When working with SQL Server, it’s not uncommon to need to store data in a temporary table or table variable. While both options can be used to accomplish the same goal, there are differences between the two that can affect performance and the ability to write efficient code. Let’s explore the differences between table variables and temporary tables, and when to use each one.
@Table Variables
Table variables are declared using the “@” symbol and are created in memory. They are similar to regular variables in that they are scoped to the batch or stored procedure they are declared in, and their values can be manipulated using standard SQL statements. Table variables are often used to store small to medium-sized data sets.
Benefits of Table Variables:
- Table variables are created in memory, which means they can be accessed more quickly than temporary tables that are created on disk.
- Table variables are automatically cleaned up when they go out of scope, which can improve performance and reduce the risk of resource contention.
- Table variables can be passed between stored procedures or functions, which can make code more modular and easier to maintain.
Limitations of Table Variables:
- Table variables are not indexed, which means they can be slower to query than temporary tables.
- Table variables cannot be used to create statistics, which can make it harder for the query optimizer to generate efficient execution plans.
- Table variables have a fixed cardinality, which means that SQL Server cannot estimate the number of rows they contain accurately.
#Temporary Tables
Temporary tables are created using the “#” symbol and are stored on disk. They can be accessed by any session that has the appropriate permissions, and they are useful for storing large data sets or data that needs to persist across multiple batches or sessions. Temporary tables can be created at the session, connection, or global level.
Benefits of Temporary Tables:
- Temporary tables can be indexed, which means they can be queried more quickly than table variables.
- Temporary tables can be used to create statistics, which can help the query optimizer generate more efficient execution plans.
- Temporary tables can be used to store large data sets or data that needs to persist across multiple batches or sessions.
Limitations of Temporary Tables:
- Temporary tables are stored on disk, which means they can be slower to access than table variables.
- Temporary tables can create resource contention, especially if they are not cleaned up properly.
- Temporary tables cannot be passed between stored procedures or functions, which can make code less modular and harder to maintain.
When to Use Table Variables vs. Temporary Tables
Table variables are best used when you need to store small to medium-sized data sets that can be manipulated quickly and don’t require indexing or statistics. If you need to pass the data between stored procedures or functions, a table variable is often the best choice.
Temporary tables are best used when you need to store large data sets or data that needs to persist across multiple batches or sessions. If you need to create indexes or statistics on the data, a temporary table is the better choice.
In conclusion, table variables and temporary tables have their own unique strengths and limitations. Choosing the right option for your specific use case will depend on factors such as the size of the data set, the need for indexing and statistics, and whether or not you need to pass the data between stored procedures or functions. By understanding the differences between the two options, you can write more efficient and maintainable SQL code.
More information on both table variables and temporary tables can be found here:
Table Variables: table (Transact-SQL) – SQL Server | Microsoft Learn
Temporary Tables: Tables – SQL Server | Microsoft Learn
