postgres-core-schema-design

name	postgres-core-schema-design
description	Use when starting a new PostgreSQL database, picking naming conventions, deciding multi-schema layout, or hardening search_path against SECURITY DEFINER injection. Prevents using SERIAL where IDENTITY is required (v10+), shipping a quoted-identifier mess ("camelCase" forces quotes everywhere), leaving public schema writable, and SECURITY DEFINER search_path hijack. Covers snake_case naming, IDENTITY vs SERIAL (v10+), multi-schema layout patterns, search_path semantics + injection risks, v15 public-schema default lockdown, multi-tenant via schema vs RLS decision tree. Keywords: schema design, naming convention, IDENTITY, GENERATED AS IDENTITY, SERIAL deprecated, search_path, SECURITY DEFINER, public schema, snake_case, multi-tenant, where should I put my tables, can I use camelCase, why does serial cause sequence issues, search path hijack, what schema layout, public schema permission denied, function ignoring search_path, identity vs serial which one
license	MIT
compatibility	Designed for Claude Code. Requires PostgreSQL 15, 16, or 17.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

postgres-core-schema-design

Quick Reference :

PostgreSQL schema design fixes four classes of mistake on day one : identifier-casing chaos ("camelCase" forces every consumer to quote every reference), the serial foot-gun (an implicit sequence with broken privilege semantics, deprecated in favor of GENERATED ... AS IDENTITY since v10), a writable public schema that lets any logged-in role create objects in the trust-by-default namespace, and SECURITY DEFINER functions whose unqualified references can be hijacked by a caller-controlled search_path. These four decisions are made once and ALWAYS at table-creation time : retrofitting them later means a coordinated migration and a recompile of every dependent function. This skill encodes the decisions you make BEFORE the first CREATE TABLE.

A PostgreSQL database is one logical container with N schemas inside it. A schema is a namespace for tables, indexes, views, functions, types, sequences, and operators. Connection-level access is per-database (one CONNECT privilege per database, no cross-database queries except via FDW or dblink). Inside a database, name resolution walks the search_path GUC left-to-right and resolves the first match. The default search_path is "$user", public : a per-user schema (created on demand, never auto-created), then the shared public schema. PostgreSQL 15 changed the security default : on a freshly initialized cluster, public no longer grants CREATE to role PUBLIC : pre-existing databases upgraded to v15 keep their pre-v15 permissive grant. Therefore : on any database that PRE-DATES v15 you MUST REVOKE CREATE ON SCHEMA public FROM PUBLIC explicitly. Every SECURITY DEFINER function MUST pin search_path via SET search_path = ... in its definition, with pg_temp last, otherwise a malicious caller can substitute objects via their own temporary schema.

When To Use This Skill :

ALWAYS use this skill when :

• Starting a new database : naming conventions, schema layout, identity-column choice are decided once
• Designing a multi-tenant database : schema-per-tenant vs RLS vs database-per-tenant tradeoff
• Writing or auditing any SECURITY DEFINER function : search_path must be pinned
• Onboarding an existing database whose public schema is wide-open and you suspect v14-or-earlier defaults
• Choosing between serial / bigserial and GENERATED ... AS IDENTITY for a new primary key column
• Picking singular vs plural table names, FK column naming, primary-key column naming

NEVER use this skill for :

• Row-Level Security policy authoring : see postgres-core-rls-policies
• Role + privilege model (CREATE ROLE, GRANT) : see postgres-core-roles-privileges
• Index naming + design : see postgres-core-indexing-strategy
• Schema migration execution (lock timing, NOT VALID + VALIDATE) : see postgres-impl-zero-downtime-ddl

Decision Trees :

Identity column : IDENTITY or SERIAL? :

PostgreSQL version >= 10?
├── No (v9.6 or earlier) : SERIAL / BIGSERIAL is the only option
└── Yes (v10+) : ALWAYS GENERATED ... AS IDENTITY. NEVER serial in new code.
    │
    Should application code be allowed to supply its own id?
    ├── Never (PK is system-issued, app must not override) :
    │       id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY
    │       (override requires INSERT ... OVERRIDING SYSTEM VALUE)
    └── Sometimes (data migration loads supply ids, app does not) :
            id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY
            (app INSERT without id : sequence allocates ; with id : honored)

Multi-tenancy : schema-per-tenant or RLS? :

Tenant count and isolation strength?
├── < 50 tenants AND tenants must NEVER share table-level access :
│   schema-per-tenant (one schema per tenant, identical DDL)
│   + Pros : strong isolation at GRANT level, per-tenant DDL freedom,
│            simple "DROP SCHEMA tenant_x CASCADE" for offboarding
│   + Cons : N copies of every DDL change, sequences/types duplicated,
│            connection pool needs SET search_path per request
├── 50 - 10,000 tenants AND DDL must stay singular :
│   RLS (one schema, tenant_id column, FORCE ROW LEVEL SECURITY policies)
│   + Pros : one DDL, planner sees one statistics set, one set of indexes,
│            CONNECT-time SET app.current_tenant
│   + Cons : every query carries tenant_id implicitly, ROLE must not BYPASSRLS,
│            cross-tenant aggregate requires a privileged role
└── > 10,000 tenants OR regulated isolation (HIPAA, GDPR DPA per-tenant) :
    database-per-tenant (one database per tenant in the cluster, or N clusters)
    + Pros : independent backup / restore / drop / encryption per tenant
    + Cons : no cross-tenant query at all without FDW, max_connections explodes

Should this name be quoted? :

Identifier matches ^[a-z_][a-z0-9_]*$ AND is not a reserved word?
├── Yes : write it unquoted. PostgreSQL folds unquoted identifiers to lowercase.
└── No : you are forcing every consumer to quote it forever.
    ├── Contains uppercase ("CustomerId") : FIX, rename to snake_case
    ├── Contains hyphen / space / special : FIX, rename to snake_case
    ├── Reserved word ("user", "order", "table") : FIX, rename
    │   (e.g. "user" -> app_user, "order" -> sales_order)
    └── Truly external system requirement (FDW to MS SQL) : quote at the
        FDW boundary, snake_case everywhere else.

Does my SECURITY DEFINER function need SET search_path? :

Function uses SECURITY DEFINER?
├── No (default SECURITY INVOKER) : caller's search_path is fine
└── Yes : ALWAYS add SET search_path = pg_catalog, <trusted_schemas>, pg_temp
    │
    Does the function reference unqualified table/operator/function names?
    ├── Yes : MANDATORY to pin search_path AND schema-qualify the references
    │        (defense-in-depth : pin search_path AND write admin.pwds, not pwds)
    └── No (every reference is schema-qualified) : pin search_path anyway as
        operators/casts also resolve via search_path. Cost : zero.

Where do I put my tables? :

Single application, one logical domain?
├── Yes : public schema is fine AFTER `REVOKE CREATE ON SCHEMA public
│         FROM PUBLIC` (v14-or-earlier databases : MANDATORY).
│         New v15+ databases : already revoked at initdb time.
└── No : multiple modules / layers / privilege boundaries?
    ├── Module separation (app, auth, billing, analytics) :
    │       one schema per module, dedicated role per module with
    │       USAGE on its own schema + SELECT on read-only schemas it needs
    ├── ETL staging vs serving :
    │       schemas etl_staging (UNLOGGED tables, BYPASSRLS service role),
    │       serving (final tables, RLS, app role has SELECT only)
    └── Multi-tenant : see "schema-per-tenant or RLS" decision above

Patterns :

Pattern 1 : IDENTITY column for every new primary key

ALWAYS use GENERATED ALWAYS AS IDENTITY for new surrogate primary keys on v10+ unless the application has a documented need to supply ids. NEVER use serial / bigserial in new code : it creates an implicit sequence whose ownership and privilege semantics surprise everyone.

-- Preferred (system-assigned, app cannot override accidentally) :
CREATE TABLE customers (
  id        bigint  GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  email     citext  NOT NULL UNIQUE,
  full_name text    NOT NULL,
  created_at timestamptz NOT NULL DEFAULT now()
);

-- Migration loads need to override : use OVERRIDING SYSTEM VALUE
INSERT INTO customers (id, email, full_name)
  OVERRIDING SYSTEM VALUE
  VALUES (42, '[e-mail]', 'Alice');

-- App is the legitimate id-owner (rare, e.g. distributed id generator) :
CREATE TABLE events (
  id   bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  ...
);
-- Now both work :
INSERT INTO events DEFAULT VALUES;            -- sequence allocates
INSERT INTO events (id) VALUES (9999);        -- app-supplied id honored

WHY : serial is documented as a "shorthand for creating a sequence and integer column" but the sequence is owned independently, ALTER COLUMN TYPE breaks the dependency, INSERT-grants leak the sequence implicitly, and the column type stays integer (not bigint) which exhausts at 2.1B rows. GENERATED AS IDENTITY is the SQL-standard spelling, is privilege-clean (the sequence is owned by the identity column, not visible to non-owners), and chooses bigint by default in modern style. Source : postgresql.org/docs/17/sql-createtable.html (Identity Columns section), Anti-Pattern 1 in vooronderzoek §19.

Pattern 2 : snake_case identifiers, never quoted

ALWAYS use lowercase snake_case for every table, column, index, sequence, and function name. NEVER ship "CamelCase" identifiers : PostgreSQL folds unquoted names to lowercase, so a quoted "CustomerId" is a DIFFERENT identifier than an unquoted CustomerId (which resolves to customerid).

-- Correct
CREATE TABLE sales_order (
  id            bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  customer_id   bigint NOT NULL REFERENCES customers(id),
  ordered_at    timestamptz NOT NULL DEFAULT now(),
  total_amount  numeric(12,2) NOT NULL
);

CREATE INDEX sales_order_customer_id_idx ON sales_order (customer_id);

-- Wrong (forces every consumer to quote forever, breaks ORM model classes,
-- breaks pg_dump --schema-only diffs across environments)
CREATE TABLE "SalesOrder" (
  "ID"          bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  "CustomerID"  bigint NOT NULL,
  ...
);

Conventions enforced project-wide :

• Tables : singular OR plural, pick one project-wide and document. Both are legitimate.
• Primary key : id (preferred — works with JOIN ... USING(id) mechanically) OR <entity>_id (preferred — self-describing in joins). NEVER mix both styles.
• Foreign keys : <referenced_entity>_id (e.g. customer_id in sales_order).
• Junction tables : <entity_a>_<entity_b> alphabetical (e.g. customer_tag, not tag_customer).
• Indexes : <table>_<column(s)>_<type>_idx (e.g. sales_order_customer_id_idx, sales_order_search_gin_idx).
• Sequences (when not implicit) : <table>_<column>_seq (PostgreSQL's auto-generated default).

WHY : an unquoted identifier is case-folded to lowercase at parse time, so SELECT * FROM Customers and SELECT * FROM customers resolve to the same relation. The MOMENT you write CREATE TABLE "Customers" (with quotes and capitals), every subsequent reference must be quoted too, and ANY tool that lowercases generates a "relation does not exist" error. Source : postgresql.org/docs/17/sql-syntax-lexical.html (Identifiers and Key Words).

Pattern 3 : Lock down public schema on v14-and-earlier databases

ALWAYS run REVOKE CREATE ON SCHEMA public FROM PUBLIC on any database that was initialized before PostgreSQL 15 (or upgraded in place from v14 or earlier). NEVER assume public is locked just because the cluster runs v15+ : pre-existing databases keep their old permissive grant. Only databases freshly created by initdb on v15+ have it revoked by default.

-- Detect the state on every database :
SELECT n.nspname,
       has_schema_privilege('public', n.nspname, 'CREATE') AS public_can_create
FROM pg_namespace n
WHERE n.nspname = 'public';

-- Lock it down (idempotent ; safe to run on already-revoked databases)
REVOKE CREATE ON SCHEMA public FROM PUBLIC;

-- Verify
SELECT has_schema_privilege('public', 'public', 'CREATE');
-- Returns f after revoke.

-- Application roles need explicit GRANT :
GRANT USAGE  ON SCHEMA public TO app_reader, app_writer;
GRANT CREATE ON SCHEMA public TO app_writer;

WHY : the historical default let any logged-in role create objects in public. That meant a low-privilege role could shadow tables, install malicious functions, or set type defaults that get picked up by search_path. CVE-2018-1058 used exactly this vector against pg_dump. The v15 change closed the default but cannot retroactively fix old databases. Source : postgresql.org/docs/17/ddl-schemas.html (Schemas section, "Public Schema"), PostgreSQL 15 release notes.

Pattern 4 : Pin search_path on every SECURITY DEFINER function

ALWAYS set search_path explicitly on SECURITY DEFINER functions, ending with pg_temp so the per-session temp schema cannot shadow trusted objects. NEVER ship a SECURITY DEFINER function without SET search_path : the caller controls their own search_path, and the function inherits it.

-- Correct : search_path is pinned, schemas are pre-qualified anyway
CREATE FUNCTION admin.check_password(uname text, pass text)
RETURNS boolean
LANGUAGE plpgsql
SECURITY DEFINER
SET search_path = pg_catalog, admin, pg_temp
AS $$
DECLARE passed boolean;
BEGIN
  SELECT (pwd = $2) INTO passed
  FROM admin.pwds
  WHERE username = $1;
  RETURN passed;
END;
$$;

-- Restrict execute privileges atomically with creation :
BEGIN;
  ALTER FUNCTION admin.check_password(text, text) OWNER TO admin_owner;
  REVOKE ALL ON FUNCTION admin.check_password(text, text) FROM PUBLIC;
  GRANT EXECUTE ON FUNCTION admin.check_password(text, text) TO app_role;
COMMIT;

-- Wrong : caller's search_path determines what `pwds` means
CREATE FUNCTION check_password(uname text, pass text)
RETURNS boolean
LANGUAGE plpgsql
SECURITY DEFINER  -- runs as owner, but resolves names via caller's path
AS $$
BEGIN
  -- attacker : SET search_path = malicious, public ;
  
   ( pwd  $  pwds  username  $);
;
$$;

WHY : a SECURITY DEFINER function executes with the OWNER's privileges, but name resolution walks the CALLER's search_path unless the function pins its own. Putting pg_temp last forbids the temporary schema (always writable) from shadowing trusted names. Pinning pg_catalog first additionally protects against operator/cast hijacking. Source : postgresql.org/docs/17/sql-createfunction.html (Writing SECURITY DEFINER Functions Safely), CVE-2007-2138.

Pattern 5 : Multi-schema layout for module separation

ALWAYS use one schema per module when the application has distinct privilege boundaries (auth vs application vs analytics). NEVER throw everything into public and try to enforce module boundaries with role-by-table grants : that scales poorly and forgets new tables.

-- One database, four schemas, four roles :
CREATE SCHEMA auth      AUTHORIZATION auth_owner;
CREATE SCHEMA app       AUTHORIZATION app_owner;
CREATE SCHEMA etl       AUTHORIZATION etl_owner;
CREATE SCHEMA reporting AUTHORIZATION reporting_owner;

-- Per-role search_path so each service sees its own schema first :
ALTER ROLE auth_service      SET search_path = auth, public;
ALTER ROLE app_service       SET search_path = app, public;
ALTER ROLE etl_service       SET search_path = etl, app, public;
ALTER ROLE reporting_service SET search_path = reporting, app, public;

-- Default privileges so newly-created objects inherit grants automatically :
ALTER DEFAULT PRIVILEGES FOR ROLE app_owner IN SCHEMA app
  GRANT SELECT, INSERT, UPDATE, DELETE ON TABLES TO app_service;
ALTER DEFAULT PRIVILEGES FOR ROLE app_owner IN SCHEMA app
  GRANT USAGE ON SEQUENCES TO app_service;

-- Cross-schema FK is fine inside one database :
CREATE TABLE app.session (
  id         bigint GENERATED ALWAYS AS  ,
  user_id       auth.user(id),
  expires_at timestamptz 
);

WHY : search_path + ALTER DEFAULT PRIVILEGES together make module separation cheap : new tables in app schema are automatically grantable to app_service, the service role's search_path resolves bare names to its own schema, and cross-schema references stay legible (auth.user). Source : postgresql.org/docs/17/ddl-schemas.html, postgresql.org/docs/17/sql-alterdefaultprivileges.html.

Pattern 6 : Set search_path correctly at every level

ALWAYS understand the four levels at which search_path can be set and which one applies. NEVER assume SET search_path inside one session affects another session.

Level	How to set	Survives reconnect?	Use case
Cluster-wide default	postgresql.conf : search_path = ...	Yes (server-wide)	Last-resort default ; rarely customized
Database default	ALTER DATABASE x SET search_path = ...	Yes (per database)	Multi-schema app database
Role default	ALTER ROLE r SET search_path = ...	Yes (per role)	Service-role isolation (Pattern 5)
Session	SET search_path = ...	Until reconnect	Ad-hoc psql work
Transaction	SET LOCAL search_path = ...	Until COMMIT/ROLLBACK	Migration scripts, multi-tenant request handlers
Function	SET search_path = ... in CREATE FUNCTION	Until function exits	SECURITY DEFINER hardening (Pattern 4)

-- Show effective resolution chain
SHOW search_path;

-- Inspect the role/database defaults that build the session value
SELECT rolname, rolconfig FROM pg_roles      WHERE rolname = current_user;
SELECT datname, datconfig FROM pg_database   WHERE datname = current_database();

WHY : a SET search_path issued in a SQL console only affects the open session. Server-side connection pools (PgBouncer transaction mode) make session SET unreliable because the next transaction may arrive on a different server connection. Use ALTER ROLE for durable per-role config, SET LOCAL inside a transaction for per-request multi-tenancy. Source : postgresql.org/docs/17/config-setting.html, postgresql.org/docs/17/runtime-config-client.html.

Anti-Patterns :

(One-liners ; full diagnosis + fix in references/anti-patterns.md.)

• serial / bigserial in new code : implicit-sequence privilege foot-gun, type stuck at integer. Fix : GENERATED ALWAYS AS IDENTITY.
• Quoted "CamelCase" identifiers : every consumer must quote forever, ORM round-tripping breaks. Fix : snake_case, no quotes.
• public schema left writable on pre-v15 databases : low-privilege roles can shadow objects, CVE-2018-1058 risk. Fix : REVOKE CREATE ON SCHEMA public FROM PUBLIC.
• SECURITY DEFINER function without SET search_path : caller can hijack name resolution. Fix : pin pg_catalog, <trusted>, pg_temp in CREATE FUNCTION.
• Reserved-word identifiers (user, order, table) : every statement must quote, breaks tooling. Fix : app_user, sales_order.
• Mixing id and <entity>_id PK styles in one schema : joins ambiguous, ORMs misalign. Fix : pick one project-wide, document in CLAUDE.md.
• Schema-per-tenant for > ~50 tenants : DDL changes become an N-pass loop, query planner stats explode. Fix : RLS (see postgres-core-rls-policies).
• Cross-database joins via dblink for normal queries : connection overhead per call, no transactional integrity. Fix : merge into one database with schemas, or use postgres_fdw carefully.
• Missing pg_temp last in pinned search_path of SECURITY DEFINER : temp-schema injection vector still open. Fix : ALWAYS end with pg_temp.

Reference Links :

• references/methods.md : IDENTITY column reference (ALWAYS vs BY DEFAULT, OVERRIDING SYSTEM/USER VALUE, sequence ownership), search_path resolution levels, GRANT matrix for schemas, naming-convention table.
• references/examples.md : End-to-end multi-schema setup, SECURITY DEFINER hardening walk-through, IDENTITY migration from serial, public-schema lockdown audit script, schema-per-tenant vs RLS comparison.
• references/anti-patterns.md : Each anti-pattern with symptom, cause, SQLSTATE (where applicable), detection query, and fix migration.

See Also :

• postgres-core-architecture : MVCC, isolation, WAL durability (the runtime model your schema lives in)
• postgres-core-version-matrix : v15 / v16 / v17 feature deltas (including v15 public-schema default)
• postgres-core-rls-policies : RLS for multi-tenant, FORCE ROW LEVEL SECURITY, BYPASSRLS audits
• postgres-core-roles-privileges : CREATE ROLE, GRANT, ALTER DEFAULT PRIVILEGES details
• postgres-impl-zero-downtime-ddl : safe execution of the schema changes you design here
• Vooronderzoek section : §2 (Full SQL Surface : IDENTITY), §13 (Security Model : search_path), §19 (Anti-Patterns : serial, public schema, search_path injection)
• Official docs : https://www.postgresql.org/docs/17/ddl-schemas.html, https://www.postgresql.org/docs/17/sql-createtable.html (Identity Columns), https://www.postgresql.org/docs/17/sql-createfunction.html (Writing SECURITY DEFINER Functions Safely)