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

@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

Answer 2

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.

Answer 3

Differences between Temporary Tables (##temp/#temp) and Table Variables (@table) are as:

1. Table variable (@table) is created in the memory. Whereas, a Temporary table (##temp/#temp) is created in the tempdb database. However, if there is a memory pressure the pages belonging to a table variable may be pushed to tempdb.

2. Table variables cannot be involved in transactions, logging or locking. This makes @table faster then #temp. So table variable is faster then temporary table.

3. Temporary table allows Schema modifications unlike Table variables.

4. Temporary tables are visible in the created routine and also in the child routines. Whereas, Table variables are only visible in the created routine.

5. Temporary tables are allowed CREATE INDEXes whereas, Table variables aren’t allowed CREATE INDEX instead they can have index by using Primary Key or Unique Constraint.

Answer 4

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.

Answer 5

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.

Answer 6

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 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             |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+