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From: dn.perl on 13 Jul 2010 00:38

On Oracle 10 hosted on a FreeBSD server, I am seeing a
disproportionate difference in response time between two queries which
should take same amount of time.

my_folder is a table with only 22,000 entries.
Query 1 : (takes 22-23 seconds)
select f.foldername from my_folder f
where f.foldername in (select f1.foldername from my_folder f1,
my_aa a, my_bb b
where some-conditions)
and f.foldername.isactive = 1 ;

Query 2 : (I aborted it after 15 minutes)
select f.foldername from my_folder f
where f.foldername not in (select f1.foldername from my_folder
f1, my_aa a, my_bb b
where some-conditions)
and f.foldername.isactive = 1 ;
The only difference is that query 2 uses 'NOT IN' clause instead of
'IN' clause.

Why should this happen?

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Next: NOT IN clause vs IN clause