How tempDB works?

Question

I am trying to understand the tempDB and following are the doubts popping in my mind.

1. What is the lifetime of data in tempDB? Say a query is doing some Order By and uses tempDB for performing that. After this query finishes, someone else also executes a query which utilizes the tempDB. Will the second query find records written by first query in the tempDB or will they be deleted?
2. Are there any visible tables created inside the tempDB by the Sql Engine? How can I know which temporary table is created because of this query? Is there any naming convention followed by the Sql engine for naming these temporary tables?

I am new to tempDB so please pardon me for asking such silly (if at all) questions :-)

It will be very nice if someone can point me to a good resource which can help me learn about tempDB.

Answer 1

Temp table is stored in tempdb until the connection is dropped (or in the case of a global temp tables when the last connection using it is dropped). You can also (and it is a good practice to do so) manually drop the table when you are finished using it with a drop table statement.

No, others cannot see your temp tables if they are local temp tables (They can see and use global temp tables) Multiple people can run commands which use the same temp table name but they will not be overlapping in a local temp table and so you can have a table named #test and so can 10,000 other users, but each one has its own structure and data.

You don't want to generally look up temp tables in tempdb. It is possible to check for existence, but that is the only time I have ever referenced tempdb directly. Simply use your temp table name. Example below of checking for existence

  IF OBJECT_ID('TempDB.dbo.#DuplicateAssignments') IS NOT NULL 
  BEGIN 
  DROP TABLE #DuplicateAssignments 
  END  

You name temp tables by prefacing the name with # (for local tables the ones you would use 999.9% of the time) and ## for global temp tables, then the rest of the name you want.

Answer 2

There's a few MSDN articles that are probably the best source of information on the tempDB database in SQL Server.

tempdb Database

The tempdb system database is a global resource that is available to all users connected to the instance of SQL Server and is used to hold the following:

• Temporary user objects that are explicitly created, such as: global or local temporary tables, temporary stored procedures, table variables, or cursors.
• Internal objects that are created by the SQL Server Database Engine, for example, work tables to store intermediate results for spools or sorting.
• Row versions that are generated by data modification transactions in a database that uses read-committed using row versioning isolation or snapshot isolation transactions.
• Row versions that are generated by data modification transactions for features, such as: online index operations, Multiple Active Result Sets (MARS), and AFTER triggers.

Operations within tempdb are minimally logged. This enables transactions to be rolled back. tempdb is re-created every time SQL Server is started so that the system always starts with a clean copy of the database. Temporary tables and stored procedures are dropped automatically on disconnect, and no connections are active when the system is shut down. Therefore, there is never anything in tempdb to be saved from one session of SQL Server to another. Backup and restore operations are not allowed on tempdb.

There's also tempdb and Index Creation, this blog post along with Working with tempdb in SQL Server 2005 which states:

The SQL Server system database, tempdb, has undergone a number of changes in SQL Server 2005. There are new tempdb usages and internal optimizations in SQL Server 2005; tempdb architecture is mostly unchanged since SQL Server 2000.

The tempdb system database is very similar to a user database. The main difference is that data in tempdb does not persist after SQL Server shuts down.

Answer 3

1. The temporary tables created in TempDB are dropped when the query is completed.

2. I'm not sure on this (I would have to try it), but I think theoretically ALL tables created in TempDB are visible, although only the user that created the table has permission to access it.