How to diagnose and fix the 25P02 in_failed_sql_transaction error code in Postgres.

The 25P02 error code in PostgreSQL is associated with the “in_failed_sql_transaction” state. This error indicates that you are trying to execute an SQL command after a previous command in the same transaction has failed. Once a transaction encounters an error, it becomes ‘tainted’, and PostgreSQL will not allow any further SQL commands to be executed until the transaction is either rolled back or the failed command is resolved with a savepoint.

To diagnose and fix the 25P02 error, you should:

  1. Identify the Failed Command: Look at the error messages preceding the 25P02 error to identify which command caused the initial failure. This is the root cause that needs to be addressed.
  2. Roll Back the Transaction: If a command has failed and you cannot or do not want to resolve it, the simplest solution is to roll back the entire transaction. This will undo all the changes made in the current transaction and allow you to start a new transaction to try again.
  1. Use Savepoints: If you have a long transaction and do not want to roll back all the changes, you can use savepoints to set markers within your transaction. If an error occurs, you can roll back to the last savepoint instead of the beginning of the transaction.
   SAVEPOINT my_savepoint;
   -- Some SQL operations
   -- If an error occurs, roll back to the savepoint
   ROLLBACK TO SAVEPOINT my_savepoint;
   -- Continue with other operations

Here are some examples of how the 25P02 error might occur and how to resolve it:

  • Example 1: Error in a Multi-Step Transaction Let’s say you are performing multiple insert operations within a transaction, and one insert fails due to a constraint violation.
  INSERT INTO employees (id, name) VALUES (1, 'John Doe'); -- Successful
  INSERT INTO employees (id, name) VALUES (1, 'Jane Doe'); -- Fails due to unique constraint on 'id'
  INSERT INTO employees (id, name) VALUES (2, 'Jim Beam'); -- Causes 25P02 error because the transaction is in a failed state

To resolve this, you could roll back the transaction or fix the second INSERT to use a unique id, then retry the third INSERT.

  • Example 2: Handling Errors with Savepoints Suppose you are performing several operations but want to ensure that if one fails, only a part of the transaction is rolled back.
  INSERT INTO accounts (account_no, balance) VALUES ('12345', 1000); -- Successful
  UPDATE accounts SET balance = balance - 100 WHERE account_no = '12345'; -- Successful
  UPDATE accounts SET balance = balance - 1100 WHERE account_no = '12345'; -- Fails due to insufficient balance
  -- At this point, you can roll back to sp2 and continue
  -- Continue with other operations

By using savepoints, you can handle errors more gracefully within a transaction and avoid the need to roll back all the changes.

  • Example 3: Automatic Rollback on Error Some client applications or ORMs automatically issue a ROLLBACK when an error is encountered. In this case, you might not see the 25P02 error directly, but you should still look for the initial error that caused the rollback. If you encounter the 25P02 error, always look for the initial failure within the transaction. Addressing the root cause will allow you to proceed with the transaction or decide to roll back and start over.

For more information on transactions and error handling in PostgreSQL, you can refer to the PostgreSQL documentation on transactions. Understanding how to use transactions and savepoints effectively will help you manage your database operations with greater precision and recover from errors more smoothly.

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