SELECT vs UPDATE performance with index

Question

If I SELECT IDs then UPDATE using those IDs, then the UPDATE query is faster than if I would UPDATE using the conditions

Answer 1

Regarding "MySQL doesn't support updating the same table you're selecting from"

UPDATE table SET field = value 
WHERE id IN (SELECT id FROM table WHERE a IS NULL LIMIT 10);

Just do this:

UPDATE table SET field = value 
WHERE id IN (select id from (SELECT id FROM table WHERE a IS NULL LIMIT 10));