At 01:20 PM 15/06/2007, you wrote:

>--- In firebird-support@yahoogroups.com, "Sean" <firebird_tmc@...> wrote:
> >
> > Hi,
> >
> > I have a table of about 3 million records. The table A has many
> > columns, among them are ID (Primary key, BigInt, auto-increasing), and
> > update_time (DateTime).
> >
> > An ascending query completes in about 1 second:
> > select first 1000 * from A
> > where update_time >= '13-JUN-2006 00:00:00'
> > and update_time <= '13-JUN-2007 00:00:00'
> > and id <=3000000
> > and id >= 2000
> >
> > Adopted plan is: PLAN(A INDEX(PK_A))
> >
> > but a descending query returns after 2 minutes:
> > select first 1000 * from A
> > where update_time >= '13-JUN-2006 00:00:00'
> > and update_time <= '13-JUN-2007 00:00:00'
> > and id <= 3000000
> > and id >= 2000
> > order by id desc
> >
> > Adopted plan is: PLAN(A ORDER A_IDX_ID_DESC)
> >
> >
> > Several relevant indexes of table A are:
> > 1. PK_A (PRIMARY KEY INDEX) on column ID, ascending
> > 2. A_IDX_ID_DESC on column ID, descending
> > 3. A_IDX_UPDATE_TIME_ASC on column update_time, ascending
> > 4. A_IDX_UPDATE_TIME_DESC on column update_time, descending
> >
> > Can anyone help to improve descending query?
> >
> > Thanks a lot!
> > Sean
> >
>
>Sorry, I wrote a wrong index name in the plan adopted by first query,
>and first query is not ordered, and it's fast. The ascending ordered
>query is fast too:
>
>select first 1000 * from A
> where update_time >= '13-JUN-2006 00:00:00'
> and update_time <= '13-JUN-2007 00:00:00'
> and id <= 3000000
> and id >= 2000
> order by id desc
>
>Adopted plan is: PLAN(A ORDER PK_A)

Looks like yet another typo. This isn't your "ascending query", is
it? The DESC ordering clause should make the plan use

> 2. A_IDX_ID_DESC on column ID, descending

Anyway, it would be interesting to see the plan and performance you
get if you use BETWEEN to avoid those GTE and LTE comparisons: I
wonder whether the optimizer might then try to do something useful
with one of those indexes on the timestamp fields, given the
opportunity to do straight-out equality evaluation.

select first 1000 * from A
where update_time BETWEEN
'13-JUN-2006 00:00:00' and '13-JUN-2007 00:00:00'
and id BETWEEN 2000 AND 3000000
order by id desc

./heLen