DROP INDEX (Transact-SQL)

Article
12/03/2008

Important

The syntax defined in <drop_backward_compatible_index> will be removed in a future version of Microsoft 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_index> instead. XML indexes cannot be dropped using backward compatible syntax.

Removes one or more relational or XML indexes from the current database. In SQL Server 2005, 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 does not 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 .

Topic link icon Transact-SQL Syntax Conventions

Syntax

DROP INDEX
{ <drop_relational_or_xml_index> [ ,...n ] 
| <drop_backward_compatible_index> [ ,...n ]
}

<drop_relational_or_xml_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 ] . | schema_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" 
            }
}

Arguments

index_name
Is the name of the index to be dropped.

database_name
Is the name of the database.

schema_name
Is the name of the schema to which the table or view belongs.

table_or_view_name
Is the name of the table or view associated with the index. To display a report of the indexes on an object, use the sys.indexes catalog view.

<drop_clustered_index_option>
Controls clustered index options. These options cannot be used with other index types.

MAXDOP = max_degree_of_parallelism
Overrides the max degree of parallelism configuration option for the duration of the index operation. For more information, see max degree of parallelism Option. Use MAXDOP to limit the number of processors used in a parallel plan execution. The maximum is 64 processors.

max_degree_of_parallelism can be:
- 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 Configuring Parallel Index Operations.

Note

Parallel index operations are available only in SQL Server 2005 Enterprise Edition.

ONLINE = ON | OFF
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 are not held. This allows queries or updates to the underlying table to continue.
- OFF
  Table locks are applied and the table is unavailable for the duration of 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 are available only in SQL Server 2005 Enterprise Edition.

MOVE TO
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 is not valid for indexed views or nonclustered indexes. If a partition scheme or filegroup is not specified, the resulting table will be 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 are not 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 may no longer be aligned with the heap. For more information about partitioned index alignment, see Special Guidelines for Partitioned Indexes.

partition_scheme_name ( column_name )
Specifies a partition scheme as the location for the resulting table. The partition scheme must have already been 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 is not restricted to the columns in the index definition. Any column in the base table can be specified.

filegroup_name
Specifies a filegroup as the location for the resulting table. If no location is specified and the table is not 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 is not a keyword. It is 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 (Transact-SQL).

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.

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

When the clustered index of an indexed view is dropped, all nonclustered indexes and auto-created statistics on the same view are automatically dropped. Manually created statistics are not dropped.

The syntax index_name ON { table_or_view_name } is new in SQL Server 2005. The syntaxtable_or_view_name**.**index_name is maintained for backward compatibility. Combining both options in a single transaction causes the statement to fail. An XML index cannot be dropped by using the backward compatible syntax.

When indexes with 128 extents or more are dropped, the SQL Server 2005 Database Engine defers the actual page deallocations, and their associated locks, until after the transaction commits. For more information, see Dropping and Rebuilding Large Objects.

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 INDEXis more efficient, especially for clustered indexes. ALTER INDEX REBUILD has optimizations to prevent the overhead of rebuilding the nonclustered indexes.

Using Options with DROP INDEX

In SQL Server 2005, 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 are not 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 (Transact-SQL).

You cannot 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 additional temporary disk space. For more information, see Determining Index Disk Space Requirements.

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, DROP INDEX may 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 Configuring Parallel Index Operations.

XML Indexes

Options cannot be specified when you drop an XML index. When a primary XML index is dropped, all associated secondary XML indexes are automatically dropped. For more information, see Indexes on xml Data Type Columns.

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

A. Dropping an index

The following example deletes the index IX_ProductVendor_VendorID on the ProductVendor table.

USE AdventureWorks;
GO
DROP INDEX IX_ProductVendor_VendorID 
    ON Purchasing.ProductVendor;
GO

B. Dropping multiple indexes

The following example deletes two indexes in a single transaction.

USE AdventureWorks;
GO
DROP INDEX
    IX_PurchaseOrderHeader_EmployeeID ON Purchasing.PurchaseOrderHeader,
    IX_VendorAddress_AddressID ON Purchasing.VendorAddress;
GO

C. Dropping a clustered index online and setting 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 was not specified, the resulting table is stored in the same filegroup as the index.

Note

This example can be executed only in SQL Server 2005 Enterprise Edition.

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

D. Dropping a clustered index online and moving 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.

USE AdventureWorks;
GO
--Create a clustered index on the PRIMARY filegroup if it does not exist.
IF NOT EXISTS (SELECT name FROM sys.indexes WHERE name = 
            N'AK_BillOfMaterials_ProductAssemblyID_ComponentID_StartDate')
    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.
-- Get the SQL Server data path
DECLARE @data_path nvarchar(256);
SET @data_path = (SELECT SUBSTRING(physical_name, 1, CHARINDEX(N'master.mdf', LOWER(physical_name)) - 1)
                  FROM master.sys.master_files
                  WHERE database_id = 1 AND file_id = 1);

-- execute the ALTER DATABASE statement 
IF NOT EXISTS (SELECT name FROM sys.filegroups
                WHERE name = N'NewGroup')
    BEGIN
    ALTER DATABASE AdventureWorks
        ADD FILEGROUP NewGroup;
    EXECUTE ('ALTER DATABASE AdventureWorks
        ADD FILE (NAME = File1,
            FILENAME = '''+ @data_path + '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. Dropping a PRIMARY KEY constraint online

Indexes that are created as the result of creating PRIMARY KEY or UNIQUE constraints cannot be dropped by using DROP INDEX. They are 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.

USE AdventureWorks;
GO
-- Set ONLINE = OFF to execute this example on editions other than Enterprise Edition.
ALTER TABLE Production.ProductCostHistory
    DROP CONSTRAINT PK_ProductCostHistory_ProductID_StartDate
        WITH (ONLINE = ON);
GO

F. Dropping an XML index

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

USE AdventureWorks;
GO
DROP INDEX PXML_ProductModel_CatalogDescription 
    ON Production.ProductModel;
GO

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