Published on

June 6, 2008

Understanding SQL Server Concepts: Temp Tables vs. Table Variables

When it comes to storing temporary data in SQL Server, developers often have to choose between using temp tables or table variables. While there are many articles discussing the advantages and disadvantages of each option, there are a few key points that are often overlooked. In this article, we will explore these points and shed light on some important considerations when deciding between temp tables and table variables.

Using TempDB

One common misconception is that table variables do not use TempDB. However, both table variables and temp tables are created in TempDB. While both options can use the data cache (memory) if available, they can also spill into TempDB when necessary. Therefore, when deciding between a temp table and a table variable, the use of TempDB should not be a determining factor.

For example, you can run the following script to see the data for a table variable in TempDB:

CREATE TABLE #TempTable (TT_Col1 INT)
DECLARE @TableVariable TABLE (TV_Col1 INT)

SELECT TOP 2 * FROM tempdb.sys.objects ORDER BY create_date DESC

This script will display the data for the table variable in TempDB, showing that both temp tables and table variables reside in the same database.

Transactions and Table Variables

Another important consideration is how transactions affect table variables. Unlike temp tables, table variables are not influenced by transactions. This can be both an advantage and a disadvantage, depending on your specific needs.

When using a transaction and a table variable, it is crucial to understand that a rollback does not always affect the data in the table variable. This means that the data you expect to retrieve may be different from the actual represented data. For example, consider the following script:

-- Insert value (1) into temp table and table variable
INSERT #TempTable VALUES (1)
INSERT @TableVariable VALUES (1)

-- Insert value (2) and rollback immediately
BEGIN TRANSACTION
INSERT #TempTable VALUES (2)
INSERT @TableVariable VALUES (2)
ROLLBACK

-- Select from temp table
SELECT * FROM #TempTable

-- Select from table variable – we expect to get only 1 record
SELECT * FROM @TableVariable

In this script, you would expect to retrieve only the value 1 from the table variable, as a rollback occurred on the insertion of the value 2. However, you will actually get two records, including the rolled back value. This behavior can significantly impact your results if you are not aware of it.

While this behavior can be seen as a disadvantage in terms of data integrity, it can also be advantageous in certain scenarios. Since a table variable is outside the transaction scope, it allows you to capture data transferred within a transaction, even if a rollback occurs.

Constraints on Temp Tables

One limitation of table variables is that you cannot add constraints to them. On the other hand, you can add constraints, such as a primary key, to temp tables just like any other table.

When adding constraints to temp tables, it is important to consider the naming of these constraints. If you define a specific constraint name, it can lead to errors due to collisions between object names. For example:

-- Create table with default constraint name
CREATE TABLE #TempTableConstraint (TTC_Col1 INT PRIMARY KEY)

-- Create table with explicit constraint name
CREATE TABLE #TempTableConstraintName (TTCN_Col1 INT CONSTRAINT PK_#TempTableConstraintName_TTCN_Col1 PRIMARY KEY)

If you run these create statements on different sessions, you will encounter an error indicating that there is already an object with the same name in the database. SQL Server can differentiate between two create temp table statements in different sessions, but it cannot handle constraints with the same name.

To avoid such errors, it is recommended to use the default constraint name when adding constraints to temp tables. For example, to create a table with a primary key constraint using the default name:

CREATE TABLE #TempTableConstraint (TTC_Col1 INT PRIMARY KEY)

Conclusion

When deciding between temp tables and table variables in SQL Server, it is crucial to consider all the differences and choose the option that best suits your needs. Remember the following key points:

  • Do not consider the use of TempDB as a deciding factor, as both temp tables and table variables reside in TempDB.
  • Be aware of how transactions can affect table variables, as rollbacks may not always impact the data in the table variable.
  • When adding constraints to temp tables, avoid using explicit constraint names to prevent collisions between object names.

By understanding these concepts and making informed decisions, you can effectively utilize temp tables and table variables in your SQL Server solutions.

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