I think it could have been a race condition: IPSConfig or Postfix throws two queries at MYSQL at the same time: Maybe to check "sender" with one query and "client" with another almost simultaneously. As there are many postfix processes (anvil, pickup, qmgr etc.) this could likely happen.
I dont think that there is any general problem in this setup as it works quite nicely on many large mailservers and I dont see such problems on our mailsystems. On website scripts in busy websites you might have hundreds of simultanious accesses to the same database table, mysql is made to handle this.
How many mail accounts do you have on that server?
Also queries in state "Abgebrochen" dont have to be same queries that were logged in the postfix table.