Recently, I have been conducting many training sessions at a leading technology company in India. During the discussion of temp table and table variable, I quite commonly hear that Table Variables are stored in memory and Temp Tables are stored in TempDB. I would like to bust this misconception by suggesting following:
Temp Table and Table Variable — both are created in TempDB and not in memory.
Let us prove this concept by running the following T-SQL script.
/* Check the difference between Temp Table and Memory Tables */
-- Get Current Session ID
SELECT @@SPID AS Current_SessionID
-- Check the space usage in page files
SELECT user_objects_alloc_page_count
FROM sys.dm_db_session_space_usage
WHERE session_id = (SELECT @@SPID )
GO
-- Create Temp Table and insert three thousand rows
CREATE TABLE #TempTable (Col1 INT)
INSERT INTO #TempTable (Col1)
SELECT TOP 3000 ROW_NUMBER() OVER(ORDER BY a.name)
FROM sys.all_objects a
CROSS JOIN sys.all_objects b
GO
-- Check the space usage in page files
SELECT user_objects_alloc_page_count
FROM sys.dm_db_session_space_usage
WHERE session_id = (SELECT @@SPID )
GO
-- Create Table Variable and insert three thousand rows
DECLARE @temp TABLE(Col1 INT)
INSERT INTO @temp (Col1)
SELECT TOP 3000 ROW_NUMBER() OVER(ORDER BY a.name)
FROM sys.all_objects a
CROSS JOIN sys.all_objects b
GO
-- Check the space usage in page files
SELECT user_objects_alloc_page_count
FROM sys.dm_db_session_space_usage
WHERE session_id = (SELECT @@SPID )
GO
-- Clean up
DROP TABLE #TempTable
GO
Let us see the resultset. It is very clear that the size of the table variable and temp table is the same and created in TempDb.
Have you ever heard of this misconception? Do you know any other method to prove that both Temp Table and TableVariable are created in TempDB.
Update : Ken Simmons have written excellent another article proving without inserting data that temp table and table variable both are created in TempDB please read here.
Reference: Pinal Dave (http://blog.SQLAuthority.com)
pinaldave
Good Explanation
About Temp Table & Table Variable
really busted the misconception of many!!!! :)
Hi Pianl,
I want to know the temp table name max length in temp db.
What is the naming convention will be used to store the temp table name?
Please explain the procees which is happening inside the SQL Server while creating temp table?
Thanks in Advance.
Regards
Prasad
Thanks for the explanation; my question now is WHICH ONE IS BETTER?
Regards,
Terry Amusa
Hi,
That is true, but when we used Temp table in place of table variable, performance much faster.
We didn’t try to prove it. But there should be some difference in storage, that’s why we got better performance.
is it when releasing memory after used?
as usual Great Explanation..
Thanks,
Tejas
Hi Tejas,
1. Yes as far as performance goes table variable does not outperform temp tables necessarily. Here is a quote from sqlmag (SQL Server Magazine)
Table Variables vs. Temporary Tables
By: Brian Moran
Quote:
A senior member of the SQL Server development team told me that table variables use internal metadata in a way that prevents the engine from using a table variable within a parallel query. He also said that SQL Server maintains statistics for queries that use temporary tables but not for queries that use table variables. Without statistics, SQL Server might choose a poor processing plan for a query that contains a table variable. The development team member added that you should limit your use of SQL Server 2000 table variables to reasonably small queries and data sets and use temporary tables for larger data sets.
2. There is a situation using a table variable is preferred.
In a user defined function if you want to access a temp table it is not possible, where you can access a table variable. For example: the following will work: the second example will not work, there will be compilation erors:
CREATE FUNCTION dbo.example1
(
)
RETURNS INT
AS
BEGIN
DECLARE @t1 TABLE (i INT)
INSERT @t1 VALUES(1)
INSERT @t1 VALUES(2)
UPDATE @t1 SET i = i + 5
DELETE @t1 WHERE i < 7
DECLARE @max INT
SELECT @max = MAX(i) FROM @t1
RETURN @max
END
GO
Example 2:
CREATE FUNCTION dbo.example2
(
)
RETURNS INT
AS
BEGIN
CREATE TABLE #t1 (i INT)
INSERT #t1 VALUES(1)
INSERT #t1 VALUES(2)
UPDATE #t1 SET i = i + 5
DELETE #t1 WHERE i < 7
DECLARE @max INT
SELECT @max = MAX(i) FROM #t1
RETURN @max
END
GO
This Q/A on stackoverflow: http://stackoverflow.com/questions/27894/whats-the-difference-between-a-temp-table-and-table-variable-in-mssql leads to a few other articles (note the accepted answer), the MS KB article seems to be down right now, but the MSDN blog article above that says that “First, the table variable is NOT necessarily memory resident. Under memory pressure, the pages belonging to a table variable can be pushed out to tempdb.”, which sounds to me like a much smaller test would need to be performed to determine if a table variable, when it is afforded plenty of RAM, will stay in memory…
Hi Pinal,
Good clarification,(there are still articles out there which claim table variables are created in memory).
Thanks for the sql…
Ramdas
Thank you very much Ramdas for your very excellent note.
Hello All,
If you attempt this experiment with very simple and small number of rows, it will still give you same result – which means both of them are created in TempDB and no in memory.
Kind Regards,
Pinal
I did not understand how the page count 6 and 12 for temp table and table variable resp shows that both uses tempdb. Can anyone explain this? Thanks.
Please note that number 6 and 12 really does not matter.
What matters is from 0 to 6 and 6 to 12 so in fact I am talking about they have same difference.
Okay, got you. Many Thanks.
Hi Pinal,
Thanks for the explanation. Have couple of questions haunting me now:
1. If both reside in tempDB, how are table variables different from temp tables? What is the advantage of using table variables in place of temp tables?
2. Why is it adviced to go for temporary tables when there is huge amount of data being stored in table variables?
Request you to throw some light on this to make things more clear which would help us in better understanding on each of these at concept and usability level.
Thanks for all your knowledge sharing.
Regards,
Phani.
Hi Phani,
In the comments section i have attempted to throw some light on the questions you have, look at the reply for tejas shah in the comments section.
Thank you
Hello Phani,
In case of large amount of data temp table is advised because sql server creates, maintains and uses the statistics of temp table while generating the execution plan. Besides this we can create index in tenp tables if that is needed.
Regards,
Pinal Dave
I have tried the sequences and it worked ,but when i drop temp #TempTable the user_objects_alloc_page_count did not clear to 0 ,WHY?
Hello sir,
Great Topic to Discuss on blog.
good explanation with example and very useful too.
Ramdas has also give very good comment and very good explanation.
Used temp (#) in this procedure. Check it out…
alter procedure sp_Insert
(
@vStr varchar(max)
)
as
Begin
Declare @GetLine varchar(max), @GetCol varchar(max)
Declare @p int, @col1 varchar(30), @col2 varchar(30)
– Select @maxid = max(InvID) from tblMaster1
– Delete from tblDetail1 where lid = @maxid
create table #temp1 (col1 varchar(20), col2 varchar(20))
While (len(@vstr) !=0)
Begin
if (len(@vstr) !=0 and charindex(‘^^’,@vstr,0) = 0)
Begin
set @getline = @vstr
set @vstr = ”
End
else
Begin
Select @getline = substring(@vstr,0,charindex(‘^^’,@vstr,0))
End
set @p = 0
While (len(@getline)!=0)
Begin
if (len(@getline) !=0 and charindex(‘^’,@getline,0) = 0)
Begin
set @getcol=@getline
set @getline=”
End
Else
Begin
print @getline
Select @getCol = substring(@getline,0,charindex(‘^’,@getline,0))
print @getcol
End
set @P=@P+1
IF(@p=1)
begin
set @col1= @getcol
end
if (@p=2)
begin
set @col2= @getcol
end
Set @getline = substring(@getline , charindex(‘^’ ,@getline,0)+1, len(@getline))
print @getline
End
– Select @maxid= isnull(max(lid) from tbldet
insert into #temp1 values(@col1,@col2)
Set @vstr = substring(@vstr,charindex(‘^^’,@vstr,0)+2 , len(@vstr))
End
Select * from #temp1
End
[...] December 28, 2009 by pinaldave Few days ago I wrote an article on the myth of table variable stored in the memory—it was very well received by the community. Read complete article here: SQL SERVER – Difference TempTable and Table Variable – TempTable in Memory a Myth. [...]
On our test/development database server (2 sql server instances) with 4GB memory had a proc using table variabes and was running over 5 minutes. Changed to temp tables and is now running under 30 seconds. In production (single instance with 4GB) ran fine with table variables – around 30 seconds. Assuming memory constraints contributed to poor performance on test/development server, rather than stats.
Hello Jakey,
Table variables performs good only for small resordset. Because there data statistics is not used while creating the execution plan and because of that sometimes fully optimized plan are not created.
Table variable should be used where they are joined with small tables only.
There are many more details which are associated with table variables but this is just one line answer here.
Regards,
Pinal Dave
Please see this query :-
SELECT TOP 3000 ROW_NUMBER() OVER(ORDER BY a.name)
gives error like
‘ROW_NUMBER’ is not a recognized function name.
when i run ito SQL 2005
Please can u tell me function for gettin rowno in
MS-SQL-2005 or 2008
@Nitin :
I think u r missing table name in the last..
It shud be:
SELECT TOP 3000 ROW_NUMBER() OVER (ORDER BY name) from TEST
Thanks a lot…
it was my mistake…
I want to do replication of two tables having two diff names and (same structure but diff records )
in single table at subscriber side.
Is it possible in SQL server 2008 or 2005.
Please give me solution.
The following stmt gives me the temp tables I created:
SELECT TABLE_NAME
FROM tempdb.INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME LIKE ‘%#%’;
But it does not show me the table variables created in tempdb.
How can I get the list of table variables created in tempdb?