Thanks Grant but I'm going to be bold and say that there is no rationale, and the optimiser is just getting it wrong this time. This should not astonish us...

@jeff, I am clearly not so skilled in irony as I had thought but I think I can now say that we have reached a perfect understanding. In...

@jeff, my 'earlier posts' were different posts and I was trying to be tongue-in cheek rather than make a serious complaint. Perhaps irony doesn't carry into the forums...

It is gratifying to have stimulated such a lively debate particularly when three of my earlier posts received no replies at all. From my perspective the matter has been satisfactorily...

Yes vala was the same in both queries. Actually, because it returned 0 rows I found a value of vala that returned > 0 rows and tried that in both...

Thanks Grant. I accept what you say about it being the FULLSCAN that made the difference, not the rebuilding per se. I would argue though that in the circumstances described...

Thanks for the suggestions. Stats were out of date but updating didn't make any difference. Rebuilding the index did, so yes I missed something obvious. Doesn't explain why the index...