kellylynch (kellylynch) wrote,
kellylynch
kellylynch

Can you create a index on a table variable in SQL Server ?

Short answer: Yes.

DECLARE @TEMPTABLE TABLE (
[ID] [INT] NOT NULL PRIMARY KEY,
[Name] [NVARCHAR] (255) COLLATE DATABASE_DEFAULT NULL,
UNIQUE NONCLUSTERED ([Name], [ID])
)
A more detailed answer is below.


http://stackoverflow.com/questions/886050/sql-server-creating-an-index-on-a-table-variable

SQL Server 2000 - 2012

Traditional tables in SQL Server can either have a clustered index or are structured as heaps.

Clustered indexes can either be declared as unique to disallow duplicate key values or default to non unique. If not unique then SQL Server silently adds a uniqueifier to any duplicate keys to make them unique.

Non clustered indexes can also be explicitly declared as unique. Otherwise for the non unique case SQL Server adds the row locator (clustered index key or RID for a heap) to all index keys (not just duplicates) this again ensures they are unique.

In SQL Server 2000 - 2012 indexes on table variables can only be created implicitly by creating a UNIQUE or PRIMARY KEY constraint. The difference between these constraint types are that the primary key must be on non nullable column(s). The columns participating in a unique constraint may be nullable. (though SQL Server's implementation of unique constraints in the presence of NULLs is not per that specified in the SQL Standard). Also a table can only have one primary key but multiple unique constraints.

Both of these logical constraints are physically implemented with a unique index. If not explicitly specified otherwise the PRIMARY KEY will become the clustered index and unique constraints non clustered but this behavior can be overridden by specifying CLUSTERED or NONCLUSTERED explicitly with the constraint declaration (Example syntax)

DECLARE @T TABLE
(
A INT NULL UNIQUE CLUSTERED,
B INT NOT NULL PRIMARY KEY NONCLUSTERED
)
As a result of the above the following indexes can be implicitly created on table variables in SQL Server 2000 - 2012.

+-------------------------------------+-------------------------------------+
| Index Type | Can be created on a table variable? |
+-------------------------------------+-------------------------------------+
| Unique Clustered Index | Yes |
| Nonunique Clustered Index | |
| Unique NCI on a heap | Yes |
| Non Unique NCI on a heap | |
| Unique NCI on a clustered index | Yes |
| Non Unique NCI on a clustered index | Yes |
+-------------------------------------+-------------------------------------+
The last one requires a bit of explanation. In the table variable definition at the beginning of this answer the non unique non clustered index on Name is simulated by a unique index on Name,Id (recall that SQL Server would silently add the clustered index key to the non unique NCI key anyway).

A non unique clustered index can also be achieved by manually adding an IDENTITY column to act as a uniqueifier.

DECLARE @T TABLE
(
A INT NULL,
B INT NULL,
C INT NULL,
Uniqueifier INT NOT NULL IDENTITY(1,1),
UNIQUE CLUSTERED (A,Uniqueifier)
)
But this is not an accurate simulation of how a non unique clustered index would normally actually be implemented in SQL Server as this adds the "Uniqueifier" to all rows. Not just those that require it.

SQL Server 2014

In addition to the methods of adding unique indexes mentioned above SQL Server 2014 also allows non unique indexes to be specified on table variables. Example syntax for that is below.

DECLARE @T TABLE (
C1 INT INDEX IX1 CLUSTERED, /*Single column indexes can be declared next to the column*/
C2 INT INDEX IX2 NONCLUSTERED,
INDEX IX3 NONCLUSTERED(C1,C2) /*Example composite index*/
);
Filtered indexes and indexes with included columns can not currently be declared with this syntax however.
Tags: sql
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