I recently had a SQL Server 2014 stored procedure that ran for several hours with high CPU usage. I narrowed the problem down to a single query with dozens of joins was stuck in compilation. Suspecting an issue with the new cardinality estimator, I changed the database compatibility level from 120 (SQL Server 2014) to 110 (SQL Server 2012) so that the legacy CE would be used. The query then compiled immediately and executed quickly, which confirmed the new CE as the culprit of the excessive compilation times.

OPTION QUERYTRACEON (9481) to the Rescue
Changing the database compatibility level was a heavy-handed option I didn’t want to pursue because some queries benefited from the new CE behavior. Instead, I specified trace flag 4891 using the OPTION QUERYTRACEON(9481) query hint to use the legacy CE for the problem query. This is an acceptable workaround for me until the bug is addressed. However, you might not be so lucky. A more common use case for OPTION QUERYTRACEON(9481) is performance regression with the new CE. Although not documented as far as I know, the QUERYTRACEON query hint requires sysadmin permissions unless executed from within a stored procedure. Consequently, QUERYTRACEON won’t be an option for those who need the hint for queries run by non-sysadmins outside stored procs. I found SQL Server MVP Ola Hallengren’s Connect suggestion to allow QUERYTRACEON with no additional permissions. I upvoted and encourage you to do so too.