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 DEFAULTs 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.