cached function calls

create table xyz
(   id int not null,
    title varchar(1000) not null
);
-- truncate xyz;
insert xyz (id,title) select qid,title from questions_mysql limit 5000;

select sum(length(title)) from xyz;
-- 1st time: 265416 in 0.032 sec
-- 2nd time: 265416 in 0.015 sec
-- 3nd time: 265416 in 0.000 sec
-- 4th time: 265416 in 0.016 sec

So far inconclusive due to title being cached potentially, not length(title) necess being cached

truncate xyz;
insert xyz (id,title) select qid,title from questions_mysql limit 5000;

select count(*)
from
(   select distinct(soundex(title))
from xyz
)xDerived;
-- 1st time: 4996 rows in 0.281 sec
-- 2nd time: 4996 rows in 0.281 sec
-- 3rd time: 4996 rows in 0.265 sec
-- 3rd time: 4996 rows in 0.266 sec

Granted xDerived on 5.6 is manifested in a temp table but if soundex is cached then temp is generated quicker


select sum(length(title)) from xyz;
-- 1st time: 265416 in 0.016 sec

Note in above line, that is new data due to truncate.