What's the difference between a temp table and table variable in SQL Server?

Question

In SQL Server 2005, we can create temp tables one of two ways:

declare @tmp table (Col1 int, Col2 int);

or

create table #tm         


        

Answer 1

Quote taken from; Professional SQL Server 2012 Internals and Troubleshooting

Statistics The major difference between temp tables and table variables is that statistics are not created on table variables. This has two major consequences, the fi rst of which is that the Query Optimizer uses a fi xed estimation for the number of rows in a table variable irrespective of the data it contains. Moreover, adding or removing data doesn’t change the estimation.

Indexes You can’t create indexes on table variables although you can create constraints. This means that by creating primary keys or unique constraints, you can have indexes (as these are created to support constraints) on table variables. Even if you have constraints, and therefore indexes that will have statistics, the indexes will not be used when the query is compiled because they won’t exist at compile time, nor will they cause recompilations.

Schema Modifications Schema modifications are possible on temporary tables but not on table variables. Although schema modifi cations are possible on temporary tables, avoid using them because they cause recompilations of statements that use the tables.

TABLE VARIABLES ARE NOT CREATED IN MEMORY

There is a common misconception that table variables are in-memory structures and as such will perform quicker than temporary tables. Thanks to a DMV called sys . dm _ db _ session _ space _ usage , which shows tempdb usage by session, you can prove that’s not the case. After restarting SQL Server to clear the DMV, run the following script to confi rm that your session _ id returns 0 for user _ objects _ alloc _ page _ count :

SELECT session_id,
database_id,
user_objects_alloc_page_count
FROM sys.dm_db_session_space_usage
WHERE session_id > 50 ;

Now you can check how much space a temporary table uses by running the following script to create a temporary table with one column and populate it with one row:

CREATE TABLE #TempTable ( ID INT ) ;
INSERT INTO #TempTable ( ID )
VALUES ( 1 ) ;
GO
SELECT session_id,
database_id,
user_objects_alloc_page_count
FROM sys.dm_db_session_space_usage
WHERE session_id > 50 ;

The results on my server indicate that the table was allocated one page in tempdb. Now run the same script but use a table variable this time:

DECLARE @TempTable TABLE ( ID INT ) ;
INSERT INTO @TempTable ( ID )
VALUES ( 1 ) ;
GO
SELECT session_id,
database_id,
user_objects_alloc_page_count
FROM sys.dm_db_session_space_usage
WHERE session_id > 50 ;

Which one to Use?

Whether or not you use temporary tables or table variables should be decided by thorough testing, but it’s best to lean towards temporary tables as the default because there are far fewer things that can go wrong.

I’ve seen customers develop code using table variables because they were dealing with a small amount of rows, and it was quicker than a temporary table, but a few years later there were hundreds of thousands of rows in the table variable and performance was terrible, so try and allow for some capacity planning when you make your decision!