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Discuss content posted by Sarvesh Singh
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Column Order in an Index
42 posts, Page 2 of 5
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Column Order in an Index
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vishalkumar_dobariya
vishalkumar_dobariya
Posted Tuesday, November 23, 2010 4:47 AM
Forum Newbie
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Last Login: Monday, May 28, 2012 10:22 PM
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Visits: 48
Good Article to understand Non clustered index usage.
Post #1025024
Ukon
Ukon
Posted Tuesday, November 23, 2010 5:23 AM
SSC Rookie
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Last Login: Today @ 10:38 AM
Points: 36,
Visits: 101
The article may be short and short on detail, however, the really good thing was that it lead me to this discussion and the links here. Thank you!
Post #1025057
ss-457805
ss-457805
Posted Tuesday, November 23, 2010 6:12 AM
Old Hand
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Last Login: Yesterday @ 2:40 AM
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Visits: 1,667
Thanks for the feedback Grant. Much appreciated. Will keep that in mind for my next article.
blog: http://sarveshsingh.com
Twitter: @sarveshsing
Post #1025093
sjimmo
sjimmo
Posted Tuesday, November 23, 2010 6:16 AM
SSCrazy
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Last Login: Friday, June 14, 2013 7:34 AM
Points: 2,865,
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Sarvesh, This was a good article, with the potential to be much better if you were to expand on it. You leave out a lot of why things work this way leaving it at a very entry level atricle.
I think it great that you did the article, but please - put more effort into it. Why does the index work this way? How do statistics impact it? What is the difference between an index scan and an index seek? etc...
Steve Jimmo
Sr DBA
“If we ever forget that we are One Nation Under God, then we will be a Nation gone under." - Ronald Reagan
Post #1025096
cengland0
cengland0
Posted Tuesday, November 23, 2010 6:44 AM
Ten Centuries
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Last Login: Thursday, January 24, 2013 9:59 PM
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Please don't laugh at me for asking this question.
Why would you create a compound index with both last name and first name together? Why not two separate indexes where one is for last name and the other is for first name. That should handle any combination that you have in your where clause.
Post #1025117
Wilfred van Dijk
Wilfred van Dijk
Posted Tuesday, November 23, 2010 6:55 AM
SSC Eights!
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Last Login: Monday, June 10, 2013 7:21 AM
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aha, the never-ending question about multicolumn or single column indexes!
Again, check Gail's excellent articles about indexes:
http://sqlinthewild.co.za/index.php/2010/09/14/one-wide-index-or-multiple-narrow-indexes/
Wilfred
The best things in life are the simple things
Post #1025128
cengland0
cengland0
Posted Tuesday, November 23, 2010 7:38 AM
Ten Centuries
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Last Login: Thursday, January 24, 2013 9:59 PM
Points: 1,354,
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Wilfred van Dijk (11/23/2010)
aha, the never-ending question about multicolumn or single column indexes!
Again, check Gail's excellent articles about indexes:
http://sqlinthewild.co.za/index.php/2010/09/14/one-wide-index-or-multiple-narrow-indexes/
Thanks for the link but I get "404 - File or directory not found."
Post #1025169
Michael Ebaya
Michael Ebaya
Posted Tuesday, November 23, 2010 8:11 AM
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cengland0 (11/23/2010)
Please don't laugh at me for asking this question.
Why would you create a compound index with both last name and first name together? Why not two separate indexes where one is for last name and the other is for first name. That should handle any combination that you have in your where clause.
Because indexes don't work the way you think they do. Indexes sort off their key expression. If that expression contains two columns, the table is sorted by both. If it contains one column only, then its only sorted by that single column, period.
Without getting into too much technical detail, SQL contains an optimization called index intersection that allows two indexes to act a wee bit like one compound index. But "a wee bit" is the operative term here; the performance is not the same.
To use your firstname, lastname example. If you have a single "John Smith" in your table, a compound index will walk the tree right to that record-- all done! No scanning required. Two indexes, one on each column, will only allow the engine to find all Smiths, then find all Johns, then laboriously intersect the result result to find which ones match both criteria.
Performance takes a further hit when you want to order by multiple columns, as a single index will return the rows in proper order, whereas results from intersected indexes must be manually sorted each time.
Post #1025195
Wilfred van Dijk
Wilfred van Dijk
Posted Tuesday, November 23, 2010 9:22 AM
SSC Eights!
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Last Login: Monday, June 10, 2013 7:21 AM
Points: 898,
Visits: 1,049
seems SSC adds something in front of it, copy/paste: http://sqlinthewild.co.za/index.php/2010/09/14/one-wide-index-or-multiple-narrow-indexes/
Wilfred
The best things in life are the simple things
Post #1025259
David Walker-278941
David Walker-278941
Posted Tuesday, November 23, 2010 9:38 AM
Valued Member
Group: General Forum Members
Last Login: Friday, April 05, 2013 12:29 PM
Points: 67,
Visits: 227
"This does not mean that the index 'NonClust_Contact_Name' was not useful. It's still much better than the Clustered scan."
*WHAT* is still much better
than
the clustered scan? I'm confused by that last sentence.
Post #1025267
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