问题
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?
回答1:
There are a few differences between Temporary Tables (#tmp) and Table Variables (@tmp), although using tempdb isn't one of them, as spelt out in the MSDN link below.
As a rule of thumb, for small to medium volumes of data and simple usage scenarios you should use table variables. (This is an overly broad guideline with of course lots of exceptions - see below and following articles.)
Some points to consider when choosing between them:
Temporary Tables are real tables so you can do things like CREATE INDEXes, etc. If you have large amounts of data for which accessing by index will be faster then temporary tables are a good option.
Table variables can have indexes by using PRIMARY KEY or UNIQUE constraints. (If you want a non-unique index just include the primary key column as the last column in the unique constraint. If you don't have a unique column, you can use an identity column.) SQL 2014 has non-unique indexes too.
Table variables don't participate in transactions and
SELECTs are implicitly withNOLOCK. The transaction behaviour can be very helpful, for instance if you want to ROLLBACK midway through a procedure then table variables populated during that transaction will still be populated!Temp tables might result in stored procedures being recompiled, perhaps often. Table variables will not.
You can create a temp table using SELECT INTO, which can be quicker to write (good for ad-hoc querying) and may allow you to deal with changing datatypes over time, since you don't need to define your temp table structure upfront.
You can pass table variables back from functions, enabling you to encapsulate and reuse logic much easier (eg make a function to split a string into a table of values on some arbitrary delimiter).
Using Table Variables within user-defined functions enables those functions to be used more widely (see CREATE FUNCTION documentation for details). If you're writing a function you should use table variables over temp tables unless there's a compelling need otherwise.
Both table variables and temp tables are stored in tempdb. But table variables (since 2005) default to the collation of the current database versus temp tables which take the default collation of tempdb (ref). This means you should be aware of collation issues if using temp tables and your db collation is different to tempdb's, causing problems if you want to compare data in the temp table with data in your database.
Global Temp Tables (##tmp) are another type of temp table available to all sessions and users.
Some further reading:
Martin Smith's great answer on dba.stackexchange.com
MSDN FAQ on difference between the two: https://support.microsoft.com/en-gb/kb/305977
MDSN blog article: http://blogs.msdn.com/sqlserverstorageengine/archive/2008/03/30/sql-server-table-variable-vs-local-temporary-table.aspx
Article: http://searchsqlserver.techtarget.com/tip/0,289483,sid87_gci1267047,00.html#
Unexpected behaviors and performance implications of temp tables and temp variables: Paul White on SQLblog.com
回答2:
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.
- Successful Insert
- Multi Row Insert where statement rolled back due to constraint violation.
- Update
- Delete
- 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 |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
回答3:
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.
回答4:
@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
回答5:
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.
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.
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.
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.
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)
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.
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.
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.
回答6:
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
回答7:
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.
回答8:
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!
回答9:
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.
回答10:
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.
回答11:
Differences between Temporary Tables (##temp/#temp) and Table Variables (@table) are as:
Table variable (@table)is created in thememory. Whereas, aTemporary table (##temp/#temp)is created in thetempdb database. However, if there is a memory pressure the pages belonging to a table variable may be pushed to tempdb.Table variablescannot be involved intransactions, logging or locking. This makes@table faster then #temp. So table variable is faster then temporary table.Temporary tableallows Schema modifications unlikeTable variables.Temporary tablesare visible in the created routine and also in the child routines. Whereas, Table variables are only visible in the created routine.Temporary tablesare allowedCREATE INDEXeswhereas,Table variablesaren’t allowedCREATE INDEXinstead they can have index by usingPrimary Key or Unique Constraint.
来源：https://stackoverflow.com/questions/27894/whats-the-difference-between-a-temp-table-and-table-variable-in-sql-server