This has nothing to do with postgres though. It’s part of a generic JDBC connection pool: > The problem is that this synchronized code might be deeply embedded

This has nothing to do with postgres though. It’s part of a generic JDBC connection pool: > The problem is that this synchronized code might be deeply embedded within the libraries you use. In our case, it was within the c3p0 library. So, the fix is straightforward: we simply wrapped the connection with a java.util.concurrent.Semaphore. I bet if you just checked out connections and slept a random amount of time you’d have the same problem.