In SQL Server, there are two ways to store and manipulate temporary data: using #TempTables or @TempVariables. While they have different purposes, their functionality overlaps to some extent. Let’s explore the similarities and differences between these two concepts.

Common Properties of #TempTable and @TempVariable

• Both #TempTables and @TempVariables are instantiated in the tempdb database.
• They are backed by physical disk, which means they consume storage space.
• Changes made to #TempTables and @TempVariables are logged in the transaction log.
• However, since tempdb always uses the simple recovery model, the transaction log records for these temporary objects only last until the next tempdb checkpoint, at which point the log is truncated.

Relation between #TempTable and @TempVariable

A table variable behaves like a local variable and has a well-defined scope within the function, stored procedure, or batch it is declared in. It can be used like a regular table within its scope. Table variables are automatically cleaned up at the end of the function, stored procedure, or batch in which they are defined. They require fewer locking and logging resources compared to #TempTables.

If a stored procedure uses temporary tables, a separate copy of the temporary table is created for each user in the system who makes use of that stored procedure. SQL Server identifies these different temporary tables by internally adding a numerical suffix to the name. On the other hand, table variables used in stored procedures cause fewer recompilations compared to temporary tables.

Recompilation occurs when DECLARE CURSOR statements or DROP TABLE statements referencing a temporary table come before other statements referencing the same temporary table. By using table variables instead of temporary tables, you can avoid repeated recompilation.

#TempTables support non-clustered indexes and create statistics on the executed query, which can help improve the performance of complex queries that require statistics and indexes. #TempTables also support input or output parameters and can be copied to another #TempTable. Additionally, the definition of a #TempTable can be changed after it is created, and it can be explicitly dropped.

Recommendation: Which one to use?

As a general rule, it is recommended to use @TempVariables by default. However, there are certain circumstances where #TempTables are more suitable:

1. If transactions need to be rolled back.
2. If the query optimizer requires statistics to run complex queries.
3. If the result sets of one table are needed for another stored procedure, such as using SELECT INTO or INSERT EXEC.
4. If you need to perform complex logic with dynamic SQL, such as creating indexes or constraints, which are not supported by table variables.
5. If the result set is very large, typically greater than 100,000 rows.

Both #TempTables and @TempVariables are valuable tools in your SQL Server toolbox, and it is important to become familiar with both concepts to leverage their benefits effectively.