Create Table: CREATE TEMPORARY TABLE `user_variables` (

  `VARIABLE_NAME` varchar(64) NOT NULL DEFAULT '',

  `VARIABLE_VALUE` varchar(2048) DEFAULT NULL,

  `VARIABLE_TYPE` varchar(64) NOT NULL DEFAULT '',

  `CHARACTER_SET_NAME` varchar(32) DEFAULT NULL

) ENGINE=MEMORY DEFAULT CHARSET=utf8

VARIABLE_NAME and VARIABLE_VALUE lengths are the same as in the tables for system variables. But unlike system variables, where we control the actual length for both, user variables don't seem to have any limit on the length of their names and values (as long as those are within max_allowed_packet). So, it's theoretically possible to have a variable with a longer name or, probably more likely, with a longer value, which will be truncated in the I_S table.