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DROP INDEX (Transact-SQL)

  • Article

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Removes one or more relational, spatial, filtered, or XML indexes from the current database. You can drop a clustered index and move the resulting table to another filegroup or partition scheme in a single transaction by specifying the MOVE TO option.

The DROP INDEX statement doesn't apply to indexes created by defining PRIMARY KEY or UNIQUE constraints. To remove the constraint and corresponding index, use ALTER TABLE with the DROP CONSTRAINT clause.

Important

The syntax defined in <drop_backward_compatible_index> will be removed in a future version of SQL Server. Avoid using this syntax in new development work, and plan to modify applications that currently use the feature. Use the syntax specified under <drop_relational_or_xml_or_spatial_index> instead. XML indexes can't be dropped using backward compatible syntax.

Transact-SQL syntax conventions

Syntax

Syntax for SQL Server (all options except filegroup and filestream apply to Azure SQL Database).

DROP INDEX [ IF EXISTS ]
{ <drop_relational_or_xml_or_spatial_index> [ , ...n ]
| <drop_backward_compatible_index> [ , ...n ]
}

<drop_relational_or_xml_or_spatial_index> ::=
    index_name ON <object>
    [ WITH ( <drop_clustered_index_option> [ , ...n ] ) ]

<drop_backward_compatible_index> ::=
    [ owner_name. ] table_or_view_name.index_name

<object> ::=
{ database_name.schema_name.table_or_view_name | schema_name.table_or_view_name | table_or_view_name }

<drop_clustered_index_option> ::=
{
    MAXDOP = max_degree_of_parallelism
  | ONLINE = { ON | OFF }
  | MOVE TO { partition_scheme_name ( column_name )
            | filegroup_name
            | "default"
            }
  [ FILESTREAM_ON { partition_scheme_name
            | filestream_filegroup_name
            | "default" } ]
}

Syntax for Azure SQL Database.

DROP INDEX
{ <drop_relational_or_xml_or_spatial_index> [ , ...n ]
}

<drop_relational_or_xml_or_spatial_index> ::=
    index_name ON <object>

<object> ::=
{ database_name.schema_name.table_or_view_name | schema_name.table_or_view_name | table_or_view_name }

Syntax for Azure Synapse Analytics and Analytics Platform System (PDW).

DROP INDEX index_name ON { database_name.schema_name.table_name | schema_name.table_name | table_name }
[ ; ]

Note

To view Transact-SQL syntax for SQL Server 2014 (12.x) and earlier versions, see Previous versions documentation.

Arguments

IF EXISTS

Applies to: SQL Server 2016 (13.x) and later versions.

Conditionally drops the index only if it already exists.

index_name

The name of the index to be dropped.

database_name

The name of the database.

schema_name

The name of the schema to which the table or view belongs.

table_or_view_name

The name of the table or view associated with the index. Spatial indexes are supported only on tables.

To display a report of the indexes on an object, use the sys.indexes catalog view.

Azure SQL Database supports the three-part name format: database_name.[schema_name].object_name when the database_name is the current database, or the database_name is tempdb and the object_name starts with #.

<drop_clustered_index_option>

Applies to: SQL Server 2008 (10.0.x) and later versions, SQL Database.

Controls clustered index options. These options can't be used with other index types.

MAXDOP = max_degree_of_parallelism

Applies to: SQL Server 2008 (10.0.x) and later versions, SQL Database (Performance Levels P2 and P3 only).

Overrides the max degree of parallelism configuration option during the index operation. For more information, see Configure the max degree of parallelism (server configuration option). Use MAXDOP to limit the number of processors used in a parallel plan execution. The maximum is 64 processors.

Important

MAXDOP isn't allowed for spatial indexes or XML indexes.

max_degree_of_parallelism can be one of the following values.

Value Description
1 Suppresses parallel plan generation
>1 Restricts the maximum number of processors used in a parallel index operation to the specified number
0 (default) Uses the actual number of processors or fewer based on the current system workload

For more information, see Configure Parallel Index Operations.

Note

Parallel index operations aren't available in every edition of SQL Server. For a list of features that are supported by the editions of SQL Server, see Editions and supported features of SQL Server 2022.

ONLINE = ON | OFF

Applies to: SQL Server 2008 (10.0.x) and later versions, Azure SQL Database.

Specifies whether underlying tables and associated indexes are available for queries and data modification during the index operation. The default is OFF.

  • ON: Long-term table locks aren't held. This allows queries or updates to the underlying table to continue.

  • OFF: Table locks are applied and the table is unavailable during the index operation.

The ONLINE option can only be specified when you drop clustered indexes. For more information, see the Remarks section.

Note

Online index operations aren't available in every edition of SQL Server. For a list of features that are supported by the editions of SQL Server, see Editions and supported features of SQL Server 2022.

MOVE TO { partition_scheme_name ( column_name ) | filegroup_name | "default" }

Applies to: SQL Server 2008 (10.0.x) and later versions. SQL Database supports "default" as the filegroup name.

Specifies a location to move the data rows that currently are in the leaf level of the clustered index. The data is moved to the new location in the form of a heap. You can specify either a partition scheme or filegroup as the new location, but the partition scheme or filegroup must already exist. MOVE TO isn't valid for indexed views or nonclustered indexes. If a partition scheme or filegroup isn't specified, the resulting table is located in the same partition scheme or filegroup as was defined for the clustered index.

If a clustered index is dropped by using MOVE TO, any nonclustered indexes on the base table are rebuilt, but they remain in their original filegroups or partition schemes. If the base table is moved to a different filegroup or partition scheme, the nonclustered indexes aren't moved to coincide with the new location of the base table (heap). Therefore, even if the nonclustered indexes were previously aligned with the clustered index, they might no longer be aligned with the heap. For more information about partitioned index alignment, see Partitioned tables and indexes.

partition_scheme_name ( column_name )

Applies to: SQL Server 2008 (10.0.x) and later versions, SQL Database.

Specifies a partition scheme as the location for the resulting table. The partition scheme must already be created, by executing either CREATE PARTITION SCHEME or ALTER PARTITION SCHEME. If no location is specified and the table is partitioned, the table is included in the same partition scheme as the existing clustered index.

The column name in the scheme isn't restricted to the columns in the index definition. Any column in the base table can be specified.

filegroup_name

Applies to: SQL Server 2008 (10.0.x) and later versions.

Specifies a filegroup as the location for the resulting table. If no location is specified and the table isn't partitioned, the resulting table is included in the same filegroup as the clustered index. The filegroup must already exist.

"default"

Specifies the default location for the resulting table.

Note

In this context, default isn't a keyword. It's an identifier for the default filegroup and must be delimited, as in MOVE TO "default" or MOVE TO [default]. If "default" is specified, the QUOTED_IDENTIFIER option must be set ON for the current session. This is the default setting. For more information, see SET QUOTED_IDENTIFIER.

FILESTREAM_ON { partition_scheme_name | filestream_filegroup_name | "default" }

Applies to: SQL Server 2008 (10.0.x) and later versions.

Specifies a location to move the FILESTREAM table that currently is in the leaf level of the clustered index. The data is moved to the new location in the form of a heap. You can specify either a partition scheme or filegroup as the new location, but the partition scheme or filegroup must already exist. FILESTREAM ON isn't valid for indexed views or nonclustered indexes. If a partition scheme isn't specified, the data is located in the same partition scheme as was defined for the clustered index.

partition_scheme_name

Specifies a partition scheme for the FILESTREAM data. The partition scheme must already be created, by executing either CREATE PARTITION SCHEME or ALTER PARTITION SCHEME. If no location is specified and the table is partitioned, the table is included in the same partition scheme as the existing clustered index.

If you specify a partition scheme for MOVE TO, you must use the same partition scheme for FILESTREAM ON.

filestream_filegroup_name

Specifies a FILESTREAM filegroup for FILESTREAM data. If no location is specified and the table isn't partitioned, the data is included in the default FILESTREAM filegroup.

"default"

Specifies the default location for the FILESTREAM data.

Note

In this context, default isn't a keyword. It's an identifier for the default filegroup and must be delimited, as in MOVE TO "default" or MOVE TO [default]. If "default" is specified, the QUOTED_IDENTIFIER option must be ON for the current session. This is the default setting. For more information, see SET QUOTED_IDENTIFIER.

Remarks

When a nonclustered index is dropped, the index definition is removed from metadata, and the index data pages (the B-tree) are removed from the database files. When a clustered index is dropped, the index definition is removed from metadata and the data rows that were stored in the leaf level of the clustered index are stored in the resulting unordered table, a heap. All the space previously occupied by the index is regained. This space can then be used for any database object.

Note

SQL Server documentation uses the term B-tree generally in reference to indexes. In rowstore indexes, SQL Server implements a B+ tree. This does not apply to columnstore indexes or in-memory data stores. For more information, see the SQL Server and Azure SQL index architecture and design guide.

An index can't be dropped if the filegroup in which it's located is offline or set to read-only.

When the clustered index of an indexed view is dropped, all nonclustered indexes and autocreated statistics on the same view are automatically dropped. Manually created statistics aren't dropped.

The syntax <table_or_view_name>.<index_name> is maintained for backward compatibility. An XML index or spatial index can't be dropped by using the backward compatible syntax.

When indexes with 128 extents or more are dropped, the Database Engine defers the actual page deallocations, and their associated locks, until after the transaction commits.

Sometimes indexes are dropped and re-created to reorganize or rebuild the index, such as to apply a new fill factor value or to reorganize data after a bulk load. To do this, using ALTER INDEX is more efficient, especially for clustered indexes. ALTER INDEX REBUILD has optimizations to prevent the overhead of rebuilding the nonclustered indexes.

Use options with DROP INDEX

You can set the following index options when you drop a clustered index: MAXDOP, ONLINE, and MOVE TO.

Use MOVE TO to drop the clustered index and move the resulting table to another filegroup or partition scheme in a single transaction.

When you specify ONLINE = ON, queries and modifications to the underlying data and associated nonclustered indexes aren't blocked by the DROP INDEX transaction. Only one clustered index can be dropped online at a time. For a complete description of the ONLINE option, see CREATE INDEX.

You can't drop a clustered index online if the index is disabled on a view, or contains text, ntext, image, varchar(max), nvarchar(max), varbinary(max), or xml columns in the leaf-level data rows.

Using the ONLINE = ON and MOVE TO options requires more temporary disk space.

After an index is dropped, the resulting heap appears in the sys.indexes catalog view with NULL in the name column. To view the table name, join sys.indexes with sys.tables on object_id. For an example query, see example D.

On multiprocessor computers that are running SQL Server 2005 Enterprise edition or later, DROP INDEX might use more processors to perform the scan and sort operations associated with dropping the clustered index, just like other queries do. You can manually configure the number of processors that are used to run the DROP INDEX statement by specifying the MAXDOP index option. For more information, see Configure Parallel Index Operations.

When a clustered index is dropped, the corresponding heap partitions retain their data compression setting unless the partitioning scheme is modified. If the partitioning scheme is changed, all partitions are rebuilt to an uncompressed state (DATA_COMPRESSION = NONE). To drop a clustered index and change the partitioning scheme requires the following two steps:

  1. Drop the clustered index.

  2. Modify the table by using an ALTER TABLE ... REBUILD ... option specifying the compression option.

When a clustered index is dropped OFFLINE, only the upper levels of clustered indexes are removed; therefore, the operation is fast. When a clustered index is dropped ONLINE, SQL Server rebuilds the heap two times, once for step 1 and once for step 2. For more information about data compression, see Data compression.

XML indexes

Options can't be specified when you drop anXML index. Also, you can't use the <table_or_view_name>.<index_name> syntax. When a primary XML index is dropped, all associated secondary XML indexes are automatically dropped. For more information, see XML indexes (SQL Server).

Spatial indexes

Spatial indexes are supported only on tables. When you drop a spatial index, you can't specify any options or use .<index_name>. The correct syntax is as follows:

DROP INDEX <spatial_index_name> ON <spatial_table_name>;

For more information about spatial indexes, see Spatial Indexes Overview.

Permissions

To execute DROP INDEX, at a minimum, ALTER permission on the table or view is required. This permission is granted by default to the sysadmin fixed server role and the db_ddladmin and db_owner fixed database roles.

Examples

The Transact-SQL code samples in this article use the AdventureWorks2022 sample database, which you can download from the Microsoft SQL Server Samples and Community Projects home page.

A. Drop an index

The following example deletes the index IX_ProductVendor_BusinessEntityID on the ProductVendor table in the AdventureWorks2022 database.

DROP INDEX IX_ProductVendor_BusinessEntityID
    ON Purchasing.ProductVendor;
GO

B. Drop multiple indexes

The following example deletes two indexes in a single transaction in the AdventureWorks2022 database.

DROP INDEX
    IX_PurchaseOrderHeader_EmployeeID ON Purchasing.PurchaseOrderHeader,
    IX_Address_StateProvinceID ON Person.Address;
GO

C. Drop a clustered index online and set the MAXDOP option

The following example deletes a clustered index with the ONLINE option set to ON and MAXDOP set to 8. Because the MOVE TO option wasn't specified, the resulting table is stored in the same filegroup as the index.

Applies to: SQL Server 2008 (10.0.x) and later versions, SQL Database.

DROP INDEX AK_BillOfMaterials_ProductAssemblyID_ComponentID_StartDate
    ON Production.BillOfMaterials WITH (ONLINE = ON, MAXDOP = 2);
GO

D. Drop a clustered index online and move the table to a new filegroup

The following example deletes a clustered index online and moves the resulting table (heap) to the filegroup NewGroup by using the MOVE TO clause. The sys.indexes, sys.tables, and sys.filegroups catalog views are queried to verify the index and table placement in the filegroups before and after the move. Beginning with SQL Server 2016 (13.x), you can use the DROP INDEX IF EXISTS syntax.

Applies to: SQL Server 2008 (10.0.x) and later versions.

--Create a clustered index on the PRIMARY filegroup if the index does not exist.
CREATE UNIQUE CLUSTERED INDEX
    AK_BillOfMaterials_ProductAssemblyID_ComponentID_StartDate
        ON Production.BillOfMaterials (ProductAssemblyID, ComponentID,
        StartDate)
    ON 'PRIMARY';
GO
-- Verify filegroup location of the clustered index.
SELECT t.name AS [Table Name], i.name AS [Index Name], i.type_desc,
    i.data_space_id, f.name AS [Filegroup Name]
FROM sys.indexes AS i
    JOIN sys.filegroups AS f ON i.data_space_id = f.data_space_id
    JOIN sys.tables as t ON i.object_id = t.object_id
        AND i.object_id = OBJECT_ID(N'Production.BillOfMaterials','U')
GO
--Create filegroup NewGroup if it does not exist.
IF NOT EXISTS (SELECT name FROM sys.filegroups
                WHERE name = N'NewGroup')
    BEGIN
    ALTER DATABASE AdventureWorks2022
        ADD FILEGROUP NewGroup;
    ALTER DATABASE AdventureWorks2022
        ADD FILE (NAME = File1,
            FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\DATA\File1.ndf')
        TO FILEGROUP NewGroup;
    END
GO
--Verify new filegroup
SELECT * from sys.filegroups;
GO
-- Drop the clustered index and move the BillOfMaterials table to
-- the Newgroup filegroup.
-- Set ONLINE = OFF to execute this example on editions other than Enterprise Edition.
DROP INDEX AK_BillOfMaterials_ProductAssemblyID_ComponentID_StartDate
    ON Production.BillOfMaterials
    WITH (ONLINE = ON, MOVE TO NewGroup);
GO
-- Verify filegroup location of the moved table.
SELECT t.name AS [Table Name], i.name AS [Index Name], i.type_desc,
    i.data_space_id, f.name AS [Filegroup Name]
FROM sys.indexes AS i
    JOIN sys.filegroups AS f ON i.data_space_id = f.data_space_id
    JOIN sys.tables as t ON i.object_id = t.object_id
        AND i.object_id = OBJECT_ID(N'Production.BillOfMaterials','U');
GO

E. Drop a PRIMARY KEY constraint online

Indexes that are created as the result of creating PRIMARY KEY or UNIQUE constraints can't be dropped by using DROP INDEX. They're dropped using the ALTER TABLE DROP CONSTRAINT statement. For more information, see ALTER TABLE.

The following example deletes a clustered index with a PRIMARY KEY constraint by dropping the constraint. The ProductCostHistory table has no FOREIGN KEY constraints. If it did, those constraints would have to be removed first.

-- Set ONLINE = OFF to execute this example on editions other than Enterprise Edition.
ALTER TABLE Production.TransactionHistoryArchive
DROP CONSTRAINT PK_TransactionHistoryArchive_TransactionID
WITH (ONLINE = ON);

F. Drop an XML index

The following example drops an XML index on the ProductModel table in the AdventureWorks2022 database.

DROP INDEX PXML_ProductModel_CatalogDescription
    ON Production.ProductModel;

G. Drop a clustered index on a FILESTREAM table

The following example deletes a clustered index online and moves the resulting table (heap) and FILESTREAM data to the MyPartitionScheme partition scheme by using both the MOVE TO clause and the FILESTREAM ON clause.

Applies to: SQL Server 2008 (10.0.x) and later versions.

DROP INDEX PK_MyClusteredIndex
    ON dbo.MyTable
    WITH (MOVE TO MyPartitionScheme,
          FILESTREAM_ON MyPartitionScheme);
GO

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