CREATE INDEX (Transact-SQL)

  • Article

Creates a relational index on a specified table or view on a specified table. An index can be created before there is data in the table. Relational indexes can be created on tables or views in another database by specifying a qualified database name.

Note

   For information about how to create an XML index, see CREATE XML INDEX (Transact-SQL). For information about how to create a spatial index, see CREATE SPATIAL INDEX (Transact-SQL). For information about how to create an xVelocity memory optimized columnstore index, see CREATE COLUMNSTORE INDEX (Transact-SQL).

Topic link icon Transact-SQL Syntax Conventions

Syntax

CREATE [ UNIQUE ] [ CLUSTERED | NONCLUSTERED ] INDEX index_name 
    ON <object> ( column [ ASC | DESC ] [ ,...n ] ) 
    [ INCLUDE ( column_name [ ,...n ] ) ]
    [ WHERE <filter_predicate> ]
    [ WITH ( <relational_index_option> [ ,...n ] ) ]
    [ ON { partition_scheme_name ( column_name ) 
         | filegroup_name 
         | default 
         }
    ]
    [ FILESTREAM_ON { filestream_filegroup_name | partition_scheme_name | "NULL" } ]

[ ; ]

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

<relational_index_option> ::=
{
    PAD_INDEX = { ON | OFF }
  | FILLFACTOR = fillfactor
  | SORT_IN_TEMPDB = { ON | OFF }
  | IGNORE_DUP_KEY = { ON | OFF }
  | STATISTICS_NORECOMPUTE = { ON | OFF }
  | DROP_EXISTING = { ON | OFF }
  | ONLINE = { ON | OFF }
  | ALLOW_ROW_LOCKS = { ON | OFF }
  | ALLOW_PAGE_LOCKS = { ON | OFF }
  | MAXDOP = max_degree_of_parallelism
  | DATA_COMPRESSION = { NONE | ROW | PAGE} 
     [ ON PARTITIONS ( { <partition_number_expression> | <range> } 
     [ , ...n ] ) ]
}

<filter_predicate> ::= 
    <conjunct> [ AND <conjunct> ]

<conjunct> ::=
    <disjunct> | <comparison>

<disjunct> ::= 
        column_name IN (constant ,...n)

<comparison> ::= 
        column_name <comparison_op> constant 

<comparison_op> ::=
    { IS | IS NOT | = | <> | != | > | >= | !> | < | <= | !< }

<range> ::= 
<partition_number_expression> TO <partition_number_expression>


Backward Compatible Relational Index 
Important   The backward compatible relational index syntax structure will be removed in a future version of SQL Server. Avoid using this syntax structure in new development work, and plan to modify applications that currently use the feature. Use the syntax structure specified in <relational_index_option> instead.

CREATE [ UNIQUE ] [ CLUSTERED | NONCLUSTERED ] INDEX index_name 
    ON <object> ( column_name [ ASC | DESC ] [ ,...n ] ) 
    [ WITH <backward_compatible_index_option> [ ,...n ] ]
    [ ON { filegroup_name | "default" } ]

<object> ::=
{
    [ database_name. [ owner_name ] . | owner_name. ] 
    table_or_view_name
}

<backward_compatible_index_option> ::=
{ 
    PAD_INDEX
  | FILLFACTOR = fillfactor
  | SORT_IN_TEMPDB
  | IGNORE_DUP_KEY
  | STATISTICS_NORECOMPUTE 
  | DROP_EXISTING 
}

Arguments

  • UNIQUE
    Creates a unique index on a table or view. A unique index is one in which no two rows are permitted to have the same index key value. A clustered index on a view must be unique.

    The Database Engine does not allow creating a unique index on columns that already include duplicate values, whether or not IGNORE_DUP_KEY is set to ON. If this is tried, the Database Engine displays an error message. Duplicate values must be removed before a unique index can be created on the column or columns. Columns that are used in a unique index should be set to NOT NULL, because multiple null values are considered duplicates when a unique index is created.

  • CLUSTERED
    Creates an index in which the logical order of the key values determines the physical order of the corresponding rows in a table. The bottom, or leaf, level of the clustered index contains the actual data rows of the table. A table or view is allowed one clustered index at a time.

    A view with a unique clustered index is called an indexed view. Creating a unique clustered index on a view physically materializes the view. A unique clustered index must be created on a view before any other indexes can be defined on the same view. For more information, see Create Indexed Views.

    Create the clustered index before creating any nonclustered indexes. Existing nonclustered indexes on tables are rebuilt when a clustered index is created.

    If CLUSTERED is not specified, a nonclustered index is created.

    Note

    Because the leaf level of a clustered index and the data pages are the same by definition, creating a clustered index and using the ON partition_scheme_name or ON filegroup_name clause effectively moves a table from the filegroup on which the table was created to the new partition scheme or filegroup. Before creating tables or indexes on specific filegroups, verify which filegroups are available and that they have enough empty space for the index.

    In some cases creating a clustered index can enable previously disabled indexes. For more information, see Enable Indexes and Constraints and Disable Indexes and Constraints.

  • NONCLUSTERED
    Creates an index that specifies the logical ordering of a table. With a nonclustered index, the physical order of the data rows is independent of their indexed order.

    Each table can have up to 999 nonclustered indexes, regardless of how the indexes are created: either implicitly with PRIMARY KEY and UNIQUE constraints, or explicitly with CREATE INDEX.

    For indexed views, nonclustered indexes can be created only on a view that has a unique clustered index already defined.

    The default is NONCLUSTERED.

  • index_name
    Is the name of the index. Index names must be unique within a table or view but do not have to be unique within a database. Index names must follow the rules of identifiers.

  • column
    Is the column or columns on which the index is based. Specify two or more column names to create a composite index on the combined values in the specified columns. List the columns to be included in the composite index, in sort-priority order, inside the parentheses after table_or_view_name.

    Up to 16 columns can be combined into a single composite index key. All the columns in a composite index key must be in the same table or view. The maximum allowable size of the combined index values is 900 bytes.

    Columns that are of the large object (LOB) data types ntext, text, varchar(max), nvarchar(max), varbinary(max), xml, or image cannot be specified as key columns for an index. Also, a view definition cannot include ntext, text, or image columns, even if they are not referenced in the CREATE INDEX statement.

    You can create indexes on CLR user-defined type columns if the type supports binary ordering. You can also create indexes on computed columns that are defined as method invocations off a user-defined type column, as long as the methods are marked deterministic and do not perform data access operations. For more information about indexing CLR user-defined type columns, see CLR User-defined Types.

  • [ ASC | DESC ]
    Determines the ascending or descending sort direction for the particular index column. The default is ASC.

  • INCLUDE **(**column [ ,... n ] )
    Specifies the non-key columns to be added to the leaf level of the nonclustered index. The nonclustered index can be unique or non-unique.

    Column names cannot be repeated in the INCLUDE list and cannot be used simultaneously as both key and non-key columns. Nonclustered indexes always contain the clustered index columns if a clustered index is defined on the table. For more information, see Create Indexes with Included Columns.

    All data types are allowed except text, ntext, and image. The index must be created or rebuilt offline (ONLINE = OFF) if any one of the specified non-key columns are varchar(max), nvarchar(max), or varbinary(max) data types.

    Computed columns that are deterministic and either precise or imprecise can be included columns. Computed columns derived from image, ntext, text, varchar(max), nvarchar(max), varbinary(max), and xml data types can be included in non-key columns as long as the computed column data types is allowable as an included column. For more information, see Indexes on Computed Columns.

    For information on creating an XML index, see CREATE XML INDEX (Transact-SQL).

  • WHERE <filter_predicate>
    Creates a filtered index by specifying which rows to include in the index. The filtered index must be a nonclustered index on a table. Creates filtered statistics for the data rows in the filtered index.

    The filter predicate uses simple comparison logic and cannot reference a computed column, a UDT column, a spatial data type column, or a hierarchyID data type column. Comparisons using NULL literals are not allowed with the comparison operators. Use the IS NULL and IS NOT NULL operators instead.

    Here are some examples of filter predicates for the Production.BillOfMaterials table:

    WHERE StartDate > '20000101' AND EndDate <= '20000630'

    WHERE ComponentID IN (533, 324, 753)

    WHERE StartDate IN ('20000404', '20000905') AND EndDate IS NOT NULL

    Filtered indexes do not apply to XML indexes and full-text indexes. For UNIQUE indexes, only the selected rows must have unique index values. Filtered indexes do not allow the IGNORE_DUP_KEY option.

  • ON partition_scheme_name**(column_name)**
    Specifies the partition scheme that defines the filegroups onto which the partitions of a partitioned index will be mapped. The partition scheme must exist within the database by executing either CREATE PARTITION SCHEME or ALTER PARTITION SCHEME. column_name specifies the column against which a partitioned index will be partitioned. This column must match the data type, length, and precision of the argument of the partition function that partition_scheme_name is using. column_name is not restricted to the columns in the index definition. Any column in the base table can be specified, except when partitioning a UNIQUE index, column_name must be chosen from among those used as the unique key. This restriction allows the Database Engine to verify uniqueness of key values within a single partition only.

    Note

    When you partition a non-unique, clustered index, the Database Engine by default adds the partitioning column to the list of clustered index keys, if it is not already specified. When partitioning a non-unique, nonclustered index, the Database Engine adds the partitioning column as a non-key (included) column of the index, if it is not already specified.

    If partition_scheme_name or filegroup is not specified and the table is partitioned, the index is placed in the same partition scheme, using the same partitioning column, as the underlying table.

    Note

    You cannot specify a partitioning scheme on an XML index. If the base table is partitioned, the XML index uses the same partition scheme as the table.

    For more information about partitioning indexes, Partitioned Tables and Indexes.

  • ON filegroup_name
    Creates the specified index on the specified filegroup. If no location is specified and the table or view is not partitioned, the index uses the same filegroup as the underlying table or view. The filegroup must already exist.

  • ON "default"
    Creates the specified index on the default filegroup.

    The term default, in this context, is not a keyword. It is an identifier for the default filegroup and must be delimited, as in ON "default" or ON [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).

  • [ FILESTREAM_ON { filestream_filegroup_name | partition_scheme_name | "NULL" } ]
    Specifies the placement of FILESTREAM data for the table when a clustered index is created. The FILESTREAM_ON clause allows FILESTREAM data to be moved to a different FILESTREAM filegroup or partition scheme.

    filestream_filegroup_name is the name of a FILESTREAM filegroup. The filegroup must have one file defined for the filegroup by using a CREATE DATABASE or ALTER DATABASE statement; otherwise, an error is raised.

    If the table is partitioned, the FILESTREAM_ON clause must be included and must specify a partition scheme of FILESTREAM filegroups that uses the same partition function and partition columns as the partition scheme for the table. Otherwise, an error is raised.

    If the table is not partitioned, the FILESTREAM column cannot be partitioned. FILESTREAM data for the table must be stored in a single filegroup that is specified in the FILESTREAM_ON clause.

    FILESTREAM_ON NULL can be specified in a CREATE INDEX statement if a clustered index is being created and the table does not contain a FILESTREAM column.

    For more information, see FILESTREAM (SQL Server).

<object>::=

Is the fully qualified or nonfully qualified object to be indexed.

  • 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 to be indexed.

    The view must be defined with SCHEMABINDING to create an index on it. A unique clustered index must be created on a view before any nonclustered index is created. For more information about indexed views, see the Remarks section.

<relational_index_option>::=

Specifies the options to use when you create the index.

  • PAD_INDEX = { ON | OFF }
    Specifies index padding. The default is OFF.

    • ON
      The percentage of free space that is specified by fillfactor is applied to the intermediate-level pages of the index.

    • OFF or fillfactor is not specified
      The intermediate-level pages are filled to near capacity, leaving sufficient space for at least one row of the maximum size the index can have, considering the set of keys on the intermediate pages.

    The PAD_INDEX option is useful only when FILLFACTOR is specified, because PAD_INDEX uses the percentage specified by FILLFACTOR. If the percentage specified for FILLFACTOR is not large enough to allow for one row, the Database Engine internally overrides the percentage to allow for the minimum. The number of rows on an intermediate index page is never less than two, regardless of how low the value of fillfactor.

    In backward compatible syntax, WITH PAD_INDEX is equivalent to WITH PAD_INDEX = ON.

  • FILLFACTOR **=**fillfactor
    Specifies a percentage that indicates how full the Database Engine should make the leaf level of each index page during index creation or rebuild. fillfactor must be an integer value from 1 to 100. If fillfactor is 100, the Database Engine creates indexes with leaf pages filled to capacity.

    The FILLFACTOR setting applies only when the index is created or rebuilt. The Database Engine does not dynamically keep the specified percentage of empty space in the pages. To view the fill factor setting, use the sys.indexes catalog view.

    Important

    Creating a clustered index with a FILLFACTOR less than 100 affects the amount of storage space the data occupies because the Database Engine redistributes the data when it creates the clustered index.

    For more information, see Specify Fill Factor for an Index.

  • SORT_IN_TEMPDB = { ON | OFF }
    Specifies whether to store temporary sort results in tempdb. The default is OFF.

    • ON
      The intermediate sort results that are used to build the index are stored in tempdb. This may reduce the time required to create an index if tempdb is on a different set of disks than the user database. However, this increases the amount of disk space that is used during the index build.

    • OFF
      The intermediate sort results are stored in the same database as the index.

    In addition to the space required in the user database to create the index, tempdb must have about the same amount of additional space to hold the intermediate sort results. For more information, see SORT_IN_TEMPDB Option For Indexes.

    In backward compatible syntax, WITH SORT_IN_TEMPDB is equivalent to WITH SORT_IN_TEMPDB = ON.

  • IGNORE_DUP_KEY = { ON | OFF }
    Specifies the error response when an insert operation attempts to insert duplicate key values into a unique index. The IGNORE_DUP_KEY option applies only to insert operations after the index is created or rebuilt. The option has no effect when executing CREATE INDEX, ALTER INDEX, or UPDATE. The default is OFF.

    • ON
      A warning message will occur when duplicate key values are inserted into a unique index. Only the rows violating the uniqueness constraint will fail.

    • OFF
      An error message will occur when duplicate key values are inserted into a unique index. The entire INSERT operation will be rolled back.

    IGNORE_DUP_KEY cannot be set to ON for indexes created on a view, non-unique indexes, XML indexes, spatial indexes, and filtered indexes.

    To view IGNORE_DUP_KEY, use sys.indexes.

    In backward compatible syntax, WITH IGNORE_DUP_KEY is equivalent to WITH IGNORE_DUP_KEY = ON.

  • STATISTICS_NORECOMPUTE = { ON | OFF}
    Specifies whether distribution statistics are recomputed. The default is OFF.

    • ON
      Out-of-date statistics are not automatically recomputed.

    • OFF
      Automatic statistics updating are enabled.

    To restore automatic statistics updating, set the STATISTICS_NORECOMPUTE to OFF, or execute UPDATE STATISTICS without the NORECOMPUTE clause.

    Important

    Disabling automatic recomputation of distribution statistics may prevent the query optimizer from picking optimal execution plans for queries involving the table.

    In backward compatible syntax, WITH STATISTICS_NORECOMPUTE is equivalent to WITH STATISTICS_NORECOMPUTE = ON.

  • DROP_EXISTING = { ON | OFF }
    Specifies that the named, preexisting clustered, or nonclustered is dropped and rebuilt. The default is OFF.

    • ON
      The existing index is dropped and rebuilt. The index name specified must be the same as a currently existing index; however, the index definition can be modified. For example, you can specify different columns, sort order, partition scheme, or index options.

    • OFF
      An error is displayed if the specified index name already exists.

    The index type cannot be changed by using DROP_EXISTING.

    In backward compatible syntax, WITH DROP_EXISTING is equivalent to WITH DROP_EXISTING = ON.

  • 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.

    Note

    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 the Editions of SQL Server 2012.

    • ON
      Long-term table locks are not held for the duration of the index operation. During the main phase of the index operation, only an Intent Share (IS) lock is held on the source table. This enables queries or updates to the underlying table and indexes to proceed. At the start of the operation, a Shared (S) lock is held on the source object for a very short period of time. At the end of the operation, for a short period of time, an S (Shared) lock is acquired on the source if a nonclustered index is being created; or an SCH-M (Schema Modification) lock is acquired when a clustered index is created or dropped online and when a clustered or nonclustered index is being rebuilt. ONLINE cannot be set to ON when an index is being created on a local temporary table.

    • OFF
      Table locks are applied for the duration of the index operation. An offline index operation that creates, rebuilds, or drops a clustered index, or rebuilds or drops a nonclustered index, acquires a Schema modification (Sch-M) lock on the table. This prevents all user access to the underlying table for the duration of the operation. An offline index operation that creates a nonclustered index acquires a Shared (S) lock on the table. This prevents updates to the underlying table but allows read operations, such as SELECT statements.

    For more information, see How Online Index Operations Work.

    Indexes, including indexes on global temp tables, can be created online with the following exceptions:

    • XML index

    • Index on a local temp table.

    • Initial unique clustered index on a view.

    • Disabled clustered indexes.

    • Clustered index if the underlying table contains LOB data types: image, ntext, text, and spatial types.

    For more information, see Perform Index Operations Online.

  • ALLOW_ROW_LOCKS = { ON | OFF }
    Specifies whether row locks are allowed. The default is ON.

    • ON
      Row locks are allowed when accessing the index. The Database Engine determines when row locks are used.

    • OFF
      Row locks are not used.

  • ALLOW_PAGE_LOCKS = { ON | OFF }
    Specifies whether page locks are allowed. The default is ON.

    • ON
      Page locks are allowed when accessing the index. The Database Engine determines when page locks are used.

    • OFF
      Page locks are not used.

  • MAXDOP = max_degree_of_parallelism
    Overrides the Configure the max degree of parallelism Server Configuration Option configuration option for the duration of the index operation. 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 or fewer based on the current system workload.

    • 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 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 the Editions of SQL Server 2012.

  • DATA_COMPRESSION
    Specifies the data compression option for the specified index, partition number, or range of partitions. The options are as follows:

    • NONE
      Index or specified partitions are not compressed.

    • ROW
      Index or specified partitions are compressed by using row compression.

    • PAGE
      Index or specified partitions are compressed by using page compression.

    For more information about compression, see Data Compression.

  • ON PARTITIONS ( { <partition_number_expression> | <range> } [ ,...n ] )
    Specifies the partitions to which the DATA_COMPRESSION setting applies. If the index is not partitioned, the ON PARTITIONS argument will generate an error. If the ON PARTITIONS clause is not provided, the DATA_COMPRESSION option applies to all partitions of a partitioned index.

    <partition_number_expression> can be specified in the following ways:

    • Provide the number for a partition, for example: ON PARTITIONS (2).

    • Provide the partition numbers for several individual partitions separated by commas, for example: ON PARTITIONS (1, 5).

    • Provide both ranges and individual partitions, for example: ON PARTITIONS (2, 4, 6 TO 8).

    <range> can be specified as partition numbers separated by the word TO, for example: ON PARTITIONS (6 TO 8).

    To set different types of data compression for different partitions, specify the DATA_COMPRESSION option more than once, for example:

    REBUILD WITH 
(
DATA_COMPRESSION = NONE ON PARTITIONS (1), 
DATA_COMPRESSION = ROW ON PARTITIONS (2, 4, 6 TO 8), 
DATA_COMPRESSION = PAGE ON PARTITIONS (3, 5)
)

Remarks

The CREATE INDEX statement is optimized like any other query. To save on I/O operations, the query processor may choose to scan another index instead of performing a table scan. The sort operation may be eliminated in some situations. On multiprocessor computers CREATE INDEX can use more processors to perform the scan and sort operations associated with creating the index, in the same way as other queries do. For more information, see Configure Parallel Index Operations.

The create index operation can be minimally logged if the database recovery model is set to either bulk-logged or simple.

Indexes can be created on a temporary table. When the table is dropped or the session ends, the indexes are dropped.

Indexes support extended properties.

Clustered Indexes

Creating a clustered index on a table (heap) or dropping and re-creating an existing clustered index requires additional workspace to be available in the database to accommodate data sorting and a temporary copy of the original table or existing clustered index data. For more information about clustered indexes, see Create Clustered Indexes.

Unique Indexes

When a unique index exists, the Database Engine checks for duplicate values each time data is added by a insert operations. Insert operations that would generate duplicate key values are rolled back, and the Database Engine displays an error message. This is true even if the insert operation changes many rows but causes only one duplicate. If an attempt is made to enter data for which there is a unique index and the IGNORE_DUP_KEY clause is set to ON, only the rows violating the UNIQUE index fail.

Partitioned Indexes

Partitioned indexes are created and maintained in a similar manner to partitioned tables, but like ordinary indexes, they are handled as separate database objects. You can have a partitioned index on a table that is not partitioned, and you can have a nonpartitioned index on a table that is partitioned.

If you are creating an index on a partitioned table, and do not specify a filegroup on which to place the index, the index is partitioned in the same manner as the underlying table. This is because indexes, by default, are placed on the same filegroups as their underlying tables, and for a partitioned table in the same partition scheme that uses the same partitioning columns. When the index uses the same partition scheme and partitioning column as the table, the index is aligned with the table.

Warning

Creating and rebuilding nonaligned indexes on a table with more than 1,000 partitions is possible, but is not supported. Doing so may cause degraded performance or excessive memory consumption during these operations. We recommend using only aligned indexes when the number of partitions exceed 1,000.

When partitioning a non-unique, clustered index, the Database Engine by default adds any partitioning columns to the list of clustered index keys, if not already specified.

Indexed views can be created on partitioned tables in the same manner as indexes on tables. For more information about partitioned indexes, see Partitioned Tables and Indexes.

In SQL Server 2012, statistics are not created by scanning all the rows in the table when a partitioned index is created or rebuilt. Instead, the query optimizer uses the default sampling algorithm to generate statistics. To obtain statistics on partitioned indexes by scanning all the rows in the table, use CREATE STATISTICS or UPDATE STATISTICS with the FULLSCAN clause.

Filtered Indexes

A filtered index is an optimized nonclustered index, suited for queries that select a small percentage of rows from a table. It uses a filter predicate to index a portion of the data in the table. A well-designed filtered index can improve query performance, reduce storage costs, and reduce maintenance costs.

Required SET Options for Filtered Indexes

The SET options in the Required Value column are required whenever any of the following conditions occur:

  • Create a filtered index.

  • INSERT, UPDATE, DELETE, or MERGE operation modifies the data in a filtered index.

  • The query optimizer uses the filtered index in the query execution plan.

    SET options

    Required value

    ANSI_NULLS

    ON

    ANSI_PADDING

    ON

    ANSI_WARNINGS*

    ON

    ARITHABORT

    ON

    CONCAT_NULL_YIELDS_NULL

    ON

    NUMERIC_ROUNDABORT

    OFF

    QUOTED_IDENTIFIER

    ON

    *Setting ANSI_WARNINGS to ON implicitly sets ARITHABORT to ON when the database compatibility level is set to 90 or higher. If the database compatibility level is set to 80 or earlier, the ARITHABORT option must explicitly be set to ON.

If the SET options are incorrect, the following conditions can occur:

  • The filtered index is not created.

  • The Database Engine generates an error and rolls back INSERT, UPDATE, DELETE, or MERGE statements that change data in the index.

  • Query optimizer does not consider the index in the execution plan for any Transact-SQL statements.

For more information about Filtered Indexes, see Create Filtered Indexes.

Spatial Indexes

For information about spatial indexes, see CREATE SPATIAL INDEX (Transact-SQL) and Spatial Indexes Overview.

XML Indexes

For information about XML indexes see, CREATE XML INDEX (Transact-SQL) and XML Indexes (SQL Server).

Index Key Size

The maximum size for an index key is 900 bytes. Indexes on varchar columns that exceed 900 bytes can be created if the existing data in the columns do not exceed 900 bytes at the time the index is created; however, subsequent insert or update actions on the columns that cause the total size to be greater than 900 bytes will fail. The index key of a clustered index cannot contain varchar columns that have existing data in the ROW_OVERFLOW_DATA allocation unit. If a clustered index is created on a varchar column and the existing data is in the IN_ROW_DATA allocation unit, subsequent insert or update actions on the column that would push the data off-row will fail.

Nonclustered indexes can include non-key columns in the leaf level of the index. These columns are not considered by the Database Engine when calculating the index key size . For more information, see Create Indexes with Included Columns.

Note

When tables are partitioned, if the partitioning key columns are not already present in a non-unique clustered index, they are added to the index by the Database Engine. The combined size of the indexed columns (not counting included columns), plus any added partitioning columns cannot exceed 1800 bytes in a non-unique clustered index.

Computed Columns

Indexes can be created on computed columns. In addition, computed columns can have the property PERSISTED. This means that the Database Engine stores the computed values in the table, and updates them when any other columns on which the computed column depends are updated. The Database Engine uses these persisted values when it creates an index on the column, and when the index is referenced in a query.

To index a computed column, the computed column must deterministic and precise. However, using the PERSISTED property expands the type of indexable computed columns to include:

  • Computed columns based on Transact-SQL and CLR functions and CLR user-defined type methods that are marked deterministic by the user.

  • Computed columns based on expressions that are deterministic as defined by the Database Engine but imprecise.

Persisted computed columns require the following SET options to be set as shown in the previous section "Required SET Options for Indexed Views".

The UNIQUE or PRIMARY KEY constraint can contain a computed column as long as it satisfies all conditions for indexing. Specifically, the computed column must be deterministic and precise or deterministic and persisted. For more information about determinism, see Deterministic and Nondeterministic Functions.

Computed columns derived from image, ntext, text, varchar(max), nvarchar(max), varbinary(max), and xml data types can be indexed either as a key or included non-key column as long as the computed column data type is allowable as an index key column or non-key column. For example, you cannot create a primary XML index on a computed xml column. If the index key size exceeds 900 bytes, a warning message is displayed.

Creating an index on a computed column may cause the failure of an insert or update operation that previously worked. Such a failure may take place when the computed column results in arithmetic error. For example, in the following table, although computed column c results in an arithmetic error, the INSERT statement works.

CREATE TABLE t1 (a int, b int, c AS a/b);
INSERT INTO t1 VALUES (1, 0);

If, instead, after creating the table, you create an index on computed column c, the same INSERT statement will now fail.

CREATE TABLE t1 (a int, b int, c AS a/b);
CREATE UNIQUE CLUSTERED INDEX Idx1 ON t1(c);
INSERT INTO t1 VALUES (1, 0);

For more information, see Indexes on Computed Columns.

Included Columns in Indexes

Non-key columns, called included columns, can be added to the leaf level of a nonclustered index to improve query performance by covering the query. That is, all columns referenced in the query are included in the index as either key or non-key columns. This allows the query optimizer to locate all the required information from an index scan; the table or clustered index data is not accessed. For more information, see Create Indexes with Included Columns.

Specifying Index Options

SQL Server 2005 introduced new index options and also modifies the way in which options are specified. In backward compatible syntax, WITH option_name is equivalent to WITH ( <option_name> = ON ). When you set index options, the following rules apply:

  • New index options can only be specified by using WITH (option_name = ON | OFF).

  • Options cannot be specified by using both the backward compatible and new syntax in the same statement. For example, specifying WITH (DROP_EXISTING, ONLINE = ON**)** causes the statement to fail.

  • When you create an XML index, the options must be specified by using WITH (option_name = ON | OFF).

DROP_EXISTING Clause

You can use the DROP_EXISTING clause to rebuild the index, add or drop columns, modify options, modify column sort order, or change the partition scheme or filegroup.

If the index enforces a PRIMARY KEY or UNIQUE constraint and the index definition is not altered in any way, the index is dropped and re-created preserving the existing constraint. However, if the index definition is altered the statement fails. To change the definition of a PRIMARY KEY or UNIQUE constraint, drop the constraint and add a constraint with the new definition.

DROP_EXISTING enhances performance when you re-create a clustered index, with either the same or different set of keys, on a table that also has nonclustered indexes. DROP_EXISTING replaces the execution of a DROP INDEX statement on the old clustered index followed by the execution of a CREATE INDEX statement for the new clustered index. The nonclustered indexes are rebuilt once, and then only if the index definition has changed. The DROP_EXISTING clause does not rebuild the nonclustered indexes when the index definition has the same index name, key and partition columns, uniqueness attribute, and sort order as the original index.

Whether the nonclustered indexes are rebuilt or not, they always remain in their original filegroups or partition schemes and use the original partition functions. If a clustered index is rebuilt to a different filegroup or partition scheme, the nonclustered indexes are not moved to coincide with the new location of the clustered index. Therefore, even the nonclustered indexes previously aligned with the clustered index, they may no longer be aligned with it. For more information about partitioned index alignment, see.

The DROP_EXISTING clause will not sort the data again if the same index key columns are used in the same order and with the same ascending or descending order, unless the index statement specifies a nonclustered index and the ONLINE option is set to OFF. If the clustered index is disabled, the CREATE INDEX WITH DROP_EXISTING operation must be performed with ONLINE set to OFF. If a nonclustered index is disabled and is not associated with a disabled clustered index, the CREATE INDEX WITH DROP_EXISTING operation can be performed with ONLINE set to OFF or ON.

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

ONLINE Option

The following guidelines apply for performing index operations online:

  • The underlying table cannot be altered, truncated, or dropped while an online index operation is in process.

  • Additional temporary disk space is required during the index operation.

  • Online operations can be performed on partitioned indexes and indexes that contain persisted computed columns, or included columns.

For more information, see Perform Index Operations Online.

Row and Page Locks Options

When ALLOW_ROW_LOCKS = ON and ALLOW_PAGE_LOCK = ON, row-, page-, and table-level locks are allowed when accessing the index. The Database Engine chooses the appropriate lock and can escalate the lock from a row or page lock to a table lock.

When ALLOW_ROW_LOCKS = OFF and ALLOW_PAGE_LOCK = OFF, only a table-level lock is allowed when accessing the index.

Viewing Index Information

To return information about indexes, you can use catalog views, system functions, and system stored procedures.

Data Compression

Data compression is described in the topic Data Compression. The following are key points to consider:

  • Compression can allow more rows to be stored on a page, but does not change the maximum row size.

  • Non-leaf pages of an index are not page compressed but can be row compressed.

  • Each nonclustered index has an individual compression setting, and does not inherit the compression setting of the underlying table.

  • When a clustered index is created on a heap, the clustered index inherits the compression state of the heap unless an alternative compression state is specified.

The following restrictions apply to partitioned indexes:

  • You cannot change the compression setting of a single partition if the table has nonaligned indexes.

  • The ALTER INDEX <index> ... REBUILD PARTITION ... syntax rebuilds the specified partition of the index.

  • The ALTER INDEX <index> ... REBUILD WITH ... syntax rebuilds all partitions of the index.

To evaluate how changing the compression state will affect a table, an index, or a partition, use the sp_estimate_data_compression_savings stored procedure.

Permissions

Requires ALTER permission on the table or view. User must be a member of the sysadmin fixed server role or the db_ddladmin and db_owner fixed database roles.

Examples

A. Creating a simple nonclustered index

The following example creates a nonclustered index on the VendorID column of the Purchasing.ProductVendor table.

USE AdventureWorks2012;
GO
IF EXISTS (SELECT name FROM sys.indexes
            WHERE name = N'IX_ProductVendor_VendorID')
    DROP INDEX IX_ProductVendor_VendorID ON Purchasing.ProductVendor;
GO
CREATE INDEX IX_ProductVendor_VendorID 
    ON Purchasing.ProductVendor (BusinessEntityID); 
GO

B. Creating a simple nonclustered composite index

The following example creates a nonclustered composite index on the SalesQuota and SalesYTD columns of the Sales.SalesPerson table.

USE AdventureWorks2012
GO
IF EXISTS (SELECT name FROM sys.indexes
            WHERE name = N'IX_SalesPerson_SalesQuota_SalesYTD')
    DROP INDEX IX_SalesPerson_SalesQuota_SalesYTD ON Sales.SalesPerson ;
GO
CREATE NONCLUSTERED INDEX IX_SalesPerson_SalesQuota_SalesYTD
    ON Sales.SalesPerson (SalesQuota, SalesYTD);
GO

C. Creating a unique nonclustered index

The following example creates a unique nonclustered index on the Name column of the Production.UnitMeasure table. The index will enforce uniqueness on the data inserted into the Name column.

USE AdventureWorks2012;
GO
IF EXISTS (SELECT name from sys.indexes
             WHERE name = N'AK_UnitMeasure_Name')
    DROP INDEX AK_UnitMeasure_Name ON Production.UnitMeasure;
GO
CREATE UNIQUE INDEX AK_UnitMeasure_Name 
    ON Production.UnitMeasure(Name);
GO

The following query tests the uniqueness constraint by attempting to insert a row with the same value as that in an existing row.

--Verify the existing value.
SELECT Name FROM Production.UnitMeasure WHERE Name = N'Ounces';
GO
INSERT INTO Production.UnitMeasure (UnitMeasureCode, Name, ModifiedDate)
    VALUES ('OC', 'Ounces', GetDate());

The resulting error message is:

Server: Msg 2601, Level 14, State 1, Line 1
Cannot insert duplicate key row in object 'UnitMeasure' with unique index 'AK_UnitMeasure_Name'. The statement has been terminated.

D. Using the IGNORE_DUP_KEY option

The following example demonstrates the effect of the IGNORE_DUP_KEY option by inserting multiple rows into a temporary table first with the option set to ON and again with the option set to OFF. A single row is inserted into the #Test table that will intentionally cause a duplicate value when the second multiple-row INSERT statement is executed. A count of rows in the table returns the number of rows inserted.

USE AdventureWorks2012;
GO
CREATE TABLE #Test (C1 nvarchar(10), C2 nvarchar(50), C3 datetime);
GO
CREATE UNIQUE INDEX AK_Index ON #Test (C2)
    WITH (IGNORE_DUP_KEY = ON);
GO
INSERT INTO #Test VALUES (N'OC', N'Ounces', GETDATE());
INSERT INTO #Test SELECT * FROM Production.UnitMeasure;
GO
SELECT COUNT(*)AS [Number of rows] FROM #Test;
GO
DROP TABLE #Test;
GO

Here are the results of the second INSERT statement.

Server: Msg 3604, Level 16, State 1, Line 5 Duplicate key was ignored.

Number of rows 
-------------- 
38

Notice that the rows inserted from the Production.UnitMeasure table that did not violate the uniqueness constraint were successfully inserted. A warning was issued and the duplicate row ignored, but the entire transaction was not rolled back.

The same statements are executed again, but with IGNORE_DUP_KEY set to OFF.

USE AdentureWorks2012;
GO
CREATE TABLE #Test (C1 nvarchar(10), C2 nvarchar(50), C3 datetime);
GO
CREATE UNIQUE INDEX AK_Index ON #Test (C2)
    WITH (IGNORE_DUP_KEY = OFF);
GO
INSERT INTO #Test VALUES (N'OC', N'Ounces', GETDATE());
INSERT INTO #Test SELECT * FROM Production.UnitMeasure;
GO
SELECT COUNT(*)AS [Number of rows] FROM #Test;
GO
DROP TABLE #Test;
GO

Here are the results of the second INSERT statement.

Server: Msg 2601, Level 14, State 1, Line 5
Cannot insert duplicate key row in object '#Test' with unique index
'AK_Index'. The statement has been terminated.

Number of rows 
-------------- 
1

Notice that none of the rows from the Production.UnitMeasure table were inserted into the table even though only one row in the table violated the UNIQUE index constraint.

E. Using DROP_EXISTING to drop and re-create an index

The following example drops and re-creates an existing index on the ProductID column of the Production.WorkOrder table by using the DROP_EXISTING option. The options FILLFACTOR and PAD_INDEX are also set.

USE AdventureWorks2012;
GO
CREATE NONCLUSTERED INDEX IX_WorkOrder_ProductID
    ON Production.WorkOrder(ProductID)
    WITH (FILLFACTOR = 80,
        PAD_INDEX = ON,
        DROP_EXISTING = ON);
GO

F. Creating an index on a view

The following example creates a view and an index on that view. Two queries are included that use the indexed view.

USE AdventureWorks2012;
GO
--Set the options to support indexed views.
SET NUMERIC_ROUNDABORT OFF;
SET ANSI_PADDING, ANSI_WARNINGS, CONCAT_NULL_YIELDS_NULL, ARITHABORT,
    QUOTED_IDENTIFIER, ANSI_NULLS ON;
GO
--Create view with schemabinding.
IF OBJECT_ID ('Sales.vOrders', 'view') IS NOT NULL
DROP VIEW Sales.vOrders ;
GO
CREATE VIEW Sales.vOrders
WITH SCHEMABINDING
AS
    SELECT SUM(UnitPrice*OrderQty*(1.00-UnitPriceDiscount)) AS Revenue,
        OrderDate, ProductID, COUNT_BIG(*) AS COUNT
    FROM Sales.SalesOrderDetail AS od, Sales.SalesOrderHeader AS o
    WHERE od.SalesOrderID = o.SalesOrderID
    GROUP BY OrderDate, ProductID;
GO
--Create an index on the view.
CREATE UNIQUE CLUSTERED INDEX IDX_V1 
    ON Sales.vOrders (OrderDate, ProductID);
GO
--This query can use the indexed view even though the view is 
--not specified in the FROM clause.
SELECT SUM(UnitPrice*OrderQty*(1.00-UnitPriceDiscount)) AS Rev, 
    OrderDate, ProductID
FROM Sales.SalesOrderDetail AS od
    JOIN Sales.SalesOrderHeader AS o ON od.SalesOrderID=o.SalesOrderID
        AND ProductID BETWEEN 700 and 800
        AND OrderDate >= CONVERT(datetime,'05/01/2002',101)
GROUP BY OrderDate, ProductID
ORDER BY Rev DESC;
GO
--This query can use the above indexed view.
SELECT  OrderDate, SUM(UnitPrice*OrderQty*(1.00-UnitPriceDiscount)) AS Rev
FROM Sales.SalesOrderDetail AS od
    JOIN Sales.SalesOrderHeader AS o ON od.SalesOrderID=o.SalesOrderID
        AND DATEPART(mm,OrderDate)= 3
        AND DATEPART(yy,OrderDate) = 2002
GROUP BY OrderDate
ORDER BY OrderDate ASC;
GO

G. Creating an index with included (non-key) columns

The following example creates a nonclustered index with one key column (PostalCode) and four non-key columns (AddressLine1, AddressLine2, City, StateProvinceID). A query that is covered by the index follows. To display the index that is selected by the query optimizer, on the Query menu in SQL Server Management Studio, select Display Actual Execution Plan before executing the query.

USE AdventureWorks2012;
GO
IF EXISTS (SELECT name FROM sys.indexes
            WHERE name = N'IX_Address_PostalCode')
    DROP INDEX IX_Address_PostalCode ON Person.Address;
GO
CREATE NONCLUSTERED INDEX IX_Address_PostalCode
    ON Person.Address (PostalCode)
    INCLUDE (AddressLine1, AddressLine2, City, StateProvinceID);
GO
SELECT AddressLine1, AddressLine2, City, StateProvinceID, PostalCode
FROM Person.Address
WHERE PostalCode BETWEEN N'98000' and N'99999';
GO

H. Creating a partitioned index

The following example creates a nonclustered partitioned index on TransactionsPS1, an existing partition scheme. This example assumes the partitioned index sample has been installed.

USE AdentureWorks2012;
GO
IF EXISTS (SELECT name FROM sys.indexes
    WHERE name = N'IX_TransactionHistory_ReferenceOrderID'
    AND object_id = OBJECT_ID(N'Production.TransactionHistory'))
DROP INDEX IX_TransactionHistory_ReferenceOrderID
    ON Production.TransactionHistory;
GO
CREATE NONCLUSTERED INDEX IX_TransactionHistory_ReferenceOrderID
    ON Production.TransactionHistory (ReferenceOrderID)
    ON TransactionsPS1 (TransactionDate);
GO

I. Creating a filtered index

The following example creates a filtered index on the Production.BillOfMaterials table. The filter predicate can include columns that are not key columns in the filtered index. The predicate in this example selects only the rows where EndDate is non-NULL.

USE AdventureWorks2012;
GO
IF EXISTS (SELECT name FROM sys.indexes
    WHERE name = N'FIBillOfMaterialsWithEndDate' 
    AND object_id = OBJECT_ID(N'Production.BillOfMaterials'))
DROP INDEX FIBillOfMaterialsWithEndDate
    ON Production.BillOfMaterials;
GO
CREATE NONCLUSTERED INDEX "FIBillOfMaterialsWithEndDate"
    ON Production.BillOfMaterials (ComponentID, StartDate)
    WHERE EndDate IS NOT NULL;
GO

J. Creating a compressed index

The following example creates an index on a nonpartitioned table by using row compression.

CREATE NONCLUSTERED INDEX IX_INDEX_1 
    ON T1 (C2)
WITH ( DATA_COMPRESSION = ROW ) ; 
GO

The following example creates an index on a partitioned table by using row compression on all partitions of the index.

CREATE CLUSTERED INDEX IX_PartTab2Col1
ON PartitionTable1 (Col1)
WITH ( DATA_COMPRESSION = ROW ) ;
GO

The following example creates an index on a partitioned table by using page compression on partition 1 of the index and row compression on partitions 2 through 4 of the index.

CREATE CLUSTERED INDEX IX_PartTab2Col1
ON PartitionTable1 (Col1)
WITH (DATA_COMPRESSION = PAGE ON PARTITIONS(1),
    DATA_COMPRESSION = ROW ON PARTITIONS (2 TO 4 ) ) ;
GO

See Also

Reference

ALTER INDEX (Transact-SQL)

CREATE PARTITION FUNCTION (Transact-SQL)

CREATE PARTITION SCHEME (Transact-SQL)

CREATE SPATIAL INDEX (Transact-SQL)

CREATE STATISTICS (Transact-SQL)

CREATE TABLE (Transact-SQL)

CREATE XML INDEX (Transact-SQL)

Data Types (Transact-SQL)

DBCC SHOW_STATISTICS (Transact-SQL)

DROP INDEX (Transact-SQL)

sys.indexes (Transact-SQL)

sys.index_columns (Transact-SQL)

sys.xml_indexes (Transact-SQL)

EVENTDATA (Transact-SQL)

Concepts

XML Indexes (SQL Server)