How to diagnose and fix the 42P05 duplicate_prepared_statement error code in Postgres.

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The 42P05 duplicate_prepared_statement error code in PostgreSQL occurs when you attempt to create a prepared statement with a name that already exists in the current session. This can happen if you inadvertently create the same prepared statement multiple times without deallocating the previous one first. Here are several examples of how this error might occur and how to resolve it:

  1. Recreating an Existing Prepared Statement:
    If you prepare a statement with a name that’s already been used in the session, you’ll encounter the 42P05 error.
   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;

   -- Trying to prepare the same plan again will cause an error:
   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;

To fix this, you can either deallocate the existing prepared statement before recreating it:

   DEALLOCATE myplan;

   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;

Or use a different name for the new prepared statement.

  1. Conflicting Prepared Statements in Different Transactions:
    Even if you are in a different transaction, you cannot create a prepared statement with the same name as one that exists in the session.
   BEGIN;
   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;
   COMMIT;

   -- Starting a new transaction doesn't allow the same name to be reused:
   BEGIN;
   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;
   COMMIT;

To resolve this, make sure to deallocate the prepared statement at the end of the transaction if you plan to reuse the name:

   DEALLOCATE myplan;
   COMMIT;
  1. Prepared Statements Persisting After Error:
    If an error occurs in a transaction after a prepared statement has been created, the prepared statement will still exist after the transaction is rolled back.
   BEGIN;
   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;
   -- Some error occurs, and the transaction is rolled back:
   ROLLBACK;

   -- The prepared statement 'myplan' still exists, so this will cause an error:
   PREPARE myplan (int) AS
   SELECT * FROM mytable WHERE id = $1;

Ensure to deallocate the prepared statement if you intend to reuse the name after a rollback:

   DEALLOCATE myplan;
  1. Prepared Statement in a Stored Procedure or Function:
    If you create a prepared statement inside a stored procedure or function, ensure that it’s deallocated at the end to avoid the 42P05 error when the function is called multiple times.
   CREATE OR REPLACE FUNCTION get_records(input_id int) RETURNS SETOF mytable AS $$
   BEGIN
     PREPARE myplan (int) AS
     SELECT * FROM mytable WHERE id = $1;

     EXECUTE myplan(input_id);

     DEALLOCATE myplan;
   END;
   $$ LANGUAGE plpgsql;

The DEALLOCATE statement here ensures that the prepared statement is cleaned up after the function executes, preventing a duplicate name error on subsequent calls.

When diagnosing the 42P05 duplicate_prepared_statement error, check for any instances where you may be creating prepared statements with the same name within the same session. Always ensure that you either deallocate a prepared statement before reusing its name or use unique names for each prepared statement. Additionally, consider the scope of your transactions and the behavior of your stored procedures or functions that may create prepared statements.

For more information on prepared statements, including how to deallocate them, you can refer to the PostgreSQL documentation on prepared statements.

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