postgresql-table-design

# PostgreSQL Table Design

## 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 (toast_tuple_target = 4096) for threshold. Case-insensitive: for locale/accent handling use non-deterministic collations; for plain ASCII use expression indexes on LOWER(col) (preferred unless column needs case-insensitive PK/FK/UNIQUE) or CITEXT.
- **Money**: NUMERIC(p,s) (never float).
- **Time**: TIMESTAMPTZ for timestamps; DATE for date-only; INTERVAL for durations. Avoid TIMESTAMP (without timezone). Use now() for transaction start time, clock_timestamp() for current wall-clock time.
- **Booleans**: BOOLEAN with NOT NULL constraint unless tri-state values are required.
- **Enums**: CREATE TYPE ... AS ENUM for small, stable sets (e.g. US states, days of week). For business-logic-driven and evolving values (e.g. order statuses) → use TEXT (or INT) + CHECK or lookup table.
- **Arrays**: TEXT[], INTEGER[], etc. Use for ordered lists where you query elements. Index with **GIN** for containment (@>, <@) and overlap (&&) queries. Access: arr[1] (1-indexed), arr[1:3] (slicing). Good for tags, categories; avoid for relations—use junction tables instead. Literal syntax: '{val1,val2}' or ARRAY[val1,val2].
- **Range types**: daterange, numrange, tstzrange for intervals. Support overlap (&&), containment (@>), operators. Index with **GiST**. Good for scheduling, versioning, numeric ranges. Pick a bounds scheme and use it consistently; prefer [) (inclusive/exclusive) by default.
- **Network types**: INET for IP addresses, CIDR for network ranges, MACADDR for MAC addresses. Support network operators (<<, >>, &&).
- **Geometric types**: POINT, LINE, POLYGON, CIRCLE for 2D spatial data. Index with **GiST**. Consider **PostGIS** for advanced spatial features.
- **Text search**: TSVECTOR for full-text search documents, TSQUERY for search queries. Index tsvector with **GIN**. Always specify language: to_tsvector('english', col) and to_tsquery('english', 'query'). Never use single-argument versions. This applies to both index expressions and queries.
- **Domain types**: CREATE DOMAIN email AS TEXT CHECK (VALUE ~ '^[^@]+@[^@]+$') for reusable custom types with validation. Enforces constraints across tables.
- **Composite types**: CREATE TYPE address AS (street TEXT, city TEXT, zip TEXT) for structured data within columns. Access with (col).field syntax.
- **JSONB**: preferred over JSON; index with **GIN**. Use only for optional/semi-structured attrs. ONLY use JSON if the original ordering of the contents MUST be preserved.
- **Vector types**: vector type by pgvector for vector similarity search for embeddings.

### Do not use the following data types

- DO NOT use timestamp (without time zone); DO use timestamptz instead.
- DO NOT use char(n) or varchar(n); DO use text instead.
- DO NOT use money type; DO use numeric instead.
- DO NOT use timetz type; DO use timestamptz instead.
- DO NOT use timestamptz(0) or any other precision specification; DO use timestamptz instead
- DO NOT use serial type; DO use generated always as identity instead.

## Table Types

- **Regular**: default; fully durable, logged.
- **TEMPORARY**: session-scoped, auto-dropped, not logged. Faster for scratch work.
- **UNLOGGED**: persistent but not crash-safe. Faster writes; good for caches/staging.

## Row-Level Security

Enable with ALTER TABLE tbl ENABLE ROW LEVEL SECURITY. Create policies: CREATE POLICY user_access ON orders FOR SELECT TO app_users USING (user_id = current_user_id()). Built-in user-based access control at the row level.

## Constraints

- **PK**: implicit UNIQUE + NOT NULL; creates a B-tree index.
- **FK**: specify ON DELETE/UPDATE action (CASCADE, RESTRICT, SET NULL, SET DEFAULT). Add explicit index on referencing column—speeds up joins and prevents locking issues on parent deletes/updates. Use DEFERRABLE INITIALLY DEFERRED for circular FK dependencies checked at transaction end.
- **UNIQUE**: creates a B-tree index; allows multiple NULLs unless NULLS NOT DISTINCT (PG15+). Standard behavior: (1, NULL) and (1, NULL) are allowed. With NULLS NOT DISTINCT: only one (1, NULL) allowed. Prefer NULLS NOT DISTINCT unless you specifically need duplicate NULLs.
- **CHECK**: row-local constraints; NULL values pass the check (three-valued logic). Example: CHECK (price > 0) allows NULL prices. Combine with NOT NULL to enforce: price NUMERIC NOT NULL CHECK (price > 0).
- **EXCLUDE**: prevents overlapping values using operators. EXCLUDE USING gist (room_id WITH =, booking_period WITH &&) prevents double-booking rooms. Requires appropriate index type (often GiST).

## Indexing

- **B-tree**: default for equality/range queries (=, <, >, BETWEEN, ORDER BY)
- **Composite**: order matters—index used if equality on leftmost prefix (WHERE a = ? AND b > ? uses index on (a,b), but WHERE b = ? does not). Put most selective/frequently filtered columns first.
- **Covering**: CREATE INDEX ON tbl (id) INCLUDE (name, email) - includes non-key columns for index-only scans without visiting table.
- **Partial**: for hot subsets (WHERE status = 'active' → CREATE INDEX ON tbl (user_id) WHERE status = 'active'). Any query with status = 'active' can use this index.
- **Expression**: for computed search keys (CREATE INDEX ON tbl (LOWER(email))). Expression must match exactly in WHERE clause: WHERE LOWER(email) = '[email protected]'.
- **GIN**: JSONB containment/existence, arrays (@>, ?), full-text search (@@)
- **GiST**: ranges, geometry, exclusion constraints
- **BRIN**: very large, naturally ordered data (time-series)—minimal storage overhead. Effective when row order on disk correlates with indexed column (insertion order or after CLUSTER).

## Partitioning

- Use for very large tables (>100M rows) where queries consistently filter on partition key (often time/date).
- Alternate use: use for tables where data maintenance tasks dictates e.g. data pruned or bulk replaced periodically
- **RANGE**: common for time-series (PARTITION BY RANGE (created_at)). Create partitions: CREATE TABLE logs_2024_01 PARTITION OF logs FOR VALUES FROM ('2024-01-01') TO ('2024-02-01'). **TimescaleDB** automates time-based or ID-based partitioning with retention policies and compression.
- **LIST**: for discrete values (PARTITION BY LIST (region)). Example: FOR VALUES IN ('us-east', 'us-west').
- **HASH**: for even distribution when no natural key (PARTITION BY HASH (user_id)). Creates N partitions with modulus.
- **Constraint exclusion**: requires CHECK constraints on partitions for query planner to prune. Auto-created for declarative partitioning (PG10+).
- Prefer declarative partitioning or hypertables. Do NOT use table inheritance.
- **Limitations**: no global UNIQUE constraints—include partition key in PK/UNIQUE. FKs from partitioned tables not supported; use triggers.

## Special Considerations

### Update-Heavy Tables

- **Separate hot/cold columns**—put frequently updated columns in separate table to minimize bloat.
- **Use fillfactor=90** to leave space for HOT updates that avoid index maintenance.
- **Avoid updating indexed columns**—prevents beneficial HOT updates.
- **Partition by update patterns**—separate frequently updated rows in a different partition from stable data.

### Insert-Heavy Workloads

- **Minimize indexes**—only create what you query; every index slows inserts.
- **Use COPY or multi-row INSERT** instead of single-row inserts.
- **UNLOGGED tables** for rebuildable staging data—much faster writes.
- **Defer index creation** for bulk loads—>drop index, load data, recreate indexes.
- **Partition by time/hash** to distribute load. **TimescaleDB** automates partitioning and compression of insert-heavy data.
- **Use a natural key for primary key** such as a (timestamp, device_id) if enforcing global uniqueness is important many insert-heavy tables don't need a primary key at all.
- If you do need a surrogate key, **Prefer BIGINT GENERATED ALWAYS AS IDENTITY over UUID**.

### Upsert-Friendly Design

- **Requires UNIQUE index** on conflict target columns—ON CONFLICT (col1, col2) needs exact matching unique index (partial indexes don't work).
- **Use EXCLUDED.column** to reference would-be-inserted values; only update columns that actually changed to reduce write overhead.
- **DO NOTHING faster** than DO UPDATE when no actual update needed.

### Safe Schema Evolution

- **Transactional DDL**: most DDL operations can run in transactions and be rolled back—BEGIN; ALTER TABLE...; ROLLBACK; for safe testing.
- **Concurrent index creation**: CREATE INDEX CONCURRENTLY avoids blocking writes but can't run in transactions.
- **Volatile defaults cause rewrites**: adding NOT NULL columns with volatile defaults (e.g., now(), gen_random_uuid()) rewrites entire table. Non-volatile defaults are fast.
- **Drop constraints before columns**: ALTER TABLE DROP CONSTRAINT then DROP COLUMN to avoid dependency issues.
- **Function signature changes**: CREATE OR REPLACE with different arguments creates overloads, not replacements. DROP old version if no overload desired.

## Generated Columns

- ... GENERATED ALWAYS AS (<expr>) STORED for computed, indexable fields. PG18+ adds VIRTUAL columns (computed on read, not stored).

## Extensions

- **pgcrypto**: crypt() for password hashing.
- **uuid-ossp**: alternative UUID functions; prefer pgcrypto for new projects.
- **pg_trgm**: fuzzy text search with % operator, similarity() function. Index with GIN for LIKE '%pattern%' acceleration.
- **citext**: case-insensitive text type. Prefer expression indexes on LOWER(col) unless you need case-insensitive constraints.
- **btree_gin/btree_gist**: enable mixed-type indexes (e.g., GIN index on both JSONB and text columns).
- **hstore**: key-value pairs; mostly superseded by JSONB but useful for simple string mappings.
- **timescaledb**: essential for time-series—automated partitioning, retention, compression, continuous aggregates.
- **postgis**: comprehensive geospatial support beyond basic geometric types—essential for location-based applications.
- **pgvector**: vector similarity search for embeddings.
- **pgaudit**: audit logging for all database activity.

## JSONB Guidance

- Prefer JSONB with **GIN** index.
- Default: CREATE INDEX ON tbl USING GIN (jsonb_col); → accelerates:
- **Containment** jsonb_col @> '{"k":"v"}'
- **Key existence** jsonb_col ? 'k', **any/all keys** ?\|, ?&
- **Path containment** on nested docs
- **Disjunction** jsonb_col @> ANY(ARRAY['{"status":"active"}', '{"status":"pending"}'])
- Heavy @> workloads: consider opclass jsonb_path_ops for smaller/faster containment-only indexes:
- CREATE INDEX ON tbl USING GIN (jsonb_col jsonb_path_ops);
- **Trade-off**: loses support for key existence (?, ?|, ?&) queries—only supports containment (@>)
- Equality/range on a specific scalar field: extract and index with B-tree (generated column or expression):
- ALTER TABLE tbl ADD COLUMN price INT GENERATED ALWAYS AS ((jsonb_col->>'price')::INT) STORED;
- CREATE INDEX ON tbl (price);
- Prefer queries like WHERE price BETWEEN 100 AND 500 (uses B-tree) over WHERE (jsonb_col->>'price')::INT BETWEEN 100 AND 500 without index.
- Arrays inside JSONB: use GIN + @> for containment (e.g., tags). Consider jsonb_path_ops if only doing containment.
- Keep core relations in tables; use JSONB for optional/variable attributes.
- Use constraints to limit allowed JSONB values in a column e.g. config JSONB NOT NULL CHECK(jsonb_typeof(config) = 'object')

## Examples

### Users

CREATE TABLE users (
  user_id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  email TEXT NOT NULL UNIQUE,
  name TEXT NOT NULL,
  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
);
CREATE UNIQUE INDEX ON users (LOWER(email));
CREATE INDEX ON users (created_at);

### Orders

CREATE TABLE orders (
  order_id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  user_id BIGINT NOT NULL REFERENCES users(user_id),
  status TEXT NOT NULL DEFAULT 'PENDING' CHECK (status IN ('PENDING','PAID','CANCELED')),
  total NUMERIC(10,2) NOT NULL CHECK (total > 0),
  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
);
CREATE INDEX ON orders (user_id);
CREATE INDEX ON orders (created_at);

### JSONB

CREATE TABLE profiles (
  user_id BIGINT PRIMARY KEY REFERENCES users(user_id),
  attrs JSONB NOT NULL DEFAULT '{}',
  theme TEXT GENERATED ALWAYS AS (attrs->>'theme') STORED
);
CREATE INDEX profiles_attrs_gin ON profiles USING GIN (attrs);