does Select values effect index selection?

[ade br]

Hi

I have been writing a query that contains a total of 9 tables, I am fairly sure that all the joins and where components have correct indexes however depending on the fields i put in the select statement the query takes up to 10 times as long.

Using the explain command I discovered that when i remove a certain field the query stops using a certain index and instead uses no index for that table(join). All the rest of the query is the same so i do not know why this would occur.

Does anyone know how the fields selected effects the optimisers choice of index??

Thanks
Ade