On 7 March 2016 at 14:27, I wrote:
> So it seems that it should in fact be usable after all. So I'm still
> stumped as to why the (scdate,sc_id) index isn't used :(
Also, while the index on sc_id will be sorted there's no guarantee
that sc_id values will be in order in the table itself, so you're
still left with (30,000) potentially random accesses to the table,
even assuming fully random distribution of scdate (with a worst-case
of 970000 random accesses). That average case is no better than the
(30,000) random accesses that were required from using an scdate
index, even ignoring the scdate/sc_id index.
So I'm afraid I'm fully back in the "I still don't get it" column.
Geoff