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NOT EXISTS and NOT IN (was RE: SQL Help finding missing records)



fixed

Sorry guys, but OS/400 is smart enough to use an index even with a NOT IN.

A test on v5r2....

SELECT SPLVL, count(*)
FROM cstspnp
group by splvl

Salesperson Level COUNT ( * )
413,797
Y 1
2 4
1 299,078
N 3

CREATE INDEX cmwtest2 ON CSTSPNP (SPLVL)

select * from cstspnp
where splvl not in ('1',' ')

Access path of file CMWTEST2 was used by query.
for reason code 1.
1 - Record selection.


I think the issue is that usually a NOT IN selects enough records that a table scan is the best
choice.

HTH,

Charles Wilt
--
Software Engineer
CINTAS Corporation - IT 92B
513.701.1307
wiltc@xxxxxxxxxx


-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
On Behalf Of DeLong, Eric
Sent: Wednesday, July 16, 2008 12:36 PM
To: RPG programming on the AS400 / iSeries
Subject: RE: SQL Help finding missing records

I don't think so..... NOT EXISTS will use an index scan *IF* the fields
referenced in the WHERE of the subquery exist within an index. I would
agree with that statement in the case of the IN predicate....

Eric DeLong

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx]On Behalf Of vhamberg@xxxxxxxxxxx
Sent: Wednesday, July 16, 2008 10:32 AM
To: RPG programming on the AS400 / iSeries
Subject: RE: SQL Help finding missing records


Caveat emptor!!

Remember that NOT EXISTS, as well as NOT IN, forces a table scan - you
can't tell if something is not there unless you look at everything.
Learned this at the last COMMON, I think.

Now probably and EXCEPTION JOIN can do better, IF you have supporting
indexes.

Regards
Vern



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