How to diagnose and fix the 25002 branch_transaction_already_active error code in Postgres.

The 25002 error code in PostgreSQL indicates a “branch_transaction_already_active” condition. This error occurs when an operation is attempted that requires a new transaction, but there is already a transaction active in the same session on a branch of a transaction. This is typically encountered in the context of transaction control within procedures or savepoints.

Here are steps to diagnose and fix this error:

  1. Identify the Active Transaction: Determine if there is an active transaction in your session. If you are using a client that supports transaction status indicators, it can help you identify whether a transaction is in progress.
  2. Review Transaction Blocks: Look at your SQL code and identify any nested transaction blocks or savepoints. PostgreSQL uses a flat transaction model, which means that savepoints can be created, but each BEGIN does not start a new transaction if one is already in progress.
  3. Ensure Proper Commit/Rollback: Make sure that every BEGIN has a corresponding COMMIT or ROLLBACK. If a transaction is left open, subsequent attempts to start a new transaction will result in the 25002 error.

Here are some examples of scenarios that might cause this error and how to resolve them:

  • Example 1: Nested Transactions Using BEGIN Suppose you have the following code:
  BEGIN; -- Start of first transaction
  -- Some SQL operations
  BEGIN; -- Attempt to start a second transaction
  -- More SQL operations
  COMMIT; -- Committing the second transaction

The second BEGIN will cause a 25002 error because a transaction is already active. To fix this, remove the second BEGIN or replace it with a SAVEPOINT to establish a point to which you can roll back without affecting the outer transaction.

  • Example 2: Transactions in Stored Procedures If you have a stored procedure that starts a transaction, and you call this procedure from within another transaction block, you will get the 25002 error.
  -- Inside a procedure
  CREATE PROCEDURE my_procedure() AS $$
    -- Procedure logic
  $$ LANGUAGE plpgsql;

  -- Calling the procedure within a transaction block
  CALL my_procedure();

To fix this, ensure that the procedure does not start its own transaction if it’s meant to be called from within another transaction. You can manage transactions outside the procedure or use savepoints within the procedure if necessary.

  • Example 3: Savepoints Without Transactions Trying to use a savepoint without an active transaction will also result in an error. For example:
  SAVEPOINT my_savepoint; -- Error if no active transaction
  -- SQL operations
  ROLLBACK TO SAVEPOINT my_savepoint; -- This needs an active transaction

To fix this, make sure to start a transaction before using savepoints:

  SAVEPOINT my_savepoint;
  -- SQL operations

Remember that PostgreSQL transactions are not nestable, and attempting to nest transactions will lead to the 25002 error. Always manage transactions carefully to ensure that they are properly opened and closed. For more detailed information on transaction errors, you can refer to resources like PostgreSQL 25002 and the PostgreSQL documentation on error codes.

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