WHERE OR performance issue

[Lee Fear]

I have two sql statements which achieve the same thing but have vastly different performance and I was just wondering why this is.

The first statement is as follows.

SELECT *
FROM tbl_sectordistances
WHERE (destinationsectorid=300 AND distance<=20) OR (originsectorid=300 AND distance<=20);

I end up aborting this query after around 4 or 5 mins as I get fed up of waiting.

However the other statement is as follows

SELECT *
FROM tbl_sectordistances
WHERE (destinationsectorid=300 AND distance<=20)
UNION
SELECT *
FROM tbl_sectordistances
WHERE (ORIGINSECTORID=300 AND distance<=20);

this returns the results in less than 1/4 or a second!

Why does the OR statement hit the performance so hard?