* *: allow different shard_row_id prefixes in one transaction * Apply suggestions from code review Co-authored-by: tangenta <tangenta@126.com> * Update sessionctx/variable/session.go Co-authored-by: tangenta <tangenta@126.com> * add comments * add unit test * add var validation * update builtinGlobalVariable Co-authored-by: tangenta <tangenta@126.com>