> I've got a query containing
>
> ... WHERE ... (MYFIELD in (1,3,4,5,7,8,9,11,12,13))
>
> which runs horribly slowly, because (quite reasonably) the query
> optimiser gives up on that, even though it's got an index on MYFIELD.

What exactly are these three dots after WHERE ?
Perhaps you should show us whole statement.

Ivan