How to slowing down my query

Question

I am new in database sql. I know that indexes make query faster. But I want to learn the ways of making query slowing down without dropping or creating objects.

    create index ind1 on students_2(surname);
        select * from students s1, students_2 s2
    where s1.name1=substr(s2.surname,1,5) and s1.dep=s2.dep and 
    s1.dep in ('Econ','Law')

Answer 1

You can embed a sleep into any expression.

select * 
from students s1 
join students_2 s2 
  on s1.name1=substr(s2.surname,1,5) and s1.dep=s2.dep
where s1.dep in ('Econ','Law')
  and sleep(0.1)=0;

This will add a 0.1 second sleep every time a row is examined. The sleep() function normally returns 0.

PS: Please use proper JOIN syntax instead of obsolete "comma" joins.

Answer 2

One solution, and probably the easiest one, would be to use an index hint to ignore the indexes you have, like this:

select * 
from 
  students s1
ignore index ind1

Alternatively, you can try to slown down the query (more) by makeing it more complex. The query below is modified a bit, especially the part that checks for the name. Instead of just matching by name, it matches two parts of the name. Because of the second condition, any index on s1.name1 cannot be efficiently used, and MySQL has to do a lot of string manipulation:

select * 
from 
  students s1, students_2 s2
where 
  substr(s1.name1, 1, 1) = substr(s2.surname, 1, 1) and 
  substr(s1.name1, 2) = substr(s2.surname, 2, 5) and 
  s1.dep = s2.dep and 
  s1.dep in ('Econ','Law')

Just like with adding indexes, forcefully not using indexes will also have varying results. Query results might still be cached, and even without indexes, a query can be pretty fast.