1---
2name: postgresql-table-design
3description: Use this skill when designing or reviewing a PostgreSQL-specific schema. Covers best-practices, data types, indexing, constraints, performance patterns, and advanced features
4---
5 
6# PostgreSQL Table Design
7 
8## Core Rules
9 
10- 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.
11- **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.
12- Add **NOT NULL** everywhere it’s semantically required; use **DEFAULT**s for common values.
13- Create **indexes for access paths you actually query**: PK/unique (auto), **FK columns (manual!)**, frequent filters/sorts, and join keys.
14- 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).
15 
16## PostgreSQL “Gotchas”
17 
18- **Identifiers**: unquoted → lowercased. Avoid quoted/mixed-case names. Convention: use `snake_case` for table/column names.
19- **Unique + NULLs**: UNIQUE allows multiple NULLs. Use `UNIQUE (...) NULLS NOT DISTINCT` (PG15+) to restrict to one NULL.
20- **FK indexes**: PostgreSQL **does not** auto-index FK columns. Add them.
21- **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.
22- **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.
23- **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.
24- **MVCC**: updates/deletes leave dead tuples; vacuum handles them—design to avoid hot wide-row churn.
25 
26## Data Types
27 
28- **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).
29- **Integers**: prefer `BIGINT` unless storage space is critical; `INTEGER` for smaller ranges; avoid `SMALLINT` unless constrained.
30- **Floats**: prefer `DOUBLE PRECISION` over `REAL` unless storage space is critical. Use `NUMERIC` for exact decimal arithmetic.
31- **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`.
32- **Money**: `NUMERIC(p,s)` (never float).
33- **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.
34- **Booleans**: `BOOLEAN` with `NOT NULL` constraint unless tri-state values are required.
35- **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.
36- **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]`.
37- **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.
38- **Network types**: `INET` for IP addresses, `CIDR` for network ranges, `MACADDR` for MAC addresses. Support network operators (`<<`, `>>`, `&&`).
39- **Geometric types**: `POINT`, `LINE`, `POLYGON`, `CIRCLE` for 2D spatial data. Index with **GiST**. Consider **PostGIS** for advanced spatial features.
40- **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.
41- **Domain types**: `CREATE DOMAIN email AS TEXT CHECK (VALUE ~ '^[^@]+@[^@]+$')` for reusable custom types with validation. Enforces constraints across tables.
42- **Composite types**: `CREATE TYPE address AS (street TEXT, city TEXT, zip TEXT)` for structured data within columns. Access with `(col).field` syntax.
43- **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.
44- **Vector types**: `vector` type by `pgvector` for vector similarity search for embeddings.
45 
46### Do not use the following data types
47 
48- DO NOT use `timestamp` (without time zone); DO use `timestamptz` instead.
49- DO NOT use `char(n)` or `varchar(n)`; DO use `text` instead.
50- DO NOT use `money` type; DO use `numeric` instead.
51- DO NOT use `timetz` type; DO use `timestamptz` instead.
52- DO NOT use `timestamptz(0)` or any other precision specification; DO use `timestamptz` instead
53- DO NOT use `serial` type; DO use `generated always as identity` instead.
54 
55## Table Types
56 
57- **Regular**: default; fully durable, logged.
58- **TEMPORARY**: session-scoped, auto-dropped, not logged. Faster for scratch work.
59- **UNLOGGED**: persistent but not crash-safe. Faster writes; good for caches/staging.
60 
61## Row-Level Security
62 
63Enable 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.
64 
65## Constraints
66 
67- **PK**: implicit UNIQUE + NOT NULL; creates a B-tree index.
68- **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.
69- **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.
70- **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)`.
71- **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).
72 
73## Indexing
74 
75- **B-tree**: default for equality/range queries (`=`, `<`, `>`, `BETWEEN`, `ORDER BY`)
76- **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.
77- **Covering**: `CREATE INDEX ON tbl (id) INCLUDE (name, email)` - includes non-key columns for index-only scans without visiting table.
78- **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.
79- **Expression**: for computed search keys (`CREATE INDEX ON tbl (LOWER(email))`). Expression must match exactly in WHERE clause: `WHERE LOWER(email) = '[email protected]'`.
80- **GIN**: JSONB containment/existence, arrays (`@>`, `?`), full-text search (`@@`)
81- **GiST**: ranges, geometry, exclusion constraints
82- **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`).
83 
84## Partitioning
85 
86- Use for very large tables (>100M rows) where queries consistently filter on partition key (often time/date).
87- Alternate use: use for tables where data maintenance tasks dictates e.g. data pruned or bulk replaced periodically
88- **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.
89- **LIST**: for discrete values (`PARTITION BY LIST (region)`). Example: `FOR VALUES IN ('us-east', 'us-west')`.
90- **HASH**: for even distribution when no natural key (`PARTITION BY HASH (user_id)`). Creates N partitions with modulus.
91- **Constraint exclusion**: requires `CHECK` constraints on partitions for query planner to prune. Auto-created for declarative partitioning (PG10+).
92- Prefer declarative partitioning or hypertables. Do NOT use table inheritance.
93- **Limitations**: no global UNIQUE constraints—include partition key in PK/UNIQUE. FKs from partitioned tables not supported; use triggers.
94 
95## Special Considerations
96 
97### Update-Heavy Tables
98 
99- **Separate hot/cold columns**—put frequently updated columns in separate table to minimize bloat.
100- **Use `fillfactor=90`** to leave space for HOT updates that avoid index maintenance.
101- **Avoid updating indexed columns**—prevents beneficial HOT updates.
102- **Partition by update patterns**—separate frequently updated rows in a different partition from stable data.
103 
104### Insert-Heavy Workloads
105 
106- **Minimize indexes**—only create what you query; every index slows inserts.
107- **Use `COPY` or multi-row `INSERT`** instead of single-row inserts.
108- **UNLOGGED tables** for rebuildable staging data—much faster writes.
109- **Defer index creation** for bulk loads—>drop index, load data, recreate indexes.
110- **Partition by time/hash** to distribute load. **TimescaleDB** automates partitioning and compression of insert-heavy data.
111- **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.
112- If you do need a surrogate key, **Prefer `BIGINT GENERATED ALWAYS AS IDENTITY` over `UUID`**.
113 
114### Upsert-Friendly Design
115 
116- **Requires UNIQUE index** on conflict target columns—`ON CONFLICT (col1, col2)` needs exact matching unique index (partial indexes don't work).
117- **Use `EXCLUDED.column`** to reference would-be-inserted values; only update columns that actually changed to reduce write overhead.
118- **`DO NOTHING` faster** than `DO UPDATE` when no actual update needed.
119 
120### Safe Schema Evolution
121 
122- **Transactional DDL**: most DDL operations can run in transactions and be rolled back—`BEGIN; ALTER TABLE...; ROLLBACK;` for safe testing.
123- **Concurrent index creation**: `CREATE INDEX CONCURRENTLY` avoids blocking writes but can't run in transactions.
124- **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.
125- **Drop constraints before columns**: `ALTER TABLE DROP CONSTRAINT` then `DROP COLUMN` to avoid dependency issues.
126- **Function signature changes**: `CREATE OR REPLACE` with different arguments creates overloads, not replacements. DROP old version if no overload desired.
127 
128## Generated Columns
129 
130- `... GENERATED ALWAYS AS (<expr>) STORED` for computed, indexable fields. PG18+ adds `VIRTUAL` columns (computed on read, not stored).
131 
132## Extensions
133 
134- **`pgcrypto`**: `crypt()` for password hashing.
135- **`uuid-ossp`**: alternative UUID functions; prefer `pgcrypto` for new projects.
136- **`pg_trgm`**: fuzzy text search with `%` operator, `similarity()` function. Index with GIN for `LIKE '%pattern%'` acceleration.
137- **`citext`**: case-insensitive text type. Prefer expression indexes on `LOWER(col)` unless you need case-insensitive constraints.
138- **`btree_gin`/`btree_gist`**: enable mixed-type indexes (e.g., GIN index on both JSONB and text columns).
139- **`hstore`**: key-value pairs; mostly superseded by JSONB but useful for simple string mappings.
140- **`timescaledb`**: essential for time-series—automated partitioning, retention, compression, continuous aggregates.
141- **`postgis`**: comprehensive geospatial support beyond basic geometric types—essential for location-based applications.
142- **`pgvector`**: vector similarity search for embeddings.
143- **`pgaudit`**: audit logging for all database activity.
144 
145## JSONB Guidance
146 
147- Prefer `JSONB` with **GIN** index.
148- Default: `CREATE INDEX ON tbl USING GIN (jsonb_col);` → accelerates:
149  - **Containment** `jsonb_col @> '{"k":"v"}'`
150  - **Key existence** `jsonb_col ? 'k'`, **any/all keys** `?\|`, `?&`
151  - **Path containment** on nested docs
152  - **Disjunction** `jsonb_col @> ANY(ARRAY['{"status":"active"}', '{"status":"pending"}'])`
153- Heavy `@>` workloads: consider opclass `jsonb_path_ops` for smaller/faster containment-only indexes:
154  - `CREATE INDEX ON tbl USING GIN (jsonb_col jsonb_path_ops);`
155  - **Trade-off**: loses support for key existence (`?`, `?|`, `?&`) queries—only supports containment (`@>`)
156- Equality/range on a specific scalar field: extract and index with B-tree (generated column or expression):
157  - `ALTER TABLE tbl ADD COLUMN price INT GENERATED ALWAYS AS ((jsonb_col->>'price')::INT) STORED;`
158  - `CREATE INDEX ON tbl (price);`
159  - Prefer queries like `WHERE price BETWEEN 100 AND 500` (uses B-tree) over `WHERE (jsonb_col->>'price')::INT BETWEEN 100 AND 500` without index.
160- Arrays inside JSONB: use GIN + `@>` for containment (e.g., tags). Consider `jsonb_path_ops` if only doing containment.
161- Keep core relations in tables; use JSONB for optional/variable attributes.
162- Use constraints to limit allowed JSONB values in a column e.g. `config JSONB NOT NULL CHECK(jsonb_typeof(config) = 'object')`
163 
164## Examples
165 
166### Users
167 
168```sql
169CREATE TABLE users (
170  user_id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
171  email TEXT NOT NULL UNIQUE,
172  name TEXT NOT NULL,
173  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
174);
175CREATE UNIQUE INDEX ON users (LOWER(email));
176CREATE INDEX ON users (created_at);
177```
178 
179### Orders
180 
181```sql
182CREATE TABLE orders (
183  order_id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
184  user_id BIGINT NOT NULL REFERENCES users(user_id),
185  status TEXT NOT NULL DEFAULT 'PENDING' CHECK (status IN ('PENDING','PAID','CANCELED')),
186  total NUMERIC(10,2) NOT NULL CHECK (total > 0),
187  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
188);
189CREATE INDEX ON orders (user_id);
190CREATE INDEX ON orders (created_at);
191```
192 
193### JSONB
194 
195```sql
196CREATE TABLE profiles (
197  user_id BIGINT PRIMARY KEY REFERENCES users(user_id),
198  attrs JSONB NOT NULL DEFAULT '{}',
199  theme TEXT GENERATED ALWAYS AS (attrs->>'theme') STORED
200);
201CREATE INDEX profiles_attrs_gin ON profiles USING GIN (attrs);
202```
203