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Index leading columns [message #486215]

Tue, 14 December 2010 05:42

pointers
Messages: 451
Registered: May 2008

Senior Member

Hi,

I have been reading Oracle documentaion about access paths, got strucked at the concept 'Leading columns in index'.
Could you please share me what is meant by 'Leading columns of an index', how to find/judge them that they are leading columns.
The url where I am reading is <<http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm#sthref976>>

Regards,
Pointers

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Re: Index leading columns [message #486217 is a reply to message #486215]

Tue, 14 December 2010 05:48

Michel Cadot
Messages: 68737
Registered: March 2007
Location: Saint-Maur, France, https...

Senior Member
Account Moderator

If an index is defined on (a,b,c) columns then "a" is the leading column, (a,b) are the leading columns, "b" alone or "c" is not a leading column.

Regards
Michel

[Updated on: Tue, 14 December 2010 06:07]

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Re: Index leading columns [message #486218 is a reply to message #486217]

Tue, 14 December 2010 05:49

pointers
Messages: 451
Registered: May 2008

Senior Member

Thanks Micheal, that is helpful to read further.

Regards,
Pointers

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Re: Index leading columns [message #486220 is a reply to message #486215]

Tue, 14 December 2010 05:51

Mahesh Rajendran
Messages: 10708
Registered: March 2002
Location: oracleDocoVille

Senior Member
Account Moderator

You have columns c1,c2,c3 in a table.
You create an index on those columns.
Something like

create index myIndex on myTable(c1,c2,c3);

c1 is the leading column here.
The optimizer will behave differently according to leading column and available stats.
The document is talking about different scenarios.
Search the forum. You will find many examples for each of those scenarios. One such is,
http://www.orafaq.com/forum/?t=msg&goto=156882&42800/&srch=leading+column#msg_156882

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Re: Index leading columns [message #486222 is a reply to message #486220]

Tue, 14 December 2010 05:53

pointers
Messages: 451
Registered: May 2008

Senior Member

Thanks Mahesh. That is helpful.

Regards,
Pointers

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