Re: Covering Indexes

On Tue, Jul 17, 2012 at 6:08 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> On 17 July 2012 16:54, David E. Wheeler <david(at)justatheory(dot)com> wrote:
> > Yeah, but that index is unnecessarily big if one will never use c or d
> in the search. The nice thing about covering indexes as described for
> SQLite 4 and implemented in MSSQL is that you can specify additional
> columns that just come along for the ride, but are not part of the indexed
> data:
> >
> > CREATE INDEX cover1 ON table1(a,b) COVERING(c,d);
> >
> > Yes, you can do that by also indexing c and d as of 9.2, but it might be
> nice to be able to include them in the index as additional row data without
> actually indexing them.
>
> Can you explain what you mean by "without actually indexing them"?
> ISTM that it is a non-feature if the index is already non-unique, and
> the difference is simply down to the amount of snake oil applied to
> the descriptive text on the release notes.
>

It would be useful in non-unique indexes to store data without ordering
operators (like xml).