SQLSTATE 42P16Severity lowLab verified

Incident brief

Multiple primary keys not allowed

A table can have at most one PRIMARY KEY; declaring PRIMARY KEY on two columns separately raises this invalid-table-definition error. Use a composite key for several columns.

Reproduced on PostgreSQL 18.4Verified 2026-07-24 (isolated lab, PostgreSQL 18.4)Reviewed by Verified against PostgreSQL 18.4 in an isolated lab environment

In 10 seconds

What: Multiple primary keys not allowed
What triggers it: Write CREATE TABLE with PRIMARY KEY on two different columns.
The fix: Keep a single PRIMARY KEY — use one column, or a composite PRIMARY KEY (a, b).
Proof: Reproduced on PostgreSQL 18.4 → A CREATE TABLE reproduces SQLSTATE 42P16; the second PRIMARY KEY clause is rejected and the LINE points at it.

The fix

What to do right now

The immediate, application-level response to this error.

Keep a single PRIMARY KEY — use one column, or a composite PRIMARY KEY (a, b).
Model additional uniqueness with UNIQUE constraints instead of extra primary keys.

Fix — SQL

-- A table has one primary key (composite if needed).
CREATE TABLE two_pk (a int primary key, b int);

Diagnose

See it live on the server

Run these against the affected instance to confirm the diagnosis before you act.

Standard triage — not specific to this error

These are canonical PostgreSQL system-catalog queries, shown as SQL to run. No sample output is attached because this is general triage, not a captured lab transcript.

This SQLSTATE does not have an error-specific live snapshot yet. These are the canonical system-catalog queries you run against the affected server to see the problem in real time — standard triage, not a reproduced transcript.

What is running right now

Active backends, how long each has been running, and what it is waiting on.

SELECT pid,
       state,
       wait_event_type,
       wait_event,
       now() - query_start AS running_for,
       left(query, 80)     AS query
FROM   pg_stat_activity
WHERE  state <> 'idle'
  AND  pid <> pg_backend_pid()
ORDER  BY running_for DESC NULLS LAST;

Who is blocking whom

Turn raw blocking PIDs into the actual queries on both sides of the wait.

SELECT blocked.pid    AS blocked_pid,
       blocked.query  AS blocked_query,
       blocking.pid   AS blocking_pid,
       blocking.query AS blocking_query
FROM   pg_stat_activity AS blocked
JOIN   LATERAL unnest(pg_blocking_pids(blocked.pid)) AS b(pid) ON true
JOIN   pg_stat_activity AS blocking ON blocking.pid = b.pid
WHERE  cardinality(pg_blocking_pids(blocked.pid)) > 0;

Locks that are still waiting

Every lock a backend has requested but not yet been granted.

SELECT l.pid,
       l.locktype,
       l.mode,
       l.granted,
       COALESCE(c.relname, l.transactionid::text) AS object
FROM   pg_locks l
LEFT   JOIN pg_class c ON c.oid = l.relation
WHERE  NOT l.granted
ORDER  BY l.pid;

Reproduce & verify

A real, single-session PostgreSQL reproduction

A literal transcript of SQL run against a live PostgreSQL instance in an isolated lab — the commands below are exactly what was executed.

1Write CREATE TABLE with PRIMARY KEY on two different columns.
2PostgreSQL processes the constraints and finds more than one primary key.
3The definition is rejected with SQLSTATE 42P16.

A schema declared PRIMARY KEY inline on two columns intending a composite key, and the table failed to create.

Schema setup

-- The error is self-contained in one statement; no schema is required.
SELECT 'no schema needed' AS setup_note;

Declaring two primary keys

CREATE TABLE two_pk (a int primary key, b int primary key);

The session continues normally

SELECT 'ok' AS session_after_error;

What PostgreSQL actually returned

Schema setup output

    setup_note    
------------------
 no schema needed
(1 row)

Declaring two primary keys output

ERROR:  multiple primary keys for table "two_pk" are not allowed
LINE 1: CREATE TABLE two_pk (a int primary key, b int primary key);
                                                      ^

The session continues normally output

 session_after_error 
---------------------
 ok
(1 row)

Related & next steps

Follow the thread

Everything this error touches — jump straight to the sibling error, term, runbook, or parameter.

Verification

PG 18.4

Last verified: 2026-07-24 (isolated lab, PostgreSQL 18.4)
Reviewed by: Verified against PostgreSQL 18.4 in an isolated lab environment
Audit status: reviewed