Fragmentation can be easily detected by running the system function sys. It can be run only against a specific index in the table or view, all indexes in the specific table or view, or vs. After the fragmentation has been detected, the next step is to determine its impact on the SQL Server and if any course of action needs to be taken.
There is no exact information on the minimal amount of fragmentation that affects the SQL Server in a specific way to cause performance congestion, especially since the SQL Server environments greatly vary from one system to another.
Here is the reasoning behind the thresholds above which will help you to determine if you should perform index rebuild or index reorganization:. Index reorganization is a process where the SQL Server goes through the existing index and cleans it up. Index rebuild is a heavy-duty process where an index is deleted and then recreated from scratch with an entirely new structure, free from all piled up fragments and empty-space pages.
However, you cannot repair nonclustered index inconsistencies by rebuilding the index online, because the online rebuild mechanism uses the existing nonclustered index as the basis for the rebuild and thus carries over the inconsistency. Rebuilding the index offline can sometimes force a scan of the clustered index or heap and so replace the inconsistent data in the nonclustered index with the data from the clustered index or heap.
To ensure that the clustered index or heap is used as the source of data, drop and recreate the nonclustered index instead of rebuilding it.
As with earlier versions, we recommend recovering from inconsistencies by restoring the affected data from a backup; however, you may be able to repair nonclustered index inconsistencies by rebuilding it offline or recreating it.
Leverage solutions such as Adaptive Index Defrag to automatically manage index fragmentation and statistics updates for one or more databases.
This procedure automatically chooses whether to rebuild or reorganize an index according to its fragmentation level, amongst other parameters, and update statistics with a linear threshold. The following scenarios cause all rowstore nonclustered indexes on a table to be automatically rebuilt:. The following scenarios do not automatically rebuild all rowstore nonclustered indexes on the same table:.
An index cannot be reorganized or rebuilt if the filegroup on which it is located is offline or read-only. When the keyword ALL is specified and one or more indexes are on an offline or read-only filegroup, the statement fails.
While an index rebuild occurs, the physical media must have enough space to store two copies of the index. When the rebuild is finished, the Database Engine deletes the original index. Rebuilding or reorganizing small rowstore indexes may not reduce fragmentation. Up to, and including, SQL Server Therefore, pages of small indexes are sometimes stored on mixed extents, which implicitly makes such indexes fragmented.
Mixed extents are shared by up to eight objects, so the fragmentation in a small index might not be reduced after reorganizing or rebuilding it. When rebuilding a columnstore index, the Database Engine reads all data from the original columnstore index, including the delta store. It combines data into new row groups, and compresses all row groups into columnstore. The Database Engine defragments the columnstore by physically deleting rows that have been marked as deleted.
This improves columnstore index quality over time. For more information about columnstore terms and concepts, see Columnstore indexes: Overview. Rebuilding the entire table takes a long time if the index is large, and requires enough disk space to store an additional copy of the entire index during the rebuild. Rebuilding a partition after loading or modifying data ensures all data is stored in compressed row groups in columnstore. When the data load process inserts data into a partition using batches smaller than , rows, the partition can end up with multiple open row groups in delta store.
Rebuilding moves all delta store rows into compressed row groups in columnstore. When reorganizing a columnstore index, the Database Engine compresses each closed row group in delta store into columnstore as a compressed row group. After performing data loads, you can have multiple small row groups in the delta store. The reorganize operation will also remove rows that have been marked as deleted from the columnstore. Reorganizing a columnstore index using Management Studio combines compressed row groups together, but does not force all row groups to be compressed into the columnstore.
Closed row groups will be compressed, but open row groups will not be compressed into columnstore. Index maintenance, performed by either reorganizing or rebuilding an index, is resource-intensive. However, depending on the database workload and other factors, the benefits it provides range from vitally important to minuscule. To avoid unnecessary resource utilization that may be detrimental to query workloads, Microsoft does not recommend performing index maintenance indiscriminately.
Instead, performance benefits from index maintenance should be determined empirically for each workload using the recommended strategy , and weighed against resource costs and workload impact needed to achieve these benefits.
The likelihood of seeing performance benefits from reorganizing or rebuilding an index is higher when the index is heavily fragmented, or when its page density is low. However, these are not the only things to consider. Now, creating these jobs can be a double-edged sword. We should always keep an eye on how long it takes for the job to finish maintenance plan.
Scripts are flexible and overall better solution while the plans are quick and a simpler solution. Ensure that an appropriate database is selected and run the code from below:. Those two tables are identical, the only difference is name, and one was created with the sequential SQL index on the ID column 1, 2, 3, etc.
Both have a clustered index on the same column. Furthermore, if we right-click on it, hit Properties and select Fragmentation , since this is a brand new index, we should see both Page fullness the average percent full of pages and Total fragmentation logical fragmentation percentage taking into account multiple files values to be 0.
Since both tables are empty, we cannot really use them unless we have some data in them. We can do this by executing the code from below:.
The above code inserts a thousand records into both tables. Both batches did the exact same thing. The only difference here is that we have our ID sequential on one table and non-sequential on the other. Just locate it in Object Explorer, right-click on it and hit Properties.
If we now check the fragmentation, notice that the fragmentation is at 5. What do you think will happen if we insert another thousands record into this table? Will it get worse or better? Head over to the index properties again, and notice the total fragmentation went down to 2.
Sure thing. It gets better because with the sequential SQL Indexes it just keeps filling out the pages. When it needs another page, it will just create it, fill it in, and so on without reorganizing or moving anything. Now, if we head over to non-sequential and check the fragmentation we can see a significantly higher value of I guarantee if we run the Insert statement one more time this number will go up even more:.
Hopefully, this example gives you an idea of how something as simple as putting an index in a field that is not sequential is essential to an SQL index. Most people rebuild them on a regular basis so that they never get to fragmented. When you need to rebuild them is based on how quickly they get fragmented. Some indexes will need to be rebuilt often, others basically never.
Check out the script the SQLFool put together that handles a lot of figuring this stuff out for you. There is no such case, but in general, doing Index Maintenance once in a week, over the weekend is the best practice to keep the environment stable. I would recommend using maintenance scripts from Ola Hallengren best maintenance scripts , customize the scripts based on your environment and schedule them to run over the weekend.
Note: Please don't forget to update stats after rebuilding indexes, because rebuilding indexes doesn't update all the statistics. As with most things in IT, it depends. What problem are you trying to fix by doing rebuilding indexes? Can you show that it actually fixes the problem? If so, then tweak the numbers until you find the least amount of maintenance you need to do to fix the problem.
If it doesn't fix the problem, or the reason you are doing it is just to appease some metric that you monitor because it might make things better, then all you are doing is burning CPU and IO and possibly making your problem worse.
There's an argument that fixing the fragmentation will not make any difference to your server, so is it worth doing regularly at all? Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. When should I rebuild indexes? Ask Question. Asked 10 years, 3 months ago. Active 3 years, 11 months ago. Viewed 87k times. Is there a case for rebuilding indexes on a regular basis? Improve this question.
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