Browsing a little history recently I came across a note I’d written about the new-style index hint. In that note I claimed that:
… the index has to start with the columns (product_group, id) in that order – with preference given to an exact match, otherwise using the lowest cost index that starts the right way.
On reading this statement I suddenly realised that I hadn’t actually proved (to myself, even) that if I had the indexes (product_group, id) and (product_group, id, other_col) then a two-column hint forced Oracle to use the two column index in all (legal) circumstances.
So, tonight’s quiz – are there any edge cases, and what easy ways can you think of to prove (or disprove) the claim for the general case.