This PostgreSQL error occurs when CREATE TABLE or ALTER TABLE statements contain conflicting clauses, syntax errors, or violate table design rules like defining multiple primary keys or mixing incompatible table types.
The 42P16 error code signals that PostgreSQL successfully parsed your CREATE TABLE or ALTER TABLE statement but rejected it because the resulting table design violates an internal rule or constraint. Unlike syntax errors (which fail during parsing), this error indicates a logical problem with the table definition itself. This error commonly appears when you attempt to define multiple primary keys on a single table, combine incompatible table features (like UNLOGGED with PARTITION BY), use identity columns as partition keys, or have syntax issues like missing commas between column definitions. PostgreSQL enforces strict rules about table structure to maintain data integrity and performance.
PostgreSQL typically provides specific details about what part of the table definition is invalid. Run your statement again and capture the full error output:
CREATE TABLE users (
id SERIAL PRIMARY KEY,
email VARCHAR(255) PRIMARY KEY -- Multiple primary keys
);The error will indicate:
ERROR: multiple primary keys for table "users" are not allowed
SQLSTATE: 42P16Look for keywords in the error message that identify the specific violation.
A table can only have one primary key. If you need multiple columns in the primary key, use a composite key:
Wrong:
CREATE TABLE orders (
order_id INT PRIMARY KEY,
customer_id INT PRIMARY KEY -- Error: multiple PKs
);Correct (composite key):
CREATE TABLE orders (
order_id INT,
customer_id INT,
PRIMARY KEY (order_id, customer_id)
);Correct (single column with unique constraint):
CREATE TABLE orders (
order_id INT PRIMARY KEY,
customer_id INT UNIQUE NOT NULL
);Missing or extra commas are common causes of this error. Review your CREATE TABLE statement carefully:
Wrong (missing comma):
CREATE TABLE products (
id SERIAL PRIMARY KEY
name VARCHAR(255) NOT NULL -- Missing comma
price DECIMAL(10,2)
);Correct:
CREATE TABLE products (
id SERIAL PRIMARY KEY,
name VARCHAR(255) NOT NULL,
price DECIMAL(10,2)
);Use a SQL formatter or linter to catch these issues automatically.
Partitioned tables cannot be UNLOGGED or TEMPORARY, and cannot use identity columns as partition keys:
Wrong:
CREATE UNLOGGED TABLE measurements (
id SERIAL,
recorded_at TIMESTAMP
) PARTITION BY RANGE (recorded_at); -- Error: UNLOGGED not allowedCorrect:
CREATE TABLE measurements (
id SERIAL,
recorded_at TIMESTAMP
) PARTITION BY RANGE (recorded_at);Wrong (identity as partition key):
CREATE TABLE events (
id INT GENERATED ALWAYS AS IDENTITY,
event_date DATE
) PARTITION BY RANGE (id); -- Error: identity columnCorrect:
CREATE TABLE events (
id INT GENERATED ALWAYS AS IDENTITY,
event_date DATE
) PARTITION BY RANGE (event_date);Ensure all data types exist and constraints reference valid columns:
-- Check available data types
SELECT typname FROM pg_type WHERE typname LIKE '%text%';
-- Verify referenced tables for foreign keys exist
SELECT tablename FROM pg_tables
WHERE schemaname = 'public' AND tablename = 'referenced_table';Wrong (invalid data type):
CREATE TABLE users (
id SERIAL PRIMARY KEY,
status USER_STATUS -- Error if enum doesn't exist
);Correct (create enum first):
CREATE TYPE user_status AS ENUM ('active', 'inactive');
CREATE TABLE users (
id SERIAL PRIMARY KEY,
status user_status
);After fixing the issue, test your statement in a transaction to ensure it works:
BEGIN;
CREATE TABLE test_table (
id SERIAL PRIMARY KEY,
name VARCHAR(255) NOT NULL
);
-- Verify it was created
\d test_table
ROLLBACK; -- Or COMMIT if satisfiedThis allows you to test without permanently modifying your schema.
Design Pattern Conflicts: The 42P16 error often indicates you're trying to combine PostgreSQL features that serve conflicting purposes. For example, UNLOGGED tables are designed for maximum write performance by skipping WAL logging, while partitioned tables are meant for managing large datasets across multiple storage units—combining them creates unclear semantics for crash recovery.
ORM and Migration Tools: Frameworks like Django, Rails, and Prisma can generate invalid DDL if their schema definitions conflict with PostgreSQL rules. Always review generated migration files before applying them. Some ORMs try to add multiple primary key definitions when handling composite keys incorrectly.
Table Inheritance vs. Partitioning: PostgreSQL supports both table inheritance (INHERITS) and declarative partitioning (PARTITION BY), but they cannot be used together on the same table. Choose one approach based on your needs: inheritance for polymorphic data structures, partitioning for scalability.
Version Differences: Some 42P16 restrictions vary between PostgreSQL versions. For example, identity columns on partitioned tables became supported in PostgreSQL 11, but using them as partition keys remained invalid. Check the official documentation for your specific PostgreSQL version.
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