The most common and widely used index that we know are nonclustered index. These indexes are created using the famous b-tree (balanced tree). B-tree is a data-structured tree where we have the root and leaves.
A nonclustered index contains the index key values and row locators that point to the storage location of the table data. Generally, nonclustered indexes should be designed to improve the performance of frequently used queries that are not covered by the clustered index.
These indexes need a regular maintenance because as data is updated, inserted or deleted these indexes are updated for the new data or delete the reference of a particular deleted data.
When this happen we called fragmentation and Brent Ozar can explain more about it.
The objective here is about maintenance the index, well not exactly who to do this, but what do not do. I’ve seen in same cases a good reindex job, everything ok, tables distributed by number of rows, cluster, then no cluster, but after all of it a step with shrink.
Do not use shrink database, this is not a good idea after a reindex. What is this? What will do in the database? In MSDN page we can see this:
Shrinking data files recovers space by moving pages of data from the end of the file to unoccupied space closer to the front of the file. When enough free space is created at the end of the file, data pages at end of the file can be deallocated and returned to the file system
The process SQL uses is ugly and results in index fragmentation again and all that work to reindex was thrown away. If don’t belive me, well Paul Randal explain something in his article.