How to improve query performance in expression related order by?

[jack su]

I have a query, which will return a list of items based on an expression, the expression depend on the input.

Such as I have a list of address, which contains lat and lng, then given one location, I want to find out the roughly nearest 25 records.

so the query will be like:

select * from mytab
where
(lat - 12.34) < 0.1 (lat-12.34) > -0.1 and
(lng - 45.78) < 0.1 and (lng - 45.78) > -0.1
order by (4774.81 * (12.34 - lat) * (12.34 - lat) + 2809 * ( 45.78 - lng) * (45.78 - lng))
limit 25

lat and lng are indexed field, and there are about 2 million records. The response time is 2 sec, apparently not good to scale.

Any suggestion?