Re: Covering Indexes

Hi all,

> On Thu, Jun 28, 2012 at 5:16 AM, David E. Wheeler <david(at)justatheory(dot)com> wrote:
> I don't see the virtue of this in this case. Since the index is not
> unique, why not just put the index on (a,b,c,d) and be done with it?
> Is there some advantage to be had in inventing a way to store c and d
> in the index without having them usable for indexing?

Why not restrict it to UNIQUE indexes ?

For not unique indexes it has no advantages (it could be in fact indexed
on all columns anyway as an implementation detail).

For the unique case the problem of many identical entries mentioned by
Tom is not relevant, so the additional data will only bloat the index
but not otherwise affect the index performance.

Would this get close enough to index-covered table ? _That_ would be
interesting - I have a really big table (table/index size: 370G/320G,
~8*10^9 rows) which is basically using double space because it's primary
key is covering all columns of the table.

Cheers,
Csaba.