即使设置了WHERE条件，MySQL仍会查询每条记录吗？

I am creating a website which is highly likely to have thousands of user records.

My interest is to find out if MySQL still would query through each and every record even if the WHERE condition is set or just JUMP straight to the CONDITION record.

for example,.. i have 1000 users and i want to get the details of a user with an id number of 900.

SELECT * FROM users WHERE id = 900;

Now, will MySQL still query through all 899 records just to find the user i am looking for or just JUMP straight to that user id... and is this the same case with an auto-incremented id and with a random one?