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

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

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

or

create table #tmp (Col1 int, Col2 int);

What are the differences between these two? I have read conflicting opinions on whether @tmp still uses tempdb, or if everything happens in memory.

In which scenarios does one out-perform the other?

@wcm - actually to nit pick the Table Variable isn't Ram only - it can be partially stored on disk.

A temp table can have indexes, whereas a table variable can only have a primary index. If speed is an issue Table variables can be faster, but obviously if there are a lot of records, or the need to search the temp table of a clustered index, then a Temp Table would be better.

[Good background article][1]


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#

For all of you who believe the myth that temp variables are in memory only

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.

Read the article here: [TempDB:: Table variable vs local temporary table][1]


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Consider also that you can often replace both with derived tables which may be faster as well. As with all performance tuning, though, only actual tests against your actual data can tell you the best approach for your particular query.

The other main difference is that table variables don't have column statistics, where as temp tables do. This means that the query optimiser doesn't know how many rows are in the table variable (it guesses 1), which can lead to highly non-optimal plans been generated if the table variable actually has a large number of rows.

Another difference:

A table var can only be accessed from statements within the procedure that creates it, not from other procedures called by that procedure or nested dynamic SQL (via exec or sp_executesql).

A temp table's scope, on the other hand, includes code in called procedures and nested dynamic SQL.

If the table created by your procedure must be accessible from other called procedures or dynamic SQL, you must use a temp table. This can be very handy in complex situations.

>In which scenarios does one out-perform the other?

For smaller tables (less than 1000 rows) use a temp variable, otherwise use a temp table.

Just looking at the claim in the accepted answer that table variables don't participate in logging.

It seems generally untrue that there is any difference in quantity of logging (at least for `insert`/`update`/`delete` operations to the table itself though I have [since found][1] that there is some small difference in this respect for cached temporary objects in stored procedures due to additional system table updates).

I looked at the logging behaviour against both a `@table_variable` and a `#temp` table for the following operations.

1. Successful Insert
2. Multi Row Insert where statement rolled back due to constraint violation.
3. Update
4. Delete
5. Deallocate

The transaction log records were almost identical for all operations.

The table variable version actually has a few **extra** log entries because it gets an entry added to (and later removed from) the `sys.syssingleobjrefs` base table but overall had a few less bytes logged purely as the internal name for table variables consumes 236 less bytes than for `#temp` tables (118 fewer `nvarchar` characters).

### Full script to reproduce (best run on an instance started in single user mode and using `sqlcmd` mode)


:setvar tablename "@T"
:setvar tablescript "DECLARE @T TABLE"

/*
--Uncomment this section to test a #temp table
:setvar tablename "#T"
:setvar tablescript "CREATE TABLE #T"
*/

USE tempdb
GO
CHECKPOINT

DECLARE @LSN NVARCHAR(25)

SELECT @LSN = MAX([Current LSN])
FROM fn_dblog(null, null)


EXEC(N'BEGIN TRAN StartBatch
SAVE TRAN StartBatch
COMMIT

$(tablescript)
(
[4CA996AC-C7E1-48B5-B48A-E721E7A435F0] INT PRIMARY KEY DEFAULT 0,
InRowFiller char(7000) DEFAULT ''A'',
OffRowFiller varchar(8000) DEFAULT REPLICATE(''B'',8000),
LOBFiller varchar(max) DEFAULT REPLICATE(cast(''C'' as varchar(max)),10000)
)


BEGIN TRAN InsertFirstRow
SAVE TRAN InsertFirstRow
COMMIT

INSERT INTO $(tablename)
DEFAULT VALUES

BEGIN TRAN Insert9Rows
SAVE TRAN Insert9Rows
COMMIT


INSERT INTO $(tablename) ([4CA996AC-C7E1-48B5-B48A-E721E7A435F0])
SELECT TOP 9 ROW_NUMBER() OVER (ORDER BY (SELECT 0))
FROM sys.all_columns

BEGIN TRAN InsertFailure
SAVE TRAN InsertFailure
COMMIT


/*Try and Insert 10 rows, the 10th one will cause a constraint violation*/
BEGIN TRY
INSERT INTO $(tablename) ([4CA996AC-C7E1-48B5-B48A-E721E7A435F0])
SELECT TOP (10) (10 + ROW_NUMBER() OVER (ORDER BY (SELECT 0))) % 20
FROM sys.all_columns
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE()
END CATCH

BEGIN TRAN Update10Rows
SAVE TRAN Update10Rows
COMMIT

UPDATE $(tablename)
SET InRowFiller = LOWER(InRowFiller),
OffRowFiller =LOWER(OffRowFiller),
LOBFiller =LOWER(LOBFiller)


BEGIN TRAN Delete10Rows
SAVE TRAN Delete10Rows
COMMIT

DELETE FROM $(tablename)
BEGIN TRAN AfterDelete
SAVE TRAN AfterDelete
COMMIT

BEGIN TRAN EndBatch
SAVE TRAN EndBatch
COMMIT')


DECLARE @LSN_HEX NVARCHAR(25) =
CAST(CAST(CONVERT(varbinary,SUBSTRING(@LSN, 1, 8),2) AS INT) AS VARCHAR) + ':' +
CAST(CAST(CONVERT(varbinary,SUBSTRING(@LSN, 10, 8),2) AS INT) AS VARCHAR) + ':' +
CAST(CAST(CONVERT(varbinary,SUBSTRING(@LSN, 19, 4),2) AS INT) AS VARCHAR)

SELECT
[Operation],
[Context],
[AllocUnitName],
[Transaction Name],
[Description]
FROM fn_dblog(@LSN_HEX, null) AS D
WHERE [Current LSN] > @LSN

SELECT CASE
WHEN GROUPING(Operation) = 1 THEN 'Total'
ELSE Operation
END AS Operation,
Context,
AllocUnitName,
COALESCE(SUM([Log Record Length]), 0) AS [Size in Bytes],
COUNT(*) AS Cnt
FROM fn_dblog(@LSN_HEX, null) AS D
WHERE [Current LSN] > @LSN
GROUP BY GROUPING SETS((Operation, Context, AllocUnitName),())

Results

+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| | | | @TV | #TV | |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| Operation | Context | AllocUnitName | Size in Bytes | Cnt | Size in Bytes | Cnt | Difference Bytes |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| LOP_ABORT_XACT | LCX_NULL | | 52 | 1 | 52 | 1 | |
| LOP_BEGIN_XACT | LCX_NULL | | 6056 | 50 | 6056 | 50 | |
| LOP_COMMIT_XACT | LCX_NULL | | 2548 | 49 | 2548 | 49 | |
| LOP_COUNT_DELTA | LCX_CLUSTERED | sys.sysallocunits.clust | 624 | 3 | 624 | 3 | |
| LOP_COUNT_DELTA | LCX_CLUSTERED | sys.sysrowsets.clust | 208 | 1 | 208 | 1 | |
| LOP_COUNT_DELTA | LCX_CLUSTERED | sys.sysrscols.clst | 832 | 4 | 832 | 4 | |
| LOP_CREATE_ALLOCCHAIN | LCX_NULL | | 120 | 3 | 120 | 3 | |
| LOP_DELETE_ROWS | LCX_INDEX_INTERIOR | Unknown Alloc Unit | 720 | 9 | 720 | 9 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysallocunits.clust | 444 | 3 | 444 | 3 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysallocunits.nc | 276 | 3 | 276 | 3 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syscolpars.clst | 628 | 4 | 628 | 4 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syscolpars.nc | 484 | 4 | 484 | 4 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysidxstats.clst | 176 | 1 | 176 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysidxstats.nc | 144 | 1 | 144 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysiscols.clst | 100 | 1 | 100 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysiscols.nc1 | 88 | 1 | 88 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysobjvalues.clst | 596 | 5 | 596 | 5 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysrowsets.clust | 132 | 1 | 132 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysrscols.clst | 528 | 4 | 528 | 4 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.clst | 1040 | 6 | 1276 | 6 | 236 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.nc1 | 820 | 6 | 1060 | 6 | 240 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.nc2 | 820 | 6 | 1060 | 6 | 240 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.nc3 | 480 | 6 | 480 | 6 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syssingleobjrefs.clst | 96 | 1 | | | -96 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syssingleobjrefs.nc1 | 88 | 1 | | | -88 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | Unknown Alloc Unit | 72092 | 19 | 72092 | 19 | |
| LOP_DELETE_ROWS | LCX_TEXT_MIX | Unknown Alloc Unit | 16348 | 37 | 16348 | 37 | |
| LOP_FORMAT_PAGE | LCX_HEAP | Unknown Alloc Unit | 1596 | 19 | 1596 | 19 | |
| LOP_FORMAT_PAGE | LCX_IAM | Unknown Alloc Unit | 252 | 3 | 252 | 3 | |
| LOP_FORMAT_PAGE | LCX_INDEX_INTERIOR | Unknown Alloc Unit | 84 | 1 | 84 | 1 | |
| LOP_FORMAT_PAGE | LCX_TEXT_MIX | Unknown Alloc Unit | 4788 | 57 | 4788 | 57 | |
| LOP_HOBT_DDL | LCX_NULL | | 108 | 3 | 108 | 3 | |
| LOP_HOBT_DELTA | LCX_NULL | | 9600 | 150 | 9600 | 150 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysallocunits.clust | 456 | 3 | 456 | 3 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.syscolpars.clst | 644 | 4 | 644 | 4 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysidxstats.clst | 180 | 1 | 180 | 1 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysiscols.clst | 104 | 1 | 104 | 1 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysobjvalues.clst | 616 | 5 | 616 | 5 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysrowsets.clust | 136 | 1 | 136 | 1 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysrscols.clst | 544 | 4 | 544 | 4 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysschobjs.clst | 1064 | 6 | 1300 | 6 | 236 |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.syssingleobjrefs.clst | 100 | 1 | | | -100 |
| LOP_INSERT_ROWS | LCX_CLUSTERED | Unknown Alloc Unit | 135888 | 19 | 135888 | 19 | |
| LOP_INSERT_ROWS | LCX_INDEX_INTERIOR | Unknown Alloc Unit | 1596 | 19 | 1596 | 19 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysallocunits.nc | 288 | 3 | 288 | 3 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.syscolpars.nc | 500 | 4 | 500 | 4 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysidxstats.nc | 148 | 1 | 148 | 1 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysiscols.nc1 | 92 | 1 | 92 | 1 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysschobjs.nc1 | 844 | 6 | 1084 | 6 | 240 |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysschobjs.nc2 | 844 | 6 | 1084 | 6 | 240 |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysschobjs.nc3 | 504 | 6 | 504 | 6 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.syssingleobjrefs.nc1 | 92 | 1 | | | -92 |
| LOP_INSERT_ROWS | LCX_TEXT_MIX | Unknown Alloc Unit | 5112 | 71 | 5112 | 71 | |
| LOP_MARK_SAVEPOINT | LCX_NULL | | 508 | 8 | 508 | 8 | |
| LOP_MODIFY_COLUMNS | LCX_CLUSTERED | Unknown Alloc Unit | 1560 | 10 | 1560 | 10 | |
| LOP_MODIFY_HEADER | LCX_HEAP | Unknown Alloc Unit | 3780 | 45 | 3780 | 45 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.syscolpars.clst | 384 | 4 | 384 | 4 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.sysidxstats.clst | 100 | 1 | 100 | 1 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.sysrowsets.clust | 92 | 1 | 92 | 1 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.sysschobjs.clst | 1144 | 13 | 1144 | 13 | |
| LOP_MODIFY_ROW | LCX_IAM | Unknown Alloc Unit | 4224 | 48 | 4224 | 48 | |
| LOP_MODIFY_ROW | LCX_PFS | Unknown Alloc Unit | 13632 | 169 | 13632 | 169 | |
| LOP_MODIFY_ROW | LCX_TEXT_MIX | Unknown Alloc Unit | 108640 | 120 | 108640 | 120 | |
| LOP_ROOT_CHANGE | LCX_CLUSTERED | sys.sysallocunits.clust | 960 | 10 | 960 | 10 | |
| LOP_SET_BITS | LCX_GAM | Unknown Alloc Unit | 1200 | 20 | 1200 | 20 | |
| LOP_SET_BITS | LCX_IAM | Unknown Alloc Unit | 1080 | 18 | 1080 | 18 | |
| LOP_SET_BITS | LCX_SGAM | Unknown Alloc Unit | 120 | 2 | 120 | 2 | |
| LOP_SHRINK_NOOP | LCX_NULL | | | | 32 | 1 | 32 |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| Total | | | 410144 | 1095 | 411232 | 1092 | 1088 |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+


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1. Temp table: A Temp table is easy to create and back up data.

Table variable: But the table variable involves the effort when we usually create the normal tables.


2. Temp table: Temp table result can be used by multiple users.

Table variable: But the table variable can be used by the current user only. 


3. Temp table: Temp table will be stored in the tempdb. It will make network traffic. When we have large data in the temp table then it has to work across the database. A Performance issue will exist.

Table variable: But a table variable will store in the physical memory for some of the data, then later when the size increases it will be moved to the tempdb.

4. Temp table: Temp table can do all the DDL operations. It allows creating the indexes, dropping, altering, etc..,

Table variable: Whereas table variable won't allow doing the DDL operations. But the table variable allows us to create the clustered index only.

5. Temp table: Temp table can be used for the current session or global. So that a multiple user session can utilize the results in the table.

Table variable: But the table variable can be used up to that program. (Stored procedure)

6. Temp table: Temp variable cannot use the transactions. When we do the DML operations with the temp table then it can be rollback or commit the transactions.

Table variable: But we cannot do it for table variable.

7. Temp table: Functions cannot use the temp variable. More over we cannot do the DML operation in the functions .

Table variable: But the function allows us to use the table variable. But using the table variable we can do that.

8. Temp table: The stored procedure will do the recompilation (can't use same execution plan) when we use the temp variable for every sub sequent calls.

Table variable: Whereas the table variable won't do like that.

Quote taken from; [Professional SQL Server 2012 Internals and Troubleshooting][1]



> **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.


[![Temporary Tables versus Table Variables][2]][2]

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!

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