uid option mysql_ssl_rsa_setup, 403 ulimit, 5297 UMASK environment variable, 632, 5298 UMASK_DIR environment variable, 632, 5298 unary minus (-), 2139 unblocking blocked hosts, 990 unbuffered option mysql, 432 UNCOMPRESS(), 2276 UNCOMPRESSED_LENGTH(), 2276 undo, 6195 undo log, 3059, 3063, 6195 undo log segment, 6195 undo tablespace, 3063, 6195 undo tablespaces, 3059 UndoDataBuffer, 4114 UndoIndexBuffer, 4113 unexpected halt replication, 3522, 3632 UNHEX(), 2183 Unicode, 1926, 6195 Unicode character (\U), 2372 Unicode Collation Algorithm, 1971 UNINSTALL COMPONENT statement, 2903 UNINSTALL PLUGIN statement, 2904 uninstalling components, 1069, 2903 uninstalling loadable functions, 1144 uninstalling plugins, 1074, 2904 UNION, 332, 2677 deprecated features, 41 parenthesized query expressions, 2681 UNIQUE, 2492 unique constraint, 6195 unique index, 6196 unique key, 6196 constraint, 82 unique keys and partitioning keys, 4720 unique_checks system variable, 913 unique_subquery join type optimizer, 1768 Unix signal handling, 634 UNIX_TIMESTAMP(), 2167 UNKNOWN testing for, 2129, 2130 Unknown column ... in ’on clause’, 2676, 2676 Unknown or incorrect time zone error, 1001 unloading tables, 312 UNLOCK INSTANCE, 2725 UNLOCK TABLES, 2726 unnamed views, 2690 5603