If set to 1, the default, secondary indexes in InnoDB tables are performed. If set to 0, storage engines can (but are not required to) assume that duplicate keys are not present in input data. Set to 0 to speed up imports of large tables to InnoDB. The storage engine will still issue a duplicate key error if it detects one, even if set to 0.
BOOLEAN (OFF, ON)
Additional information on this topic may be found in the MariaDB Public Knowledge Base.
Information specific to MariaDB SkySQL can be found on the unique_checks page in the SkySQL Documentation.