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how is this query resolved?

 
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jctgf
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Joined: 05 Nov 2006
Posts: 89
Topics: 36
PostPosted: Sat Mar 07, 2009 6:07 pm    Post subject: how is this query resolved? Reply with quote
Code:

SELECT ...
FROM TABLE1
WHERE COLUMNA = ...
    AND COLUMNB < ...
ORDER BY COLUMNB
FETCH FIRST 200 ROWS ONLY


Hi there,
In this query I select lines from a table, order it and limit the result cursor to 250 lines.
I wonder if these steps are taken in the sequence described above, please.
If so, and if the number of lines selected in the first step is huge, it probably won
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kolusu
Site Admin
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Joined: 26 Nov 2002
Posts: 12394
Topics: 75
Location: San Jose
PostPosted: Sat Mar 07, 2009 10:50 pm    Post subject: Reply with quote
jctgf,

If both the FETCH FIRST clause and the ORDER BY clause are specified, the ordering is performed on the entire result table prior to returning the first n rows.

Check this link

http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/DSNSQJ12/4.4.3?

Kolusu
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jsharon1248
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Joined: 08 Aug 2007
Posts: 291
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Location: Chicago
PostPosted: Sun Mar 08, 2009 10:15 pm    Post subject: Reply with quote
If you have an index that matches the ORDER BY columns exactly, DB2 could bypass the physical sort. You'd have to look at the access path for the query to make sure.
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jctgf
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Posts: 89
Topics: 36
PostPosted: Mon Mar 09, 2009 5:12 pm    Post subject: Reply with quote
thanks.
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