SET @start_global_value = @@global.alter_algorithm;

SELECT @@global.alter_algorithm;
SET GLOBAL alter_algorithm=2;
SELECT @@global.alter_algorithm;
--error ER_WRONG_VALUE_FOR_VAR
SET GLOBAL alter_algorithm=5;
SELECT @@global.alter_algorithm;

SET SESSION alter_algorithm=INSTANT;
SHOW SESSION VARIABLES LIKE 'alter_algorithm';