Guidelines for online index operations
Applies to: SQL Server (all supported versions) Azure SQL Database Azure SQL Managed Instance
When you perform online index operations, the following guidelines apply:
- Clustered indexes must be created, rebuilt, or dropped offline when the underlying table contains the following large object (LOB) data types: image, ntext, and text.
- Nonunique nonclustered indexes can be created online when the table contains LOB data types but none of these columns are used in the index definition as either key or nonkey (included) columns.
- Indexes on local temp tables cannot be created, rebuilt, or dropped online. This restriction does not apply to indexes on global temp tables.
- Indexes can be resumed from where it stopped after an unexpected failure, database failover, or a PAUSE command. See Create Index and Alter Index.
Online index operations are not available in every edition of Microsoft SQL Server. For a list of features that are supported by the editions of SQL Server, see Features supported by editions.
The following table shows the index operations that can be performed online, the indexes that are excluded from these online operations, and resumable index restrictions. Additional restrictions are also included.
|Online index operation||Excluded indexes||Other restrictions|
|ALTER INDEX REBUILD||Disabled clustered index or disabled indexed view
Index on a local temp table
|Specifying the keyword ALL may cause the operation to fail when the table contains an excluded index.
Additional restrictions on rebuilding disabled indexes apply. For more information, see Disable Indexes and Constraints.
|CREATE INDEX||XML index
Initial unique clustered index on a view
Index on a local temp table
|CREATE INDEX WITH DROP_EXISTING||Disabled clustered index or disabled indexed view
Index on a local temp table
|DROP INDEX||Disabled index
Index on a local temp table
|Multiple indexes cannot be specified within a single statement.|
|ALTER TABLE ADD CONSTRAINT (PRIMARY KEY or UNIQUE)||Index on a local temp table
|Only one subclause is allowed at a time. For example, you cannot add and drop PRIMARY KEY or UNIQUE constraints in the same ALTER TABLE statement.|
|ALTER TABLE DROP CONSTRAINT (PRIMARY KEY or UNIQUE)||Clustered index|
The underlying table cannot be modified, truncated, or dropped while an online index operation is in process.
The online option setting (ON or OFF) specified when you create or drop a clustered index is applied to any nonclustered indexes that must be rebuilt. For example, if the clustered index is built online by using CREATE INDEX WITH DROP_EXISTING, ONLINE=ON, all associated nonclustered indexes are re-created online also.
When you create or rebuild a UNIQUE index online, the index builder and a concurrent user transaction may try to insert the same key, therefore violating uniqueness. If a row entered by a user is inserted into the new index (target) before the original row from the source table is moved to the new index, the online index operation will fail.
Although not common, the online index operation can cause a deadlock when it interacts with database updates because of user or application activities. In these rare cases, the SQL Server Database Engine will select the user or application activity as a deadlock victim.
You can perform concurrent online index DDL operations on the same table or view only when you are creating multiple new nonclustered indexes, or reorganizing nonclustered indexes. All other online index operations performed at the same time fail. For example, you cannot create a new index online while rebuilding an existing index online on the same table.
An online operation cannot be performed when an index contains a column of the large object type, and in the same transaction there are update operations before this online operation. To work around this issue, place the online operation outside the transaction or place it before any updates in the transaction.
Disk space considerations
Online index operations require more disk space requirements than offline index operations.
- During index creation and index rebuild operations, additional space is required for the index being built (or rebuilt).
- In addition, disk space is required for the temporary mapping index. This temporary index is used in online index operations that create, rebuild, or drop a clustered index.
- Dropping a clustered index online requires as much space as creating (or rebuilding) a clustered index online.
For more information, see Disk Space Requirements for Index DDL Operations.
Although online index operations permit concurrent user update activity, the index operations will take longer if the update activity is very heavy. Typically, online index operations will be slower than equivalent offline index operations regardless of the concurrent update activity level.
Because both the source and target structures are maintained during the online index operation, the resource usage for insert, update, and delete transactions is increased, potentially up to double. This could cause a decrease in performance and greater resource usage, especially CPU time, during the index operation. Online index operations are fully logged.
Although we recommend online operations, you should evaluate your environment and specific requirements. It may be optimal to run index operations offline. In doing this, users have restricted access to the data during the operation, but the operation finishes faster and uses fewer resources.
On multiprocessor computers that are running SQL Server 2016, index statements may use more processors to perform the scan and sort operations associated with the index statement just like other queries do. You can use the MAXDOP index option to control the number of processors dedicated to the online index operation. In this way, you can balance the resources that are used by index operation with those of the concurrent users. For more information, see Configure Parallel Index Operations. For more information about the editions of SQL Server that support Parallel indexed operations, see Features Supported by editions.
Because an S-lock or Sch-M lock is held in the final phase of the index operation, be careful when you run an online index operation inside an explicit user transaction, such as BEGIN TRANSACTION...COMMIT block. Doing this causes the lock to be held until the end of the transaction, therefore impeding user concurrency.
Online index rebuilding may increase fragmentation when it is allowed to run with
MAX DOP > 1 and
ALLOW_PAGE_LOCKS = OFF options. For more information, see How It Works: Online Index Rebuild - Can Cause Increased Fragmentation.
Transaction log considerations
Large-scale index operations, performed offline or online, can generate large data loads that can cause the transaction log to quickly fill. This is because both offline and online index rebuild operations are fully logged. To make sure that the index operation can be rolled back, the transaction log cannot be truncated until the index operation has been completed; however, the log can be backed up during the index operation. Therefore, the transaction log must have sufficient space to store both the index operation transactions and any concurrent user transactions for the duration of the index operation. For more information, see Transaction Log Disk Space for Index Operations.
Resumable index considerations
The resumable index option for create index and index rebuild applies to SQL Server (index rebuild starting with SQL Server 2017, with create index also supported in SQL Server 2019) and SQL Database. See Create Index and Alter Index.
When you perform resumable online index create or rebuild, the following guidelines apply:
- Managing, planning and extending of index maintenance windows. You can pause and restart an index create or rebuild operation multiple times to fit your maintenance windows.
- Recovering from index create or rebuild failures (such as database failovers or running out of disk space).
- When an index operation is paused, both the original index and the newly created one require disk space and need to be updated during DML operations.
- Enables truncation of transaction logs during an index create or rebuild operation.
- SORT_IN_TEMPDB=ON option is not supported
Resumable index create or rebuild does not require you to keep open a long running transaction, allowing log truncation during this operation and a better log space management. With the new design, we managed to keep necessary data in a database together with all references required to restart the resumable operation.
Generally, there is no performance difference between resumable and non-resumable online index rebuild. For create resumable index, there is a constant overhead that causes a small performance difference between resumable and non-resumable index create. This difference is mostly noticeable only for smaller tables.
When you update a resumable index while an index operation is paused:
- For read-mostly workloads, the performance impact is insignificant.
- For update-heavy workloads, you may experience some throughput degradation (our testing shows less than 10% degradation).
Generally, there is no difference in defragmentation quality between resumable and non-resumable online index create or rebuild.
Online default options
These options are in public preview for SQL Server 2019 (15.x).
You can set default options for online or resumable at a database level by setting the ELEVATE_ONLINE or ELEVATE_RESUMABLE database scoped configuration options. With these default options, you can avoid accidentally performing an operation that takes your database table offline. Both options will cause the engine to automatically elevate certain operations to online or resumable execution.
You can set either option as FAIL_UNSUPPORTED, WHEN_SUPPORTED, or OFF using the ALTER DATABASE SCOPED CONFIGURATION command. You can set different values for online and resumable.
Both ELEVATE_ONLINE and ELEVATE_RESUMABLE only apply to DDL statements that support the online and resumable syntax respectively. For example, if you attempt to create an XML index with ELEVATE_ONLINE=FAIL_UNSUPORTED, the operation will run offline since XML indexes don't support the ONLINE= syntax. The options only effect DDL statements that are submitted without specifying an ONLINE or RESUMABLE option. For example, by submitting a statement with ONLINE=OFF or RESUMABLE=OFF, the user can override a FAIL_UNSUPPORTED setting and run a statement offline and/or non-resumably.
ELEVATE_ONLINE and ELEVATE_RESUMABLE does not apply to XML index operations.
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