RE: NOT EXISTS and NOT IN (was RE: SQL Help finding missing records) -- RPG400-L

Eric

True enough.

The matter of supporting indexes still holds, I believe. The caution against using NOT IN was given within the last year by a knowledgeable IBM person. So it was given fully in the context of running in SQE.

Again, if there is no index, the choices are not good for the optimizer.

Vern

-------------- Original message --------------
From: "DeLong, Eric" <EDeLong@xxxxxxxxxxxxxxx>

Vern,

I suspect that this is yet another "legacy" issue, that has been corrected on
newer releases. I'm certain that CQE had serious issues with NOT IN, but I'm
not so sure about SQE... Perhaps some of these performance related statements
need to be re-reviewed so that we can un-learn these notions that no longer
apply....

Eric DeLong

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx]On Behalf Of vhamberg@xxxxxxxxxxx
Sent: Thursday, July 17, 2008 10:38 AM
To: RPG programming on the AS400 / iSeries
Subject: Re: NOT EXISTS and NOT IN (was RE: SQL Help finding missing
records)

Charles

I was probably mistaken on the NOT EXISTS, which would take advantage of an
index, if it exists.

And it's good that NOT IN can use and index. I will still stand by what was said
by the IBMer, however, that NOT IN can be a performance drag; better, if
possible, to have a predicate without the NOT. Without an index to use, the
optimizer would have either to create one (table scan PLUS writing the index
out) or to do the table scan.

Lots of ad hoc queries do not have indexes to support them. Of course, that's
another issue.

Vern

-------------- Original message --------------
From: "Wilt, Charles"

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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