RE: Outer Joins are Evil?

From: Rudy Zung <Rudy.Zung_at_efi.com>
Date: Tue, 31 Aug 2004 12:03:53 -0400
Message-ID: <4FBAA533C7C64940A7921F3000736B120217C515@pghexmb01.printcafe.efi.internal>

A lack of understanding of outer-joins is what usually drives people to think that all outer-joins are bad.

Outer-joins can degenerate into an FTS if the outer-join columns can skip over leading columns of an index: Table A and B both have columns C1 and C2; both are indexed on C1 and C2. This is OK:

select A.*,

          B.*
      from A,
           B
      where A.C1 =3D B.C1 and
            A.C2 =3D B.C2(+);

The query still can use the C1 of the index to narrow the list of records before deciding if there is a matchin B.C2(+).
This is bad:

select A.*,

          B.*
      from A,
           B
      where A.C1 =3D B.C1(+) and
            A.C2 =3D B.C2(+);

What if you have a B where A.C2 =3D B.C2, also where there=20 are no records where A.C1 =3D B.C1? For Oracle to find the matching A.C2 =3D B.C2, Oracle would need to FTS.

Classically, this is the question of find me all the words in the dictionary that begin with the letter "O" versus=20 find me all the words in the dictionary where the second letter in the word is "O".

-----Original Message-----
From: oracle-l-bounce_at_freelists.org =
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Karen Morton
Sent: Tuesday, August 31, 2004 11:54 AM
To: oracle-l_at_freelists.org
Subject: RE: Outer Joins are Evil?

Sounds like excuses...not reasons. What evidence do they provide to support their conclusion that outer joins are bad? It looks a bit like an attempt to disguise a fear of doing full table scans or something like that.

Outer joins, like pretty much everything else, are not "inherently evil". They are another option/tool to be used appropriately when and where needed. While I don't disagree with using default values in FK columns and the like, doing it only with the justification of avoiding outer joins is a bit near-sighted. Eliminating any one thing out of fear of what "it" may do seems to me to be more a fear of poorly written code as a result of misusing the feature. If it's really the fear of bad code, then teach people how to properly use the tool and do not take the tool out of the box entirely instead.

Karen Morton
Hotsos Enterprises, Ltd.
http://www.hotsos.com
Upcoming events at http://www.hotsos.com/education/schedule.html

=20

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Barr, Stephen Sent: Tuesday, August 31, 2004 8:04 AM
To: Oracle-L_at_freelists.org
Subject: Outer Joins are Evil?

I'm currently working on a datawarehouse project (~5 Tb) where the decision has been made to avoid performing outer joins.
=20

The reasons given for this seem to be -
=20

1. Simplifies user navigation of the structures - i.e. avoids outer joins.=20

2.	Outer joins are slow and should be avoided at all costs.=20
3.	If an FK is missing it is populated with a default value which

will
relate to an actual row in the target table, hence no rows will ever be dropped - again, supposedly this is to simply SQL and avoid outer joins.

=20

What they actually do is populate each table in the structure with three default rows with an SK of 0, 1 & 2. Any FK's which are missing, not applicable or invalid will point to one of these rows.
=20

Now....my question is....what is so inherently evil about outer joins that we go to this extreme to avoid them?
=20

AND...has anyone else seen something like this deployed in other places?
=20

Thanks,
=20

Steve.
=20
=20
=20

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Received on Tue Aug 31 2004 - 11:00:50 CDT