RE: SQL question

[Walden Leverich <walden@xxxxxxxxxxxxxxx>]

Strangely enough, I had the same problem this morning on V4R1 over
465,000 records. I actually killed the SQL after 15 minutes trying to
count(distinct field) from file. After reading your e-mail I tried the
select without the count and it ran almost instantly. 

Now, the original count would have resulted in close to 465,000 records
(a nearly unique field), I ran another count distinct that resulted in
only 3 records and it ran almost instantly. The only thing I can think
of is that the count is being performed in such a way that it is
optimized for a very low count value and takes FOREVER for a large
number of distinct values.

Try your SQL again counting distinct values for a very non-unique field
and post the results. If we see the same result I think we can report
this as a database bug.

-Walden

-----Original Message-----
From: Ravi [mailto:ravi@spacestar.com]
Sent: Wednesday, June 17, 1998 5:31 PM
To: MIDRANGE-L@midrange.com
Subject: SQL question


Howdy folks,

I have a file with about 1/2 million records on our development
machine (V4R2).

I ran a SQL query from ISQL:
-  Select distinct FIELD from FILE.
The query ran in about 2 minutes and gave me the result set with 
about 200,000 records. FIELD is not a key field.

I ran another query from ISQL over the same file/field:
- Select count(distinct FIELD) from FILE.
This query ran for over 10 minutes and returned the count.
Why would this query take so much longer than the first query? I ran the
test a few times when there was hardly any activity on the system and 
got the same results. The 2nd query takes atleast 5 times as long 
to complete. 
We have a similar table(few hundred thousand rcds)on an RS6000/Oracle
database. I ran similar queries over Oracle and there was no noticeable
difference in run times. I chose a non-indexed field.
What is so different about DB2/400 that would cause this? Just
curious...

Thanks

Ravi
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