The difference between the index clustered et Non-clustered in a database is one of the most popular questions in SQL.
Indexes are a very important concept, it makes the execution of your queries fast and if you are comparing a query SELECT which uses an indexed column than one that doesn't, you will see a big difference in performance.
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To understand how SQL Server defragments an index, consider a simplified example of pages after many insertions, updates, and deletes, as shown in the following figure.
Read More Defragmentation of an index (REORGANIZE)
What is fragmentation? How to detect fragmentation and how to eliminate it?
A fragmented table is a table where some of its data pages point to pages that are not immediately following pages in the extent, and if all its pages are contiguous in both the allocation plan and the extents, then the table is not fragmented.
Read More What is fragmentation?