Thursday, April 10, 2008

index-only scan

"index-only" means "no data page reads".

Before we explain index-only query, we need to explain that Most indexed queries first read the index then[2] read the data pages [1].

a typical index-only query uses a composite index [3]. Another post about "narrow index key" explains that the index key width is column width added up.

See also
http://books.google.com/books?id=JSVhe-WLGZ0C&pg=PA298&lpg=PA298&dq=index-only+scan&source=web&ots=Llbg8PAiJ9&sig=PhRIwHc78MNuso1xMJTLDnkJn-U&hl=en#PPA297,M1

http://www-1.ibm.com/support/docview.wss?uid=swg1IY94275
http://bioivlab.ils.unc.edu/course_wikis/INLS_623/ClassWork_INLS623_Spring2008/index.php/Kyle http://www.google.com/search?q=index%2Donly+scan

[1] for structure of a page, read the db2 training manual P171
[2] follows the index leaf nodes
[3] we have no time to cover single-column index-only queries

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