Re: Key Compression vs. Selectivity

Jonathan Lewis a écrit :
> "Robert Klemme" <shortcutter_at_googlemail.com> wrote in message
> >
> > The general rule of thumb for indexing is to put the most selective
> > columns first in order to foster fast lookups. (Of course there are
> > other considerations involved, which single index covers most queries
> > etc.)
>
> The "most selective first" has always been a little
> flexible (at least since 6.0 appeared)

Jonathan and Robert....

Why would the rule of thumb "put the most selective column first" be a good one to any degree ?

(let's assume that permutations of index columns don't impact the number of queries covered)

• Raoul