is this usage of User-defined Variables guaranteed?

hi.

I need to always have the same results for the following query even if some execution plan is changed.(in mariaDB 5.5)

SELECT A.*

,IF(@PREV_JOB=JOB,@ROWNUMBER:=@ROWNUMBER+1,@ROWNUMBER:=1+LEAST(0,@PREV_JOB:=JOB)) AS ROWNUMBER

FROM (SELECT EMP.*

FROM EMP

,(SELECT @ROWNUMBER:=0, @PREV_JOB:='') X

ORDER BY JOB, SAL) A;

this query was "SELECT A.*, ROW_NUMBER() OVER(PARTITION BY JOB ORDER BY SAL) FROM EMP A;" in ORACLE DB.

I worry about that “It is unsafe to read a user-defined variable and set its value in the same statement (unless the command is SET), because the order of these actions is undefined.”(in document: User-defined Variables).

is this usage of User-defined Variables guaranteed?

If the execution plan was changed, can the result also be changed?

Thanks you.