Postgresql

# PostgreSQL Table Design

Use this skill when

- Designing a schema for PostgreSQL

- Selecting data types and constraints

- Planning indexes, partitions, or RLS policies

- Reviewing tables for scale and maintainability

Do not use this skill when

- You are targeting a non-PostgreSQL database

- You only need query tuning without schema changes

- You require a DB-agnostic modeling guide

Instructions

1. Capture entities, access patterns, and scale targets (rows, QPS, retention).

2. Choose data types and constraints that enforce invariants.

3. Add indexes for real query paths and validate with `EXPLAIN`.

4. Plan partitioning or RLS where required by scale or access control.

5. Review migration impact and apply changes safely.

Safety

- Avoid destructive DDL on production without backups and a rollback plan.

- Use migrations and staging validation before applying schema changes.

Core Rules

- Define a **PRIMARY KEY** for reference tables (users, orders, etc.). Not always needed for time-series/event/log data. When used, prefer `BIGINT GENERATED ALWAYS AS IDENTITY`; use `UUID` only when global uniqueness/opacity is needed.

- **Normalize first (to 3NF)** to eliminate data redundancy and update anomalies; denormalize **only** for measured, high-ROI reads where join performance is proven problematic. Premature denormalization creates maintenance burden.

- Add **NOT NULL** everywhere it’s semantically required; use **DEFAULT**s for common values.

- Create **indexes for access paths you actually query**: PK/unique (auto), **FK columns (manual!)**, frequent filters/sorts, and join keys.

- Prefer **TIMESTAMPTZ** for event time; **NUMERIC** for money; **TEXT** for strings; **BIGINT** for integer values, **DOUBLE PRECISION** for floats (or `NUMERIC` for exact decimal arithmetic).

PostgreSQL “Gotchas”

- **Identifiers**: unquoted → lowercased. Avoid quoted/mixed-case names. Convention: use `snake_case` for table/column names.

- **Unique + NULLs**: UNIQUE allows multiple NULLs. Use `UNIQUE (...) NULLS NOT DISTINCT` (PG15+) to restrict to one NULL.

- **FK indexes**: PostgreSQL **does not** auto-index FK columns. Add them.

- **No silent coercions**: length/precision overflows error out (no truncation). Example: inserting 999 into `NUMERIC(2,0)` fails with error, unlike some databases that silently truncate or round.

- **Sequences/identity have gaps** (normal; don't "fix"). Rollbacks, crashes, and concurrent transactions create gaps in ID sequences (1, 2, 5, 6...). This is expected behavior—don't try to make IDs consecutive.

- **Heap storage**: no clustered PK by default (unlike SQL Server/MySQL InnoDB); `CLUSTER` is one-off reorganization, not maintained on subsequent inserts. Row order on disk is insertion order unless explicitly clustered.

- **MVCC**: updates/deletes leave dead tuples; vacuum handles them—design to avoid hot wide-row churn.

Data Types

- **IDs**: `BIGINT GENERATED ALWAYS AS IDENTITY` preferred (`GENERATED BY DEFAULT` also fine); `UUID` when merging/federating/used in a distributed system or for opaque IDs. Generate with `uuidv7()` (preferred if using PG18+) or `gen_random_uuid()` (if using an older PG version).

- **Integers**: prefer `BIGINT` unless storage space is critical; `INTEGER` for smaller ranges; avoid `SMALLINT` unless constrained.

- **Floats**: prefer `DOUBLE PRECISION` over `REAL` unless storage space is critical. Use `NUMERIC` for exact decimal arithmetic.

- **Strings**: prefer `TEXT`; if length limits needed, use `CHECK (LENGTH(col) <= n)` instead of `VARCHAR(n)`; avoid `CHAR(n)`. Use `BYTEA` for binary data. Large strings/binary (>2KB default threshold) automatically stored in TOAST with compression. TOAST storage: `PLAIN` (no TOAST), `EXTENDED` (compress + out-of-line), `EXTERNAL` (out-of-line, no compress), `MAIN` (compress, keep in-line if possible). Default `EXTENDED` usually optimal. Control with `ALTER TABLE tbl ALTER COLUMN col SET STORAGE strategy` and `ALTER TABLE tbl SET (