How to Index this query (1 reply)

select
`id`,
`a`+`b`+`c` as `def`
from
`table`
where
`a`+`b`+`c`>="1000"
order by
`a`+`b`+`c` desc
limit 10

How can can i index a+b+c?

ALTER TABLE `table` ADD INDEX ( `a` , `b` , `c` ) ;

does nothing



any help appreciated !
Thanks