How to diagnose and fix the 55P03 lock_not_available error code in Postgres.

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The 55P03 error code in PostgreSQL indicates a “lock_not_available” error. This error occurs when a query attempts to acquire a lock that is not available at that moment because another session holds a conflicting lock. The error is commonly encountered when using the NOWAIT option or the SKIP LOCKED clause in a locking query, which tells PostgreSQL to immediately raise an error or skip the locked rows instead of waiting for the lock to become available.

To diagnose and fix this error, you’ll need to understand the context in which the locks are being acquired and manage the transaction isolation and lock acquisition accordingly. Here are some scenarios where this error might occur, along with examples and potential fixes:

Scenario 1: Using NOWAIT in a SELECT FOR UPDATE

If you use the NOWAIT option with a SELECT FOR UPDATE, and the row you’re trying to lock is already locked by another transaction, PostgreSQL will return a 55P03 error.

Example:

BEGIN;
SELECT * FROM my_table WHERE id = 1 FOR UPDATE NOWAIT;
-- If another transaction has already locked row with id = 1, this will raise a 55P03 error.

Fix:
You can handle this error in your application code by catching the exception and deciding on the next steps, such as retrying the transaction after a delay or taking alternative actions.

Example Fix:

BEGIN;
-- Use a loop or application logic to retry the transaction.
SELECT * FROM my_table WHERE id = 1 FOR UPDATE;
-- Process the row.
COMMIT;

Scenario 2: Concurrent Transactions Modifying the Same Data

When multiple transactions are trying to modify the same data, they will require exclusive locks, leading to 55P03 errors if NOWAIT is used.

Example:

BEGIN;
UPDATE my_table SET column = value WHERE id = 1;
-- If another transaction is already updating the same row, using NOWAIT in another query will cause a 55P03 error.
COMMIT;

Fix:
Design your transactions to minimize the time they hold locks, and avoid using NOWAIT if it’s acceptable for the transaction to wait for the lock.

Scenario 3: SKIP LOCKED in a Queue-like System

In a queue-like system where you process rows and skip locked ones, you might encounter 55P03 if you incorrectly use NOWAIT.

Example:

BEGIN;
SELECT * FROM queue_table WHERE processed = FALSE FOR UPDATE SKIP LOCKED;
-- Process the rows.
COMMIT;

Fix:
Ensure that you are using SKIP LOCKED correctly and not combining it with NOWAIT in a way that causes conflicts.

General Tips for Diagnosis and Fixes:

  1. Understand Locking Mechanisms:
    Familiarize yourself with PostgreSQL’s locking mechanisms and how different types of locks interact.
  2. Review Application Logic:
    Review your application’s transaction logic to ensure that it acquires locks in a consistent order, which can help prevent deadlocks and reduce lock contention.
  3. Handle Exceptions:
    Implement exception handling in your application code to catch 55P03 errors and respond appropriately, such as by retrying the transaction after a delay.
  4. Optimize Transaction Sizes:
    Keep transactions as small and as short as possible to reduce the time they hold locks, which can minimize the chances of lock contention.
  5. Monitor Locks:
    Use monitoring tools or queries to check for lock contention in your database. The pg_locks view can provide information about the current locks held by transactions.

By carefully managing transactions and handling cases where locks are not available, you can prevent the 55P03 lock_not_available error from disrupting your application’s workflow. For more detailed information on lock management, consider reviewing the official PostgreSQL documentation on concurrency control.

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