How to create/choose a suitable index

[Tim Streater]

I am having trouble figuring out a suitable index(es) in the following case:

select * from circuit where (col1a=val1 and col2a=val2) or (col1b=val1 and col2b=val2);

(it happens that I use val1 and val2 as above).

Every choice I make for index doesn't, according to EXPLAIN, get used by the SELECT. The table has a few thousand entries and I expect the SELECT to return one or two at most. Well, it does, but quite slowly (in fact the SELECT is a bit more complex) and I would really like to improve this particular combination as its at the inner-most loop of my PHP code :-)

INDEXes seem to work for ANDs but not when there's an OR in the month (sorry). So an index of columns A,B gets used if you have A AND B in the WHERE but not A OR B.

I can't find any suitable pointers in the docs, so any clues would be appreciated.

Thanks -- tim