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What is use of pad Indexs?

Posted on 2006-07-12
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Hi,
I'm curious what is the use of Pad Index?? how it can boost sql server performance by enable this option? and what is the great advantages by using Pad Index?
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Question by:motioneye
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http://www.sqlservercentral.com/columnists/chedgate/clusterthatindexparttwo.asp

FILL FACTOR & PAD INDEX
One thing that I intentionally left out of the prior article was a description of fill factor and pad index, in order to keep focus on the problems of not having a clustered index on a table. One thing I did mention was that one slight disadvantage you might run into by having a clustered index on a table are the page splits that can occur when adding data (or sometimes when changing existing data) to the table. This is a part from that article:
 
"Because the data is stored in the order of the index, to insert a new row SQL Server must find the page with the two rows between which the new row shall be placed. Then, if there is not room to fit the row on that page, a split occurs and some of the rows get moved from this page to a newly created one."
 
As some of you readers commented, one way to counter this (at least to a degree) is to specify a value for the fill factor and possibly pad index for the clustered index. Fill factor specifies the percentage that SQL Server should fill every data page to when creating an index. Specifying a fill factor value of 50 therefore means that each data page will only be half full, in effect meaning that the storage space needed for the table is doubled (since there are twice as many data pages). The good thing is that page splits are not as necessary as with a higher fill factor, since there will usually be room on a page to insert a new row without splitting the page into two and moving rows between them. If fill factor is specified you can also specify the option to use pad index (it uses the same value as fill factor), which works in the same way as fill factor only it is used for index pages in the intermediate levels (between the root level and leaf level).
 
Note that both options are only used when creating or rebuilding an index. They are not maintained after initial 'filling' by SQL Server, as that would defeat the original purpose to use them, i.e. avoiding page splits (as SQL Server would have to use page splits to maintain them). Also note that these options apply for non-clustered indexes as well, the index pages of non-clustered indexes use page splits in the same way as clustered indexes of course. As always, think carefully and test the effects of changing the fill factor and pad index values.
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