Using temporary tables or table variables for SQL tuning

26 Sep 2023

When it comes to optimizing SQL queries, one of the strategies that can be employed is the use of temporary tables or table variables. These can help improve query performance by reducing the amount of data that needs to be accessed or processed.

Temporary Tables

Temporary tables are similar to regular tables, but they are not persisted in the database. They are created in memory or in a temporary space and can be used for storing intermediate results during query execution.

Temporary tables can be particularly useful when:

There is a need to store and manipulate a large amount of data for complex calculations or multiple stages of processing.
The data needs to be accessed multiple times by different parts of the query.
Joining or filtering conditions can be simplified by precomputing or preselecting the data.

Here is an example of creating and using a temporary table in SQL Server:

CREATE TABLE #TempTable (
    ID INT,
    Name VARCHAR(50)
)

INSERT INTO #TempTable (ID, Name)
SELECT ID, Name
FROM OriginalTable
WHERE ...

SELECT ID, Name 
FROM #TempTable
WHERE ...

DROP TABLE #TempTable

Table Variables

Table variables are another option for improving SQL query performance. They are similar to temporary tables in terms of their usage, but they have some differences in behavior and limitations. Table variables are created and used within the scope of a single batch, function, or stored procedure.

Table variables can be a good choice when:

The amount of data is relatively small.
The data does not need to be accessed multiple times by different parts of the query.
The query optimization can benefit from the use of in-memory data held in a table variable.

Here is an example of creating and using a table variable in SQL Server:

DECLARE @TempTable TABLE (
    ID INT,
    Name VARCHAR(50)
)

INSERT INTO @TempTable (ID, Name)
SELECT ID, Name
FROM OriginalTable
WHERE ...

SELECT ID, Name
FROM @TempTable
WHERE ...

#sql #optimization