Sergey Gigoyan opens a can of worms:
When data is modified in a Microsoft SQL database, the corresponding indexes are also modified. These modifications cause indexes to become fragmented. Fragmentation means that the logical order of the data inside the index pages does not match its physical order. The more the fragmentation level is increased, the more performance is affected for SELECT statements. With fragmented indexes, the information is not spread logically, making the index’s data retrieving operations more time-consuming resulting in query performance issues. Thus, fragmentation of the indexes should be fixed periodically in order to maintain high performance. Rebuild and Reorganize index operations are aimed at defragmenting indexes.
Click through to learn a bit about why we may need index maintenance, followed by a comparison between index rebuilding versus index reorganization. All that said, I do not have room in my indexing philosophy (or my heart) for reorganization. Reorganization simply doesn’t provide enough value as it is. If you want a really detailed dive into the topic, Jeff Moden put together an amazing talk that we were lucky enough to be able to record. Specifically, about an hour and a quarter in (link to timestamp), Jeff gets to the topic of reorganization and how little it actually does in practice.