Is the table too small?
Fragmentation figures are meaningless for small tables that span up only a few extents
This is captured by @minPageCount
in the script. The default is 8: meaning it will be skipped if one extent in size.
As you perform inserts updates and deletes, your indexes will become fragmented both internally and externally.
Internal fragmentation is you have a high percentage of free space on your index pages, meaning that SQL Server needs to read more pages when scanning the index.
External fragmentation is when the pages of the index are not in order any more, so SQL Server has to do more work, especially in IO terms to read the index.
If your indexes become too fragmented, at best, your queries will be less efficient but at worst, SQL Server will just stop using the indexes all together, meaning virtually all queries would have to perform a table scan or clustered index scan. This will hurt your performance a lot!
When you reorganise an index, then SQL Server uses the existing index pages and just shuffles data around on those ages. This will alleviate internal fragmentation and can also remove a small amount of external fragmentation. It is a lighter weight operation than rebuild and is always online.
When you rebuild an index, SQL Server actually resorts the data of the index and uses a new set of index pages. This will obviously alleviate both internal and external fragmentation but is a more heavy weight operation and by default causes the index to go offline, although it can be performed as an online operation, depending on your SQL Server version and settings.
Please do not expect to have 0 fragmentation after a Rebuild however. Unless you use a MAXDOP query hint, SQL Server will parallelise the rebuild operation and the more processors involved, the more fragmentation there is likely to be, because each processor or core, will rebuild their section or fragment of the index individually, without regard for each other. This is a trade off between best fragmentation levels and time taken to rebuild the index. For near 0 fragmentation, use MAXDOP 1 and sort the results in TempDB.
Best Answer
It's really hard to say how long your rebuild will take, as SQL itself doesn't really know in advance and cant give you an estimate.
You can use the following query to use the dm_exec_requests dmv to view how long your index rebuild has been going on for, and to verify that SQL doesn't really have an estimate:
However when it comes to a real estimate on time required you can read this nice blog post from sqlmunkee, which summarises this by saying "..it depends. ":
Frustrating, but true, sadly.