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SQL Server Performance: Differences between Temp Table nd Table Variable

In this post, we'll do a comparison of table variables and temporary tables with references to Microsoft's official documentation pages.

Differences Between Temp Table and Table Variable​

Table Variables:

  • Definition: Table variables are declared using the DECLARE statement and are essentially variables that hold a table of data.
  • Scope: They are scoped to the batch, stored procedure, or function in which they are defined. They are also cached in memory.
  • Resource Usage: Table variables are stored in memory, and their usage does not count against the transaction log.
  • Statistics: Table variables have limited statistics, and the query optimizer might generate less optimal plans.
  • Indexes: They do not support explicitly defined indexes, but they come with a unique clustered index by default.
  • Transaction Behavior: They participate in transactions but cannot have explicit transactions defined for them.
  • Dropping: They are automatically dropped when the scope ends.
  • Documentation Reference: Microsoft's documentation on table variables: Table Variables

Temporary Tables:

  • Definition: Temporary tables are created using the CREATE TABLE statement with # or ## prefix.
  • Scope: Local temporary tables (#) are visible only to the session that creates them. Global temporary tables (##) are visible across all sessions but are dropped when the last session using them ends.
  • Resource Usage: They are created in the tempdb database and may involve disk I/O.
  • Statistics: Temporary tables have better statistics, leading to improved query optimization.
  • Indexes: They support explicitly defined indexes, primary keys, constraints, and triggers.
  • Transaction Behavior: They participate in transactions and can be explicitly included in transactions.
  • Dropping: They are automatically dropped when the session ends or when the scope ends (for local temporary tables). Global temporary tables are dropped when the last session using them ends.
  • Documentation Reference: Microsoft's documentation on temporary tables: Temporary Tables

In terms of official references, Microsoft's documentation provides detailed information on the usage, benefits, and limitations of both table variables and temporary tables. Depending on your specific use case and requirements, you can refer to the provided documentation to make an informed decision on whether to use table variables or temporary tables in your SQL queries.

When to Use Temp Table or Table Variable​

Choosing between table variables and temporary tables depends on the specific requirements of your task:

  • For small datasets (less than 100), relatively simple operations, and when memory consumption is a concern, table variables can be more efficient.
  • For larger datasets (greater than 100), complex operations, the need for accurate statistics, indexing, and better control over transactional behavior, temporary tables might be a better choice.

In summary, table variables are suitable for lightweight tasks within a limited scope, while temporary tables offer more versatility and optimization options, especially for handling larger and more complex data sets.

Create and Drop Local Temp Table Example​

Here is an example of how you can create and drop a local temporary table:

Create and drop local temp table
-- Create a local temporary table
CREATE TABLE #TempTable (ID INT, Name VARCHAR(50));

-- Do some operations with the temporary table

-- Drop the temporary table when it's no longer needed
DROP TABLE #TempTable;

Create and Drop Global Temp Table Example​

Here is an example of how you can create and drop a global temporary table:

Create and drop global temp table
-- Create a global temporary table
CREATE TABLE ##TempTable (ID INT, Name VARCHAR(50));

-- Do some operations with the temporary table

-- Drop the global temporary table when it's no longer needed
DROP TABLE ##TempTable;

Should You Drop Temp Table Explicitly?​

Temporary tables are automatically dropped when the session that created them ends. You do not necessarily need to manually drop temporary tables; they will be removed automatically as part of the cleanup process.

Temporary tables have a scope tied to the session or batch that created them, and they are automatically cleaned up by the system when that scope ends. However, if you are done using a temporary table and want to release the resources associated with it before the session ends, you can choose to manually drop it using the DROP TABLE statement as shown in the above code snippets.

While you do not have to manually drop temporary tables, doing so can be beneficial if you want to free up resources before the session ends or if you're working with a long-running session and want to release resources sooner.

Create Table Variable Example​

Here is an example of how you can create and use a table variable:

Create and using a table variable
DECLARE @MyTableVariable TABLE (ID INT, Name VARCHAR(50));

INSERT INTO @MyTableVariable (ID, Name)
VALUES (1, 'Alice'), (2, 'Bob');

SELECT * FROM @MyTableVariable;