Hello all.
I read a question concerning the use of the MINUS function earlier and I had
not heard of it before today. After some experimenting with it, I was quite
amazed at the speed of this function compared to using NOT IN.
Considering the following:
Query One
select distinct icons from inpatient
minus select distinct icons from ptca;
Query 2
select distinct icons from inpatient
where icons NOT IN(select distinct icons from ptca)
Query number one began to run in about 5 seconds whereas query 2 took
forever to run. My question is why is the MINUS so much faster?
My understanding of the NOT IN is that it probably builds the record set in
the second part of the SQL query each time it comes to a new icons number in
the first part of the query. Therefore, if there are one thousand records to
be searched, it probably builds the record set one thousand times. Is this
correct? If so, how does the MINUS function build and compare the record
set?
And finally, where do I get these wonderful little tidbits? Would a SQL
tuning book exist that might talk about the strengths and limitations of
different functions? I really appreciate your time and input.
Petre Hazelton
Halifax, Nova Scotia CANADA
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Author: Peter Hazelton
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