For most purposes it is true that ascending/descecnding perform the
same.

min, max, and order by operations are where index direction becomes
important.

--- In firebird-support@yahoogroups.com, "unordained"
<unordained_00@c...> wrote:

> Errr, are you sure about this?
>
> From talking to Ann about how indexing is achieved, I got the

distinct impression all indexes are

> bidirectional ... the choice of asc vs. desc only matters when

updating an index, deciding which

> pointers in the doubly-linked list will be updated first. During an

update, when the index

> is 'broken', it guarantees that the doubly-linked list between

index pages will be valid in at

> least one direction, which is the direction chosen for the index.

The engine knows when the index

> is being updated, and will avoiding using it in the 'wrong'

direction during that time ... that was

> my understanding, at least. So asc/desc indexes, most of the time,

are pretty much identical

> performance-wise?
>
> -Philip
>
> ---------- Original Message -----------
> From: David Johnson <johnson_d@c...>
> To: firebird-support@yahoogroups.com
> Sent: Wed, 17 Aug 2005 08:17:37 -0500
> Subject: Re: [firebird-support] Re: Performance of Firebird vs.

other DBMS

>
> > On Wed, 2005-08-17 at 13:09 +0000, laurenz_brein wrote:
> > > The article did not make it clear to me why a SELECT MAX() could
> > > not profit from an ascending index.
> >
> > An ascending index would have to be traversed backwards to get a

true

> > max. Firebird opted for unidirectional indexes (like DB2, which

is a

> > locking database).
> >
> > A descending index finds the most likely candidate first, and

then if

> > there are issues it can go to the next most likely candidate.
> >
> > Use ascending indexes for min (), and descending indexes for max

() in

> > any system with unidirectional indexes.
> >
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> >
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> > on the main (top) menu. Try Knowledgebase and FAQ links !
> >
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