... according to the indexing strategies written by Mike Cain and Kent
Milligan, the most selective key should be the first.
... if we are speaking from SQL using indexes, SQL can use multiple indexes
(especially EVIs) for the same query and build dynamic bitmaps (index anding
and oring) before accessing the data.
When using SQL the main goal is to access the data as fast as possible. In
this way SQL may select the data first into temporary objects and sort the
result after.
For more information just have a look at the following white paper:
IBM DB2 for i indexing methods and strategies
Learn how to use DB2 indexes to boost performance
https://www-304.ibm.com/partnerworld/wps/servlet/ContentHandler/stg_ast_sys_
wp_db2_i_indexing_methods_strategies
Mit freundlichen Grüßen / Best regards
Birgitta Hauser
"Shoot for the moon, even if you miss, you'll land among the stars." (Les
Brown)
"If you think education is expensive, try ignorance." (Derek Bok)
"What is worse than training your staff and losing them? Not training them
and keeping them!"
-----Ursprüngliche Nachricht-----
Von: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] Im Auftrag von Schutte, Michael D
Gesendet: Friday, 02. September 2011 14:10
An: Midrange Systems Technical Discussion
Betreff: RE: Order of key fields
I believe you were asking about ordering, not selecting. The answer is it
depends. If your order by clause matches an index key then sql will take
advantage of that index. You sql could run slower if you order by a key
that doesn't match any indices. Logically, lot, case, piece makes more
sense.
-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of George Kinney
Sent: Thursday, September 01, 2011 4:50 PM
To: midrange-l@xxxxxxxxxxxx
Subject: Order of key fields
Here's the situation, I have a table that has a 12 field index. Some of
those fields are far more variable than others, and at the moment they
are specified in the DDS in the order they appear in the source data.
What I'm wondering is if there is a signifigant performance advantage
(or penalty) to ordering them according to their variability. In other
words, given a situation where you have a lot#, case#, piece# with many
pieces# per case, per lot, would a key order of (lot, case, piece)
affect performance differently than (piece, case, lot)?
Anybody know, or have any pointers to enlightening online resources?
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