Re: AW: Index use vs Table Scan!

> Hello again!
> 
> I have real data in the relation and the EXPLAIN-statement states a huge
> page count and a table scan, when I use the <= or < operator in the
> statement 
> 
>       explain select * from s_event where (eventtime < 1111056567313)
> 
> and a page count that is much more smaller and the use of an index, when I
> use the  statement 
> 
>       explain select * from s_event where (eventtime = 1111056567313)
> 
> The manual and the section you mentioned doesn't explain this behaviour,
> at
> least I haven't found the explanation.
> 
> Any further hints/tipps?

Did you update statistics?

Also, you could try with a clustered index on column eventtime.  My guess
would be that that would be used for this type of query as it is sorted and
thus allows for easier extraction of ranges.

Kind regards

robert

> 
> Thanx a lot!
> 
> Chris
> 
> -----Ursprüngliche Nachricht-----
> Von: Grossmann, Gert [mailto:gert.grossmann@xxxxxxx]
> Gesendet: Freitag, 18. März 2005 09:07
> An: maxdb
> Betreff: AW: Index use vs Table Scan!
> 
> 
> Simple check access path with "EXPLAIN SELECT ... ". Now you can see if
> index or key is used. For further information please read manual section
> 'Background Knowledge/SQL Optimizer'.
> 
> Gert
> 
> -----Ursprüngliche Nachricht-----
> Von: Christoph Weser [mailto:Christoph.Weser@xxxxxxxxx] 
> Gesendet: Freitag, 18. März 2005 08:50
> An: Grossmann, Gert
> Betreff: AW: Index use vs Table Scan!
> 
> 
> Hello Gert!
> 
> Thanx for your quick replay and a simple answer.....how do I check the
> access paths and how to interpret results?
> There are right now some real data in the table...a few million rows.
> 
> Thanx a lot for your help!
> 
> Chris
> 
> -----Ursprüngliche Nachricht-----
> Von: Grossmann, Gert [mailto:gert.grossmann@xxxxxxx]
> Gesendet: Freitag, 18. März 2005 07:02
> An: maxdb (E-Mail)
> Betreff: AW: Index use vs Table Scan!
> 
> 
> MaxDB has a cost based optimizer. Without any data in relations there is
> no
> difference between table scan vs. index access.
> Fill your releation "s_event" with real data and then check access pathes.
> 
> Gert
> 
> -----Ursprüngliche Nachricht-----
> Von: Christoph Weser [mailto:Christoph.Weser@xxxxxxxxx] 
> Gesendet: Donnerstag, 17. März 2005 17:29
> An: maxdb (E-Mail)
> Betreff: Index use vs Table Scan!
> 
> 
> Hello All!
> 
> I'm running 7.4.3.32 on a W2K system.
> 
> I have the table: 
> 
> create table s_event (
>        EID FIXED(38,0) NOT NULL, 
>        ETID FIXED(38,0) NOT NULL,
>        SID FIXED(38,0) NOT NULL,
>        CID FIXED(38,0),
>        eventtime FIXED(38,0) NOT NULL,
>        data LONG BYTE,
> 
>        FOREIGN KEY(ETID) references s_eventtype(ETID)
> )
> 
> //
> create index s_event_x1 on s_event(EID)
> //
> create index s_event_x2 on s_event(ETID)
> //
> create index s_event_x3 on s_event(SID)
> //
> create index s_event_x4 on s_event(CID)
> //
> create index s_event_x5 on s_event(eventtime)
> 
> When I do an 
> 
>       explain select * from s_event where (eventtime < 1111056567313)
> 
> the result is a table scan!
> 
> On the other hand, when I do an 
> 
>       explain select * from s_event where (eventtime = 1111056567313)
> 
> The DB does an index scan and it uses the index s_event_x5.
> 
> 
> Is there a way for the first statement, that the db also does an index
> scan
> and not the expensive table scan???
> Why isn't an index also used in the first statement? Any explanations?
> 
> Thanx a lot to everyone!!!
> 
> Chris
> 
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