SA0148 : Consider using a temporary table instead of a table variable

The use of table variables in T-SQL can lead to performance issues and incorrect query behavior.

Description

In T-SQL code, table variables are often used as an alternative to temporary tables. However, they have certain limitations and characteristics that can result in inefficient query plans and unexpected results.

For example:

SQL
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1-- Example of potentially problematic use of a table variable
2DECLARE @TableVar TABLE (ID INT, Name NVARCHAR(50));
3
4INSERT INTO @TableVar (ID, Name) VALUES (1, 'Sample');
5
6SELECT * FROM @TableVar WHERE ID = 1;

In this example, using a table variable might limit SQL Server’s ability to generate optimal query plans because statistics are not maintained on table variables. Hence, query performance might degrade, especially with larger datasets.

  • Performance issues: Without automatic statistics updating, SQL Server lacks information for executing efficient query plans, potentially slowing down operations.

  • Limited indexing: Table variables do not support the indexing capabilities of temporary tables, which can further lead to inefficient data access patterns.

How to fix

Avoid using table variables when storing more than 100 rows, as they may lead to suboptimal execution plans. Instead, consider using temporary tables or appropriate query hints to improve performance.

Follow these steps to address the issue:

  1. Assess the current usage of the @TableVar by evaluating the number of rows it holds and its impact on performance.

  2. If the @TableVar contains more than 100 rows, consider replacing it with a temporary table. Create a temporary table by using the CREATE TABLE #TempTable syntax:

  3. Rewrite the query to populate the temporary table instead of the table variable:

For example:

SQL
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1-- Example of using a temporary table instead of a table variable
2CREATE TABLE #TempTable (ID INT, Name NVARCHAR(50));
3INSERT INTO #TempTable (ID, Name) VALUES (1, 'Sample');
4SELECT * FROM #TempTable WHERE ID = 1;

  1. If using a temporary table is not feasible, use the USE PLAN query hint to potentially improve the execution plan stability.

  2. In cases where performance is acceptable and suppression is warranted, use the rule suppression mark to ignore the warning for specific queries.

Scope

The rule has a Batch scope and is applied only on the SQL script.

Parameters

Rule has no parameters.

Remarks

The rule does not need Analysis Context or SQL Connection.

Effort To Fix
20 minutes per issue.
Categories

Design Rules, Bugs

Additional Information
Example Test SQL
SQL
Copy
 1DECLARE @mail TABLE 
 2(      
 3        toAddress NVARCHAR( 100 ) ,
 4        fromAddres NVARCHAR( 100 ) ,
 5        subject NVARCHAR( 256 ) ,
 6        body NVARCHAR( 4000 )
 7);
 8
 9INSERT INTO @mail(  toAddress ,
10                          fromAddres ,
11                          subject ,
12                          body )
13VALUES(  'support@ubitsoft.com' ,
14         'sqlenight_user@gmail.com' ,
15         'Body' ,
16         'Put your subject here.' );
17
18SELECT     toAddress ,
19           fromAddres ,
20           subject ,
21           body
22FROM       @mail
23
24DECLARE @mail2 TABLE 
25(      
26        toAddress NVARCHAR( 100 ) ,
27        fromAddres NVARCHAR( 100 ) ,
28        subject NVARCHAR( 256 ) ,
29        body NVARCHAR( 4000 ))
Analysis Results
  Message Line Column
1 SA0148 : Consider using a temporary table instead of a table variable. 2 8
2 SA0148 : Consider using a temporary table instead of a table variable. 25 8
See Also

Other Resources

Analysis Rules