Larger indexes have more intermediate levels and pages. These pages can get empty space on them and become out of order over time as well. But hey, in some systems microseconds do matter! In that case, breaking up these giant indexes into partitions is probably desirable more on that soon. The bigger the transaction, the worse the rollback can be. You definitely want to handle the indexes individually.
This is a little counter-intuitive when you have a short maintenance window, but it can ease some pains. Many popular index maintenance scripts have an option for this. But table partitioning can be excellent for data management. By breaking up the table into smaller chunks, partitioning makes it much easier to take advantage of all the strengths of rebuilds without so many downsides.
For both rowstore and columnstore indexes, it is especially important to review index or heap fragmentation and page density after a large number of rows has been deleted or updated.
For heaps, if there are frequent updates, it may also be needed to review fragmentation periodically to avoid proliferation of forwarding records. For more information about heaps, see Heaps Tables without Clustered Indexes. See Examples for sample queries to determine fragmentation and page density.
You can reduce index fragmentation and increase page density by using one of the following methods:. For partitioned indexes, you can use either of the following methods on all partitions or a single partition of an index.
Reorganizing an index is less resource intensive than rebuilding an index. For that reason it should be your preferred index maintenance method, unless there is a specific reason to use index rebuild. Reorganize is always an online operation. Starting with SQL Server This improves the columnstore index quality over time. If you cancel a reorganize operation, or if it is otherwise interrupted, the progress it made to that point is persisted in the database.
To reorganize large indexes, the operation can be started and stopped multiple times until it completes. Rebuilding an index drops and re-creates the index. Depending on the type of index and the Database Engine version, a rebuild operation can be done offline or online.
An offline index rebuild usually takes less time than an online rebuild, but it holds object-level locks for the duration of the rebuild operation, blocking queries from accessing the table or view.
An online index rebuild does not require object-level locks until the end of the operation, when a lock must be held for a short duration to complete the rebuild. Depending on the version of the Database Engine, an online index rebuild can be started as a resumable operation.
A resumable index rebuild can be paused, keeping the progress made to that point. A resumable rebuild operation can be resumed after having been paused or interrupted, or aborted if completing the rebuild becomes unnecessary. For more information about online index rebuilds, see Perform Index Operations Online.
While an index is being rebuilt online, every modification of data in indexed columns must update an additional copy of the index. This may result in a minor performance degradation of data modification statements during online rebuild. If an online resumable index operation is paused, this performance impact persists until the resumable operation either completes or is aborted.
If you do not intend to complete a resumable index operation, abort it instead of pausing it. For rowstore indexes , rebuilding removes fragmentation in all levels of the index, and compacts pages based on the specified or current fill factor.
When ALL is specified, all indexes on the table are dropped and rebuilt in a single operation. When indexes with or more extents are rebuilt, the Database Engine defers page deallocations and acquiring the associated locks until after the rebuild completes. For syntax examples, see Examples - Rowstore rebuild. For columnstore indexes , rebuilding removes fragmentation, moves any delta store rows into columnstore, and physically deletes rows that have been marked for deletion.
For syntax examples, see Examples - Columnstore rebuild. In earlier versions of SQL Server, you could sometimes rebuild a rowstore nonclustered index to correct inconsistencies due to data corruption in the index. Starting with SQL Server , you may still be able to repair such inconsistencies in the nonclustered index by rebuilding a nonclustered index offline.
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.
Every so often, we need to rebuild indexes in Oracle, because indexes become fragmented over time. This causes their performance - and by extension - that of your database queries, to degrade. Hence, rebuilding indexes every now and again can be quite beneficial.
Having said that, indexes should not be rebuilt to often, because it's a resource intensive task. Worse, as an index is being rebuilt, locks will be placed on the index, preventing anyone from accessing it while the rebuilding occurs.
Any queries trying to access this index in order to return the required results will be temporarily blocked, until the rebuild is complete. In today's blog, we'll learn how often to build indexes and how to determine when an index needs to be rebuilt. As mentioned in the introduction, rebuilding indexes is both a resource intensive and blocking task.
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