Determine Users within X Radius by Postcode

[eversonj90]

I have two database tables:

 

The first which is a table of postcodes (around 1.7 million records) and has a postcode, longitude and latitude field.

The second which is a table of user data, which includes a postcode field - their postcode on registration.

 

I need to be able to determine which users fall within X radius of Y (another users postcode) without ideally running a query to compare distances with all 1.7 million postcodes to shortlist possibles.

 

I was thinking maybe it'd be possible to take Y users postcode and to use their longitude and latitude, doing some math (not my strong point) to determine all possible postcodes - by adjusting the longitude and latitude degrees relative to the distance being searched.

 

Where to start?

[InterwebsGuy]

This might be helpful:

Avoiding a full table scan - comp.databases.mysql | Google Groups

 

I came across it by chance awhile back and bookmarked it for future reference. Interesting stuff!