git:20260602.d5545f4 to git:20260610.c7292cf

2 added, 0 removed. Audit A to A.

---

name: postgresdb

description: Use when designing or reviewing a PostgreSQL schema (types, constraints, normalization), writing or optimizing SQL, choosing or adding indexes, reading EXPLAIN/ANALYZE, planning a migration (expand-contract, concurrent index, batched backfill), or operating/securing Postgres (roles, RLS, PgBouncer pooling, VACUUM/autovacuum, pg_stat_statements, declarative partitioning, backups/PITR). Triggers - "slow query", "add an index", "EXPLAIN", "design this table", "zero-downtime migration", "row level security", "connection pool", "N+1", "JSONB", "full-text search", "pgvector". PostgreSQL 16; ORM-agnostic (SQLAlchemy/Alembic, Prisma, golang-migrate, raw SQL).

tags: [postgres, sql, database, migrations]

recommends: [secure-coding]

origin: risco

---

# PostgreSQL — schema, indexing, queries, ops

Engine-level PostgreSQL 16 guidance: design correct schemas, pick the right index, read EXPLAIN and

fix slow SQL, run zero-downtime migrations, and operate/secure the database. Tooling-agnostic; every

example is runnable.

## When to use / When NOT to use

+ > **⚠️ SDD new-feature gate — read this first.** If this skill fired on a **new, non-trivial feature or behaviour change** and there is **no approved spec + plan** under `02-DOCS/wiki/sdd/`, STOP — do **not** write feature code yet. Hand off to `../specify/SKILL.md` first: it runs brainstorm → spec → plan → tasks before any code, then routes back here once the plan is approved. Build here directly only for a genuinely one-line / low-risk change. Method: `../sdd/SKILL.md`.

**When to use:**

- Schema/DDL decisions: types, keys, constraints, generated/identity columns, enum-vs-lookup, jsonb-vs-column.

- Any query that is slow, scans too much, or returns wrong cardinality; reading a plan.

- Index decisions: which kind, column order, partial/covering, and when NOT to add one.

- Migrations against tables with real data or live traffic.

- Concurrency: isolation, locking, deadlocks, queues.

- Ops: pooling, vacuum, monitoring, partitioning, backups, RLS, least-privilege roles.

**When NOT to use:**

- ORM-API ergonomics (Prisma `updateMany` count trap, SQLAlchemy session lifecycle) → your ORM's own docs. This skill owns the **SQL the ORM emits and the engine behavior underneath**.

- Non-Postgres engines (MySQL/SQLite/ClickHouse) — different MVCC, locking, planner.

- App-layer caching / Redis / Kafka as products; only Postgres-as-queue via `SKIP LOCKED` is in scope.

- Cloud-vendor console clicks — we give the SQL/params, not the RDS/Cloud SQL UI path.

Deep dives: [schema-and-indexing](references/schema-and-indexing.md) (types, constraints, every index

kind, bloat) · [query-optimization](references/query-optimization.md) (EXPLAIN, joins, concurrency,

JSONB/FTS/pgvector) · [migrations](references/migrations.md) (zero-downtime DDL, per-ORM) ·

[operations-and-security](references/operations-and-security.md) (roles, RLS, pooling, vacuum,

partitioning, backups).

## Non-negotiables

1. `timestamptz` always for events; store UTC. Never naive `timestamp`.

2. Money is `numeric(19,4)`, never `float`/`double precision`.

3. Every FK gets a covering index — Postgres does **not** auto-index FK columns.

4. `text` + `CHECK (length(...) <= n)`, not `varchar(n)` as a length hack.

5. Index creation on a live table is **always** `CONCURRENTLY` (so it cannot run inside a txn).

6. Never `ADD COLUMN ... NOT NULL` without a default/backfill plan; never add a volatile default on a large table without a batched backfill.

7. Read the plan before adding an index: `EXPLAIN (ANALYZE, BUFFERS)` or it didn't happen.

8. Migrations are forward-only in prod; never edit an applied migration.

9. `SET lock_timeout` + `SET statement_timeout` around DDL on hot tables.

10. RLS is opt-in per table and the **table owner bypasses it** — verify with a non-owner role (use `FORCE ROW LEVEL SECURITY` for the owner too).

## Decision rules

Fast lookups; runnable DDL lives in the references.

### Pick the column type

| --- | --- | --- | --- |

### Pick the index

| --- | --- | --- | --- |

Hash indexes: almost never — equality-only, no multicolumn, rarely beats btree even though WAL-logged

since PG10.

### When NOT to add an index

- Low-selectivity boolean / `status` with few distinct values (planner ignores it; seq scan wins).

- Tiny tables (a seq scan reads one page; the index adds maintenance for nothing).

- Write-heavy columns rarely filtered — every index is a write tax.

- A column already the **left prefix** of an existing composite index.

- Redundant with a `UNIQUE` constraint — the constraint already created an index.

## Copy-paste patterns

### Canonical table (types + constraints + identity)

```sql

-- GOOD: identity PK, public uuid, FK with action, numeric money, timestamptz,

-- status via lookup FK, generated tsvector, CHECK constraints.

CREATE TABLE orders (

id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,

public_id uuid NOT NULL DEFAULT gen_random_uuid(), -- v4; see schema ref for v7

user_id bigint NOT NULL REFERENCES users (id) ON DELETE RESTRICT,

status text NOT NULL REFERENCES order_statuses (code) ON UPDATE CASCADE,

amount numeric(19,4) NOT NULL CHECK (amount >= 0),

currency text NOT NULL CHECK (length(currency) = 3),

note text,

search tsvector GENERATED ALWAYS AS (to_tsvector('simple', coalesce(note, ''))) STORED,

created_at timestamptz NOT NULL DEFAULT now(),

updated_at timestamptz NOT NULL DEFAULT now(),

UNIQUE (public_id)

);

CREATE INDEX ix_orders_user_id ON orders (user_id); -- FKs are NOT auto-indexed

CREATE INDEX ix_orders_status ON orders (status);

```

```sql

-- BAD: every column is a future migration or a bug.

CREATE TABLE orders (

id serial PRIMARY KEY, -- sequence-ownership gotchas; use IDENTITY

user_id int REFERENCES users(id), -- int overflows at 2.1B; no FK index

status varchar(20), -- length hack; no constraint on values

amount float, -- money drift

created_at timestamp DEFAULT now() -- naive: loses the zone

);

```

`updated_at` is not auto-maintained — add a `BEFORE UPDATE` trigger (see schema ref) or set it in the

app; Postgres has no `ON UPDATE` clause.

### Composite index column order

Equality columns first, then the range/sort column.

```sql

-- Query: WHERE user_id = $1 AND created_at >= $2 ORDER BY created_at DESC

-- GOOD: equality (user_id) then range (created_at)

CREATE INDEX ix_orders_user_created ON orders (user_id, created_at DESC);

-- BAD: range-first index cannot satisfy the equality efficiently for this query

CREATE INDEX ix_orders_created_user ON orders (created_at, user_id);

```

Confirm with `EXPLAIN` that the plan shows `Index Cond: (user_id = ... AND created_at >= ...)`, not a

`Filter:`.

### Partial + covering (INCLUDE) index

```sql

-- Partial: index only the rows you query (smaller, hotter)

CREATE INDEX ix_orders_active ON orders (user_id, created_at DESC)

WHERE status <> 'cancelled';

-- Covering: INCLUDE non-key columns to enable an index-only scan

CREATE INDEX ix_orders_user_cover ON orders (user_id) INCLUDE (amount, created_at);

```

Index-only scan requires a recently-`VACUUM`ed table; confirm `Heap Fetches: 0` in `EXPLAIN (ANALYZE)`.

### Keyset (cursor) pagination — not OFFSET

```sql

-- GOOD: keyset on a stable composite sort; uses ix_orders_user_created

SELECT id, amount, created_at

FROM orders

WHERE user_id = $1

AND (created_at, id) < ($2, $3) -- row-value comparator = last row of prev page

ORDER BY created_at DESC, id DESC

LIMIT 20;

-- BAD: OFFSET scans and discards 100000 rows every page (O(n))

SELECT * FROM orders WHERE user_id = $1 ORDER BY created_at DESC LIMIT 20 OFFSET 100000;

```

The index must match the `ORDER BY` direction exactly; include the tiebreaker (`id`).

### UPSERT done right

```sql

-- GOOD: insert-or-update; EXCLUDED is the row that failed to insert

INSERT INTO inventory (sku, qty)

VALUES ($1, $2)

ON CONFLICT (sku)

DO UPDATE SET qty = inventory.qty + EXCLUDED.qty

WHERE inventory.qty + EXCLUDED.qty >= 0 -- guard

RETURNING id, qty;

-- DO NOTHING returns no row on conflict; wrap to always get the row:

WITH ins AS (

INSERT INTO tags (name) VALUES ($1)

ON CONFLICT (name) DO NOTHING

RETURNING id

)

SELECT id FROM ins

UNION ALL

SELECT id FROM tags WHERE name = $1 LIMIT 1;

```

### Queue with SKIP LOCKED

```sql

-- GOOD: contention-free job claim; concurrent workers never block each other

UPDATE jobs

SET status = 'processing', locked_at = now()

WHERE id = (

SELECT id FROM jobs

WHERE status = 'pending'

ORDER BY created_at

FOR UPDATE SKIP LOCKED

LIMIT 1

)

RETURNING id, payload;

```

`SKIP LOCKED` skips rows another txn holds; `FOR UPDATE` alone would serialize all workers.

### Kill the N+1

```sql

-- BAD: application loops, one query per order (N+1)

-- for o in orders: SELECT * FROM order_items WHERE order_id = o.id

-- GOOD: one set-based query

SELECT o.id, json_agg(i.*) AS items

FROM orders o

JOIN order_items i ON i.order_id = o.id

WHERE o.user_id = $1

GROUP BY o.id;

-- Top-N-per-group: JOIN LATERAL, not a window-filter scan

SELECT u.id, recent.*

FROM users u

JOIN LATERAL (

SELECT id, amount, created_at FROM orders

WHERE user_id = u.id ORDER BY created_at DESC LIMIT 3

) recent ON true;

```

An ORM emitting N queries is the same bug — fix it at the SQL boundary, not with a cache.

### EXPLAIN, the right way

```sql

EXPLAIN (ANALYZE, BUFFERS, SETTINGS, FORMAT TEXT)

SELECT * FROM orders WHERE user_id = $1 AND created_at >= $2;

```

Read these four first:

1. **Estimated vs actual rows** — a large gap means stale stats; run `ANALYZE`.

2. **The most expensive node** — highest `actual time` × `loops`.

3. **`Seq Scan` on a big table** where you expected an index.

4. **`Rows Removed by Filter`** — the predicate was not pushed to an index.

Full method in [query-optimization](references/query-optimization.md).

## Anti-patterns / rationalizations → STOP

| Rationalization | Reality → STOP |

| --- | --- |

| "I'll add the FK index later, the query works now" | Unindexed FK = seq scan + heavy lock cascade on parent `DELETE`/`UPDATE`. Index it now. |

| "UUID PK is fine everywhere" | Random v4 fragments the B-tree and bloats WAL. Use `IDENTITY` internally or uuid **v7**. |

| "`SELECT count(*)` to check existence" | Counts the whole match. Use `EXISTS (SELECT 1 ...)`. |

| "CTEs are just for readability" | Pre-12 they were optimization fences; PG12+ inlines unless `MATERIALIZED`. Know which you want. |

| "`NOT IN (subquery)`" | NULL-unsafe (one NULL → empty result) and slow. Use `NOT EXISTS`. |

| "Store money as float, round on display" | Silent drift across arithmetic. `numeric(19,4)`. |

| "`ADD COLUMN ... NOT NULL DEFAULT now()`" | Volatile default rewrites the table under ACCESS EXCLUSIVE. Non-volatile constant is instant (PG11+). |

| "One big `jsonb` blob beats columns" | No constraints, no per-key stats, GIN bloat. Promote hot keys to typed columns. |

| "RLS policy calling `auth.uid()` per row" | Re-evaluated per row. Wrap: `(SELECT auth.uid())` so it runs once. |

| "`CREATE INDEX` in the migration is fine" | Blocks writes for the whole build. `CREATE INDEX CONCURRENTLY` (outside a txn). |

| "`VACUUM FULL` will fix bloat" | Takes ACCESS EXCLUSIVE, rewrites the table. Use autovacuum tuning / `REINDEX CONCURRENTLY`. |

## Quick reference

### Isolation levels

| --- | --- | --- | --- |

Retry the txn on SQLSTATE `40001` (serialization_failure) and `40P01` (deadlock_detected).

### Lock modes (DDL)

`CREATE INDEX CONCURRENTLY` takes SHARE UPDATE EXCLUSIVE (allows writes); plain `CREATE INDEX`,

`ALTER TABLE ... TYPE`, `ADD COLUMN` with a volatile default, and `VACUUM FULL` take ACCESS EXCLUSIVE

(blocks everything). Full table in [migrations](references/migrations.md#lock-impact-reference).

### Diagnostic one-liners

```sql

-- Unindexed foreign keys

SELECT c.conrelid::regclass AS tbl, a.attname AS col

FROM pg_constraint c

JOIN pg_attribute a ON a.attrelid = c.conrelid AND a.attnum = ANY (c.conkey)

WHERE c.contype = 'f'

AND NOT EXISTS (

SELECT 1 FROM pg_index i

WHERE i.indrelid = c.conrelid AND (i.indkey::int2[])[0] = a.attnum

);

-- Top slow queries (needs pg_stat_statements)

SELECT calls, round(mean_exec_time::numeric, 2) AS mean_ms,

round(total_exec_time::numeric, 2) AS total_ms, query

FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 20;

-- Dead tuples / bloat candidates

SELECT relname, n_dead_tup, n_live_tup, last_autovacuum

FROM pg_stat_user_tables WHERE n_dead_tup > 1000 ORDER BY n_dead_tup DESC;

-- Blocking locks

SELECT blocked.pid AS blocked_pid, blocking.pid AS blocking_pid, blocked.query AS blocked_query

FROM pg_stat_activity blocked

JOIN pg_locks bl ON bl.pid = blocked.pid AND NOT bl.granted

JOIN pg_locks gl ON gl.locktype = bl.locktype AND gl.database IS NOT DISTINCT FROM bl.database

AND gl.relation IS NOT DISTINCT FROM bl.relation AND gl.granted

JOIN pg_stat_activity blocking ON blocking.pid = gl.pid;

-- Cache hit ratio (aim > 0.99)

SELECT sum(heap_blks_hit) / nullif(sum(heap_blks_hit + heap_blks_read), 0) AS ratio

FROM pg_statio_user_tables;

-- Unused indexes

SELECT relname, indexrelname, idx_scan

FROM pg_stat_user_indexes WHERE idx_scan = 0 ORDER BY relname;

```

## Verify

Run `scripts/verify.sh` from your project root. It lints discovered SQL with `sqlfluff` (if

configured), syntax-sanity-checks migration files (the quote/paren balance check is dollar-quote and

block-comment aware) and flags foot-guns (`CREATE INDEX` without `CONCURRENTLY` in a migration,

`ADD COLUMN ... NOT NULL` without `DEFAULT`, `VACUUM FULL`), and — only if `DATABASE_URL` and `psql`

are present — checks that `pg_stat_statements` is enabled. It exits non-zero **only** on a real

`sqlfluff` lint error; everything else (missing tools, heuristic quote/paren warnings, DB unreachable)

is advisory `[skip]`/`[warn]`, never a failure. Runs on stock macOS bash 3.2; it never writes and

never connects without `DATABASE_URL`.

## Project grounding (02-DOCS + CLAUDE.md)

When this skill runs in a project with a `02-DOCS/` layer (the

[`harness`](../harness/SKILL.md) Karpathy wiki), record this

project's database decisions there and index them from the root `CLAUDE.md`, so the next

agent inherits the conventions instead of re-deriving them.

1. **Find the article** `02-DOCS/wiki/stack/postgresdb.md`, linked from a `## Knowledge map` section in the root

`CLAUDE.md`.

2. **If missing or stale**, create/update it with the project's real choices — schema and naming conventions, the migration tool, indexing/partitioning decisions, the pooling setup, and any RLS policies —

then add/refresh the `CLAUDE.md` link (create the `## Knowledge map` section, and

`CLAUDE.md` itself, if absent).

3. **Read it first on every use** and stay consistent; when a convention changes, update the

article (bump its `Updated` date) in the same change.

No `02-DOCS/` layer? Skip silently (optionally suggest `harness`). Unlike the

brand study, technical conventions are *recorded, not gated* — never block the task on this.

## See Also

- [references/schema-and-indexing.md](references/schema-and-indexing.md) — types, constraints, all index kinds, bloat.

- [references/query-optimization.md](references/query-optimization.md) — EXPLAIN, joins, concurrency, JSONB/FTS/pgvector.

- [references/migrations.md](references/migrations.md) — zero-downtime DDL, lock-impact table, per-ORM.

- [references/operations-and-security.md](references/operations-and-security.md) — roles, RLS, pooling, vacuum, partitioning, backups.

- Sibling skills: [`harness`](../harness/SKILL.md) (scaffolds the `01-TOOLS/POSTGRES` operational tool). Stack siblings: [`fastapi`](../fastapi/SKILL.md), [`nextjs`](../nextjs/SKILL.md), [`go`](../go/SKILL.md), [`flutter`](../flutter/SKILL.md), [`secure-coding`](../secure-coding/SKILL.md), [`deployment`](../deployment/SKILL.md).

- Out of scope here — these are external tools with their own upstream documentation, not skills in this collection: ORM-surface traps (Prisma's `updateMany` count, serverless connection exhaustion) and per-runner migration wiring (Prisma, Alembic, golang-migrate, Drizzle, …). This skill owns the engine-level SQL and migration mechanics those tools sit on top of; for the tool's own API/CLI, read that tool's docs.