invalid_table_definition multiple primary keys for table “…” are not allowed — 42P16

Symptoms

A table definition (or ALTER TABLE) declared a primary key when the table already has one. A table can have at most one primary key, so PostgreSQL raises SQLSTATE 42P16 (invalid_table_definition).

• A second PRIMARY KEY was declared on the same table.
• Common with both a column-level and a table-level PK.
• Use UNIQUE for additional uniqueness constraints.

What the server log shows

ERROR:  multiple primary keys for table "orders" are not allowed

Why PostgreSQL raises this — what the manual says

Section 5.5.4 Primary Keys:

“A table can have at most one primary key.”

A primary key is the table’s single canonical identifier. Declaring more than one is contradictory, so PostgreSQL rejects the definition with 42P16.

Common causes

• Both a column-level and a table-level PRIMARY KEY clause.
• Adding a PK via ALTER TABLE when one already exists.
• A migration that re-declares the primary key.

How to fix it

1. Keep a single PRIMARY KEY; use composite PK syntax if you need multiple columns.
2. Use UNIQUE constraints for additional uniqueness requirements.
3. Drop the existing PK first if you intend to replace it.

Related & next steps

• SQLSTATE 42P16 — Invalid Table Definition (full reference)
• Message: column referenced in foreign key constraint does not exist
• Glossary: Primary key

Reference: PostgreSQL 18 Section 5.4 “Constraints”.