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SQL Server interview questions: - What is Fragmentation?

By Shiv Prasad Koirala in SQL
Jan 25th, 2012
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This is the SQL Server interview questions which are asked during the interview not so frequent. But still go through it once before going for the interview.

Speed issues occur because of two major things

Fragmentation.
Splits.

Splits have been covered in the first questions. But one other big issue is fragmentation. When database grows it will lead to splits, but what happens when you delete something from the database...HeHeHe life has lot of turns right. Ok let's say you have two extents and each have two pages with some data. Below is a graphical representation. Well actually that's now how things are inside but for sake of clarity lot of things have been removed.

Figure: - Data Distribution in Initial Stages

Now over a period of time some Extent and Pages data undergo some delete. Here is the modified database scenario. Now one observation you can see is that some page's are not removed even when they do not have data. Second If SQL server wants to fetch all "Females" it has to span across to two extent and multiple pages within them. This is called as "Fragmentation" i.e. to fetch data you span across lot of pages and extents. This is also termed as "Scattered Data".

Figure: - Data Distribution after Deletes

What if the fragmentation is removed, you only have to search in two extent and two pages. Definitely, this will be faster as we are spanning across less entities