Using IDENTITY to create surrogate keys in Azure SQL Data Warehouse
Recommendations and examples for using the IDENTITY property to create surrogate keys on tables in Azure SQL Data Warehouse.
What is a surrogate key
A surrogate key on a table is a column with a unique identifier for each row. The key is not generated from the table data. Data modelers like to create surrogate keys on their tables when they design data warehouse models. You can use the IDENTITY property to achieve this goal simply and effectively without affecting load performance.
Creating a table with an IDENTITY column
The IDENTITY property is designed to scale out across all the distributions in the data warehouse without affecting load performance. Therefore, the implementation of IDENTITY is oriented toward achieving these goals.
You can define a table as having the IDENTITY property when you first create the table by using syntax that is similar to the following statement:
CREATE TABLE dbo.T1 ( C1 INT IDENTITY(1,1) NOT NULL , C2 INT NULL ) WITH ( DISTRIBUTION = HASH(C2) , CLUSTERED COLUMNSTORE INDEX ) ;
You can then use
INSERT..SELECT to populate the table.
This remainder of this section highlights the nuances of the implementation to help you understand them more fully.
Allocation of values
The IDENTITY property doesn't guarantee the order in which the surrogate values are allocated, which reflects the behavior of SQL Server and Azure SQL Database. However, in Azure SQL Data Warehouse, the absence of a guarantee is more pronounced.
The following example is an illustration:
CREATE TABLE dbo.T1 ( C1 INT IDENTITY(1,1) NOT NULL , C2 VARCHAR(30) NULL ) WITH ( DISTRIBUTION = HASH(C2) , CLUSTERED COLUMNSTORE INDEX ) ; INSERT INTO dbo.T1 VALUES (NULL); INSERT INTO dbo.T1 VALUES (NULL); SELECT * FROM dbo.T1; DBCC PDW_SHOWSPACEUSED('dbo.T1');
In the preceding example, two rows landed in distribution 1. The first row has the surrogate value of 1 in column
C1, and the second row has the surrogate value of 61. Both of these values were generated by the IDENTITY property. However, the allocation of the values is not contiguous. This behavior is by design.
The range of values for the data type are spread evenly across the distributions. If a distributed table suffers from skewed data, then the range of values available to the datatype can be exhausted prematurely. For example, if all the data ends up in a single distribution, then effectively the table has access to only one-sixtieth of the values of the data type. For this reason, the IDENTITY property is limited to
BIGINT data types only.
When an existing IDENTITY column is selected into a new table, the new column inherits the IDENTITY property, unless one of the following conditions is true:
- The SELECT statement contains a join.
- Multiple SELECT statements are joined by using UNION.
- The IDENTITY column is listed more than one time in the SELECT list.
- The IDENTITY column is part of an expression.
If any one of these conditions is true, the column is created NOT NULL instead of inheriting the IDENTITY property.
CREATE TABLE AS SELECT
CREATE TABLE AS SELECT (CTAS) follows the same SQL Server behavior that's documented for SELECT..INTO. However, you can't specify an IDENTITY property in the column definition of the
CREATE TABLE part of the statement. You also can't use the IDENTITY function in the
SELECT part of the CTAS. To populate a table, you need to use
CREATE TABLE to define the table followed by
INSERT..SELECT to populate it.
Explicitly inserting values into an IDENTITY column
SQL Data Warehouse supports
SET IDENTITY_INSERT <your table> ON|OFF syntax. You can use this syntax to explicitly insert values into the IDENTITY column.
Many data modelers like to use predefined negative values for certain rows in their dimensions. An example is the -1 or "unknown member" row.
The next script shows how to explicitly add this row by using SET IDENTITY_INSERT:
SET IDENTITY_INSERT dbo.T1 ON; INSERT INTO dbo.T1 ( C1 , C2 ) VALUES (-1,'UNKNOWN') ; SET IDENTITY_INSERT dbo.T1 OFF; SELECT * FROM dbo.T1 ;
The presence of the IDENTITY property has some implications to your data-loading code. This section highlights some basic patterns for loading data into tables by using IDENTITY.
To load data into a table and generate a surrogate key by using IDENTITY, create the table and then use INSERT..SELECT or INSERT..VALUES to perform the load.
The following example highlights the basic pattern:
--CREATE TABLE with IDENTITY CREATE TABLE dbo.T1 ( C1 INT IDENTITY(1,1) , C2 VARCHAR(30) ) WITH ( DISTRIBUTION = HASH(C2) , CLUSTERED COLUMNSTORE INDEX ) ; --Use INSERT..SELECT to populate the table from an external table INSERT INTO dbo.T1 (C2) SELECT C2 FROM ext.T1 ; SELECT * FROM dbo.T1 ; DBCC PDW_SHOWSPACEUSED('dbo.T1');
It's not possible to use
CREATE TABLE AS SELECT currently when loading data into a table with an IDENTITY column.
For more information on loading data, see Designing Extract, Load, and Transform (ELT) for Azure SQL Data Warehouse and Loading best practices.
You can use the sys.identity_columns catalog view to identify a column that has the IDENTITY property.
To help you better understand the database schema, this example shows how to integrate sys.identity_column` with other system catalog views:
SELECT sm.name , tb.name , co.name , CASE WHEN ic.column_id IS NOT NULL THEN 1 ELSE 0 END AS is_identity FROM sys.schemas AS sm JOIN sys.tables AS tb ON sm.schema_id = tb.schema_id JOIN sys.columns AS co ON tb.object_id = co.object_id LEFT JOIN sys.identity_columns AS ic ON co.object_id = ic.object_id AND co.column_id = ic.column_id WHERE sm.name = 'dbo' AND tb.name = 'T1' ;
The IDENTITY property can't be used:
- When the column data type is not INT or BIGINT
- When the column is also the distribution key
- When the table is an external table
The following related functions are not supported in SQL Data Warehouse:
This section provides some sample code you can use to perform common tasks when you work with IDENTITY columns.
Column C1 is the IDENTITY in all the following tasks.
Find the highest allocated value for a table
MAX() function to determine the highest value allocated for a distributed table:
SELECT MAX(C1) FROM dbo.T1
Find the seed and increment for the IDENTITY property
You can use the catalog views to discover the identity increment and seed configuration values for a table by using the following query:
SELECT sm.name , tb.name , co.name , ic.seed_value , ic.increment_value FROM sys.schemas AS sm JOIN sys.tables AS tb ON sm.schema_id = tb.schema_id JOIN sys.columns AS co ON tb.object_id = co.object_id JOIN sys.identity_columns AS ic ON co.object_id = ic.object_id AND co.column_id = ic.column_id WHERE sm.name = 'dbo' AND tb.name = 'T1' ;