At an old job we had a mammoth awful database running on MSSQLSERVER and we at one point ran into concurrency using the MERGE statement in a high-traffic area o

At an old job we had a mammoth awful database running on MSSQLSERVER and we at one point ran into concurrency using the MERGE statement in a high-traffic area of the database. It turns out MERGE isn’t transactional so in between the check to see if a row exists and the insert, a concurrent call would sometimes create the row and we would get a constraint violation (it was a unique index basically). Tracking down the source of the error was only the first of many horrors on the way to fixing the problem for good. Fun times.