Question

I want to insert a row, but if a conflict occurs (example below) I’d like the database to lock the existing row so I can log its contents for debugging purposes. I am using READ_COMMITTED transaction isolation.

For example:

CREATE TABLE users(id BIGINT AUTO_INCREMENT, name VARCHAR(30),
  count INT NOT NULL, PRIMARY KEY(id), UNIQUE(name));

Thread 1:
  INSERT INTO users(username, count) VALUES('joe', 1000);
  transaction.commit();

Thread 2:
  // Insert fails due to conflict with above record
  INSERT INTO users(username, count) VALUES('joe', 0);

  // Get the conflicting row and log its properties
  SELECT * FROM users WHERE username = 'joe';

If the conflicting row is not locked, it may be modified by the time I check it. The only workaround I found is invoking SELECT id FROM users WHERE username = 'joe' FOR UPDATE before the insert. Is it possible way to implement this without any overhead when a conflict does not occur?

UPDATE: I am not asking to avoid the conflict or the resulting SQLException. I am just asking for the conflicting row to get locked so I can look up what values triggered the conflict. Yes, I know that the conflicting record contains joe but I want to log all its other columns.

Answer 1

There doesn’t seem to be a portable way of doing this and looking at MVCC there is a strong indication that this cannot be implemented without a substantial performance impact.

So in conclusion: you’re going to have to settle for knowing that a conflict occurred but have no way of being 100% sure of the cause (there is no thread-safe to verify).