Incident brief
Duplicate cursor
Declaring a cursor with a name already open in the same transaction raises this error; cursor names must be unique within a transaction.
In 10 seconds
- What
- Duplicate cursor
- What triggers it
- Open a transaction and DECLARE a cursor with a given name.
- The fix
- Give each cursor a unique name within the transaction.
- Proof
- Reproduced on PostgreSQL 18.4 → Inside one transaction the reproduction declares a cursor, then re-declares the same name: the second DECLARE raises SQLSTATE 42P03.
The fix
What to do right now
The immediate, application-level response to this error.
- Give each cursor a unique name within the transaction.
- Or CLOSE the existing cursor before re-declaring the name.
-- Use a distinct name for each open cursor.
BEGIN;
DECLARE dc CURSOR FOR SELECT 1;
DECLARE dc2 CURSOR FOR SELECT 2;
COMMIT;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;Why it happens
What PostgreSQL is telling you
The mechanism behind the error, grounded in the official manual — not paraphrased.
PostgreSQL 18 Documentation — Appendix A. PostgreSQL Error Codes (Table A.1, Class 42 — Syntax Error or Access Rule Violation)
42P03 → duplicate_cursorRead the full section on postgresql.org →
Re-declaring an open cursor name
The cursor 'dc' is already open in this transaction, so the second DECLARE raises 'cursor dc already exists'.Recovering the aborted transaction
The error aborted the open transaction, so the session must ROLLBACK before it accepts new work; the next statement then runs normally.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.
- 1Open a transaction and DECLARE a cursor with a given name.
- 2DECLARE another cursor with the same name in the same transaction.
- 3The second DECLARE aborts with SQLSTATE 42P03: 'cursor already exists'.
A routine looped and re-declared a fixed cursor name within the same transaction, colliding on the second pass.
-- The error is self-contained in one statement; no schema is required.
SELECT 'no schema needed' AS setup_note;BEGIN;
DECLARE dc CURSOR FOR SELECT 1;
DECLARE dc CURSOR FOR SELECT 2;ROLLBACK;
SELECT 'ok' AS session_after_rollback;What PostgreSQL actually returned
setup_note
------------------
no schema needed
(1 row)BEGIN
DECLARE CURSOR
ERROR: cursor "dc" already existsROLLBACK
session_after_rollback
------------------------
ok
(1 row)Two cursors coexist once they have distinct names.
We declare two differently-named cursors in one transaction to show both succeed.
Without this
Before: re-declaring the name collides
With this, tested
After: two distinct names both declare
What Pro unlocks here
- The exact prevention SQL — copy-paste ready
- Raw psql output captured from the Docker lab
- A senior-DBA action list to take it further
- Live monitoring queries to catch it in production
- The deeper audit: fix-that-fails counterexample, GUC before/after, server-log evidence
Related & next steps
Follow the thread
Everything this error touches — jump straight to the sibling error, term, runbook, or parameter.
Verification
- 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