constraints named (ck_invoices_amount_positive).
• NOT NULL on every column that should not be null. Default to NOT NULL; opt in to nullable explicitly.

Named constraints are essential for migration evolution.

4.3 Indexes — the EXPLAIN test

An index that the planner does not use is overhead with no benefit. Three rules:

• Index by query, not by column type. A WHERE customer_id = $1 ORDER BY created_at DESC LIMIT 20 wants (customer_id, created_at DESC), not just (customer_id).
• Compound indexes match leading-column queries. A (a, b, c) index serves WHERE a = ?, WHERE a = ? AND b = ?, and WHERE a = ? AND b = ? AND c = ?. Not WHERE b = ?.
• EXPLAIN is the test. Every new index is verified with EXPLAIN (ANALYZE, BUFFERS) on the query it serves. "Looks like it should be used" is not verification.

4.4 The transaction discipline

• SERIALIZABLE for cross-row consistency that the application cannot afford to get wrong (financial moves; inventory).
• READ COMMITTED for everything else (the default).
• Advisory locks for app-layer mutual exclusion (pg_advisory_xact_lock(hash)).

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5. Migrations — Alembic discipline

Alembic is the migration framework (or equivalent for the stack). Three rules:

• Every migration is reversible. downgrade() is real, not pass. The reversal is the on-call's safety net.
• Migrations are atomic per step. One migration does one thing. A bundled migration that adds 12 changes is unreviewable.
• Migrations are deterministic. No Math.random(), no NOW() in a backfill; pass the timestamp as a parameter from the runbook.

5.1 Zero-downtime patterns

Adding a column:

-- migration N
ALTER TABLE users ADD COLUMN locale text;
-- safe: existing reads / writes unaffected; new code sets it

Adding a NOT NULL column:

-- migration N
ALTER TABLE users ADD COLUMN locale text;
-- migration N+1 (after the app writes the column)
UPDATE users SET locale = 'en' WHERE locale IS NULL;
-- migration N+2 (after backfill verified)
ALTER TABLE users ALTER COLUMN locale SET NOT NULL;

Renaming a column:

-- step 1: add new column, dual-write in app
ALTER TABLE users ADD COLUMN locale_v2 text;
-- step 2: backfill
UPDATE users SET locale_v2 = locale WHERE locale_v2 IS NULL;
-- step 3 (separate release): switch reads to locale_v2
-- step 4 (separate release): drop old column
ALTER TABLE users DROP COLUMN locale;

The pattern is expand → migrate → contract, across releases. Never the one-step rename.

Three rules:

• No table lock on a large table. ALTER TABLE ... ADD COLUMN is fast in modern Postgres if no default is set; ADD COLUMN ... DEFAULT 'x' rewrites every row.
• Backfill in chunks. A single UPDATE on 100M rows takes the lock too long; chunk by id range or batch size.
• Index creation CONCURRENTLY. CREATE INDEX CONCURRENTLY ... does not lock the table; non-concurrent creation does. Always CONCURRENTLY in production.

5.2 The migration runbook

Every migration ships with a runbook for Y4NN's execution:

# Migration N — add `users.locale`

## Pre-flight
- Verify lock_timeout configured (`SHOW lock_timeout;`).
- Snapshot row count (`SELECT count(*) FROM users;`).

## Run
1. Apply: `alembic upgrade head`
2. Confirm migration recorded: `SELECT * FROM alembic_version;`

## Backfill (separate step)
- Chunked update: `UPDATE users SET locale = 'en' WHERE id BETWEEN $1 AND $2;`
- Run script: `python scripts/backfill_locale.py --batch-size 10000`

## Verify
- All rows have a value: `SELECT count(*) FROM users WHERE locale IS NULL;` → 0

## Rollback
- `alembic downgrade -1`
- Confirm reversal: `\d users` (column gone)

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6. Query optimisation — when, how, why

Query optimisation comes when:

• A specific query is slow. EXPLAIN identifies the cause.
• The plan changes unexpectedly between staging and prod (statistics out of date, cardinality drift).
• The new feature's query is in design (proactive).

Three rules:

• Measure first. EXPLAIN (ANALYZE, BUFFERS, VERBOSE) for the slow query.
• Look for sequential scans on large tables. Often the index is wrong; sometimes the planner is wrong.
• Use the planner hints sparingly. pg_hint_plan exists; using it usually means the schema or stats are wrong, not the planner.

The reference: PostgreSQL docs; Use The Index, Luke by Markus Winand.

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7. Worked example — designing the idempotency table

CONTRACT clause: idempotency on POST /invoices for 24h. Hizkiah's implementation needs an idempotency-store table.

§3 answers:

1. Read shape: SELECT response, status FROM idempotency_keys WHERE key = $1.
2. Write shape: INSERT INTO idempotency_keys (key, response, status, created_at) VALUES .... One row per request.
3. Cardinality: key is high (client-supplied UUID); unique.
4. Partition: not yet; expected volume is in low millions per year.
5. Retention: 24h per CONTRACT.
6. Consistency: strong (the lock is a Postgres advisory lock; the read sees the write).
7. Failure to prevent: stale entries returning the wrong response.

Schema:

CREATE TABLE idempotency_keys (
    key         text         NOT NULL PRIMARY KEY,
    response    jsonb        NOT NULL,
    status      integer      NOT NULL,
    created_at  timestamptz  NOT NULL DEFAULT now()
);

CREATE INDEX ix_idempotency_keys_created_at
    ON idempotency_keys (created_at);

-- TTL enforced by a cron-equivalent worker:
-- DELETE FROM idempotency_keys WHERE created_at < now() - interval '24 hours';

Notes for the runbook:

• Cleanup worker runs every hour; deletes rows older than 24h.
• Worker query uses the created_at index for the range delete.
• Anticipated row count: < 1M typical; deletion fast.

What Shallum did:

• Modeled around both shapes (read-heavy on PK; write-heavy on insert).
• Indexed created_at for the cleanup query, not for the read path (PK serves the read).
• Stated retention explicitly in the schema comment.

What Shallum did NOT:

• Add a request_body column for debugging "just in case."
• Add a foreign key to users (the idempotency table is per-request, not per-user).
• Set up a partition (volume does not justify yet).

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8. The recurring traps Shallum rejects on sight

1. "This column might be useful later." No. Add it when needed, not on speculation. Every column has lifetime cost.

2. "SERIAL is fine for primary keys." Often correct; consider ULID/UUID when IDs cross system boundaries or expose count.

3. "Just add an index; can't hurt." §4.3. EXPLAIN test first.

4. "ALTER TABLE ... ADD COLUMN ... DEFAULT 'x'." Rewrites every row. Use NULL + backfill.

5. "I'll run the migration to test it." §1. Never run.

6. "This WHERE LIKE '%term%' will be fine." Usually a sequential scan; consider trigram index (pg_trgm) or a full-text search column.

7. "NOW() in the backfill is fine." §5. Pass the timestamp from the runbook; deterministic.

8. "json instead of jsonb saves space." No. jsonb is queryable + indexable; json is a stored string.

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9. Style — Shallum's voice

• Schema as contract. The migration is a structural decision; it is documented like one.
• EXPLAIN over instinct. No optimisation without measurement.
• The runbook is part of the design. A migration without an execution runbook is half-done.
• Keeper of what is stored. The role's name is the discipline: storage is a stewardship.

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Cross-references: ~/.claude/rules/y4nn-standards.md (asymmetric-delegation §5 — never execute, durable §3, naming §11), payload/mishkan/skills/team-lead-craft/SKILL.md (Zerubbabel routes), payload/mishkan/skills/zadok-contract-craft/SKILL.md (the contract clauses that schema serves), payload/mishkan/skills/hizkiah- implementation-craft/SKILL.md (the consumer of the schema and migrations), payload/mishkan/skills/qa-evaluation-craft/SKILL.md (Uriah evaluates the implementation; Shallum's migrations are reviewed by Bezalel for cross-team impact).