---
name: design-postgres-tables
description: Design PostgreSQL table schemas. Use when modeling new data, adding tables, or reviewing/improving existing schema design.
---

# Design Postgres Tables Skill

Use when: designing new tables, modeling relationships, reviewing schema, or planning migrations.

## Column Conventions
- **Primary key**: `id UUID PRIMARY KEY DEFAULT gen_random_uuid()` (prefer UUID over serial for distributed systems; use BIGSERIAL if you need sortable IDs)
- **Timestamps**: always include `created_at TIMESTAMPTZ DEFAULT NOW()` and `updated_at TIMESTAMPTZ DEFAULT NOW()`
- **Soft delete**: `deleted_at TIMESTAMPTZ DEFAULT NULL` (NULL = active)
- **Foreign keys**: name as `<table_singular>_id`, e.g. `user_id`, `project_id`
- Use `TEXT` over `VARCHAR(n)` unless you need DB-enforced length limits
- Use `BOOLEAN` not `SMALLINT` for true/false
- Use `NUMERIC(precision, scale)` for money, never `FLOAT`

## Naming Conventions
- Tables: `snake_case`, plural: `users`, `project_sessions`, `api_tokens`
- Columns: `snake_case`, singular
- Indexes: `idx_<table>_<column(s)>`: `idx_users_email`
- Foreign keys: `fk_<table>_<referenced_table>`: `fk_orders_users`
- Unique constraints: `uq_<table>_<column(s)>`: `uq_users_email`

## Standard Table Template
```sql
CREATE TABLE items (
    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    
    -- Business columns
    name        TEXT NOT NULL,
    description TEXT,
    status      TEXT NOT NULL DEFAULT 'active'
                CHECK (status IN ('active', 'inactive', 'archived')),
    
    -- Relationships
    user_id     UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    
    -- Metadata
    metadata    JSONB DEFAULT '{}',
    
    -- Audit
    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    deleted_at  TIMESTAMPTZ
);

-- Indexes
CREATE INDEX idx_items_user_id ON items(user_id);
CREATE INDEX idx_items_status ON items(status) WHERE deleted_at IS NULL;
CREATE INDEX idx_items_created_at ON items(created_at DESC);

-- Auto-update updated_at
CREATE TRIGGER items_updated_at
    BEFORE UPDATE ON items
    FOR EACH ROW EXECUTE FUNCTION update_updated_at_column();
```

## Relationship Patterns
- **One-to-many**: FK on the "many" side
- **Many-to-many**: junction table with `<a>_id` + `<b>_id` + its own PK + timestamps
- **Polymorphic**: `entity_type TEXT + entity_id UUID` (add check constraint for valid types)

## Index Strategy
- Index all foreign keys
- Index columns used in WHERE clauses with high selectivity
- Partial indexes for filtered queries: `WHERE deleted_at IS NULL`
- Composite indexes: put equality columns first, range columns last
- JSONB: use GIN index for `@>` containment queries

## Design Checklist
- [ ] Every table has UUID PK + created_at + updated_at
- [ ] Foreign keys have explicit ON DELETE behavior (CASCADE vs RESTRICT vs SET NULL)
- [ ] Enum-like columns use CHECK constraints or a lookup table
- [ ] Money stored as NUMERIC not FLOAT
- [ ] Indexes on all FK columns
- [ ] No storing arrays as comma-separated strings (use ARRAY[] or junction table)
- [ ] JSONB for truly dynamic/schemaless data only — not as a crutch
