How to diagnose and fix the 34000 invalid_cursor_name error code in Postgres.

by

The 34000 invalid_cursor_name error in PostgreSQL indicates that an invalid cursor name was specified in a statement. This typically occurs when a cursor name is not found in the current session because it either does not exist, has not been declared, or is not accessible due to scope or transaction boundaries.

Here are some common scenarios that can lead to this error, along with examples and sample code to diagnose and fix the issue:

1. Cursor Not Declared

Before you can fetch from a cursor, it must be declared. Forgetting to declare a cursor or misspelling its name can cause this error.

Example:

BEGIN;
FETCH ALL IN non_existent_cursor;

Fix:
Declare the cursor before fetching from it.

BEGIN;
DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
FETCH ALL IN my_cursor;

2. Cursor Declared in a Different Scope

A cursor declared within a PL/pgSQL block or a function is only visible within that block or function.

Example:

DO $$
BEGIN
  DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
END;
$$;
FETCH ALL IN my_cursor; -- Error: my_cursor is not visible here

Fix:
Make sure to fetch from the cursor within the same scope it was declared.

DO $$
BEGIN
  DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
  FETCH ALL IN my_cursor;
END;
$$;

3. Cursor Declared in a Different Transaction

Cursors are transaction-specific. If you commit the transaction in which a cursor was declared and then try to fetch from it in a new transaction, you will get an invalid_cursor_name error.

Example:

BEGIN;
DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
COMMIT; -- Ends the transaction where my_cursor was declared
BEGIN;
FETCH ALL IN my_cursor; -- Error: my_cursor does not exist in this transaction

Fix:
Fetch from the cursor within the same transaction it was declared.

BEGIN;
DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
FETCH ALL IN my_cursor;
COMMIT;

4. Cursor Closed

If you close a cursor and then attempt to fetch from it, you will encounter the invalid_cursor_name error.

Example:

BEGIN;
DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
CLOSE my_cursor;
FETCH ALL IN my_cursor; -- Error: my_cursor is closed

Fix:
Do not close the cursor before you have finished fetching from it.

BEGIN;
DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
FETCH ALL IN my_cursor;
CLOSE my_cursor;

Diagnosing the Issue

To diagnose the issue, ensure that:

  • The cursor is correctly declared before any fetch operation.
  • The cursor name is spelled correctly.
  • The cursor is used within the scope and transaction where it was declared.
  • The cursor has not been closed before fetching from it.

By checking each of these areas, you can identify the cause of the 34000 invalid_cursor_name error and apply the appropriate fix. Remember that cursors in PostgreSQL are sensitive to transaction and block scope, so always pay attention to where and when they are being used.

Leave a Comment