By Peter He, 2010/01/15 (first published: 2008/09/01)
Briefcase
There are lots of articles, blogs, forums discussed the differences and similarities between table variables and local temporary tables (ref. as temp tables hereafter). E.g.:
- Sql server table variable vs. local temporary table talks more about the difference from execution perspective.
- http://databases.aspfaq.com/database/should-i-use-a-temp-table-or-a-table-variable.html talks more about how to use them.
- And the recent article in SQLServerCentral.com
A summarization of table variable vs. temp table is attached in the appendix. Instead of repeating those well known facts, I'll focus on several differences that are hardly mentioned so far between table variable and temp table definitions.
XML Collection
The AdventureWorks sample database has an xml collection HumanResources.HRResumeSchemaCollection, which is used by the [Resume] column in the table HumanResources.JobCandidate.
The following script queries all the resumes into a table variable:
USE AdventureWorks GO DECLARE @TestXmls TABLE (JobCandidateID int NOT NULL, [Resume] xml(HumanResources.HRResumeSchemaCollection) PRIMARY KEY CLUSTERED(JobCandidateID)) INSERT @TestXmls (JobCandidateID,[Resume]) SELECT JobCandidateID,[Resume] FROM HumanResources.JobCandidate SELECT * FROM @TestXmls GO USE AdventureWorks GO
It works perfectly. However, if you change the table variable to a temp table:
CREATE TABLE #TestXmls (JobCandidateID int NOT NULL, [Resume] xml(HumanResources.HRResumeSchemaCollection) PRIMARY KEY CLUSTERED(JobCandidateID))
You will get the following error:
Msg 6314, Level 16, State 1, Line 2
Collection specified does not exist in metadata : 'HumanResources.HRResumeSchemaCollection'
This is because a temp table is created in TempDB, which does not have the xml collection HumanResources.HRResumeSchemaCollection. Though you can create the xml collection in the TempDB, it's not practical because:
- TempDB is recreated whenever SQL server restarts;
- TempDB is shared by all user/system databases and it is easily to have naming conflicts;
The work around is to use un-typed xml in the temporary table.
CREATE TABLE #TestXmls (JobCandidateID int NOT NULL, [Resume] xml PRIMARY KEY CLUSTERED(JobCandidateID)) INSERT #TestXmls (JobCandidateID,[Resume]) SELECT JobCandidateID,[Resume] FROM HumanResources.JobCandidate SELECT * FROM #TestXmls GO
User Defined DataTypes and User Defined Types (UDTs)
A user defined data type and UDT (CLR type) is similar to xml collection. It only resides in the database it is created. The AdventureWorks sample database has a user defined data type dbo.name, whose native data type is nvarchar(50). The following script queries all the address type names:
USE AdventureWorks GO DECLARE @TestUDTs TABLE (AddressTypeID int NOT NULL, [Name] dbo.Name NOT NULL PRIMARY KEY CLUSTERED(AddressTypeID)) INSERT @TestUDTs(AddressTypeID,[Name]) SELECT AddressTypeID,[Name] FROM Person.AddressType SELECT * FROM @TestUDTs
It works fine. If you change to use temp table:
CREATE TABLE #TestUDTs (AddressTypeID int NOT NULL, [Name] nvarchar(50) NOT NULL PRIMARY KEY CLUSTERED(AddressTypeID))
It will fail with the following error:
Msg 2715, Level 16, State 7, Line 1
Column, parameter, or variable #2: Cannot find data type dbo.Name.
The work around is to use the native data type of the user defined data type:
CREATE TABLE #TestUDTs (AddressTypeID int NOT NULL, [Name] nvarchar(50) NOT NULL PRIMARY KEY CLUSTERED(AddressTypeID))
UDTs are also scoped to a single database. If you use UDTs in a table variable, they are used in the same way as native data types. However since there is no equivalent native data type, if you need to use a UDT in a temp table, the UDT must be registered in tempdb the same way as for a user database, i.e. you need to CREATE the ASSEMLY and CREATE the TYPE in the TempDB. Due to the reasons mentioned in XML Collection
Collation
The collations for the string type (char, nchar, varchar, nvarchar) columns, if not given explicitly, will take the database collation of TempDB in a temp table, and inherit the collation of the current user database in a table variable. If not handled correctly, string comparing will fail.
Let's first find out the collation of the TempDB:
SELECT collation_name FROM sys.databases WHERE database_id = DB_ID('tempdb')
To list all available collations:
SELECT * FROM fn_helpcollations()
Now let's create a test database, remember to use a different collation than the TempDB:
CREATE DATABASE CollateTest COLLATE Albanian_BIN GO USE CollateTest GO
Create a table and populate some data:
CREATE TABLE dbo.Test1 (C2 int NOT NULL, C1 nvarchar(100) NOT NULL PRIMARY KEY CLUSTERED(C2))
GOINSERT dbo.Test1 (C1,C2) SELECT N'R1',100
Now use a table variable to join with the table:
DECLARE @TestCollations TABLE (RID int identity not null, [Name] nvarchar(100) NOT NULL
PRIMARY KEY CLUSTERED(RID))
INSERT @TestCollations ([Name]) VALUES (N'R2')
INSERT @TestCollations ([Name]) VALUES (N'R3')
SELECT A.* FROM dbo.Test1 A, @TestCollations B WHERE A.C1=B.[Name]
GO
It works fine. If you change to use temp table:
CREATE TABLE #TestCollations (RID int identity not null, [Name] nvarchar(100) NOT NULL
PRIMARY KEY CLUSTERED(RID))
INSERT #TestCollations ([Name]) VALUES (N'R2')
INSERT #TestCollations ([Name]) VALUES (N'R3')
SELECT A.* FROM dbo.Test1 A, #TestCollations B WHERE A.C1=B.[Name]
GO
You get the following error:
Msg 468, Level 16, State 9, Line 6
Cannot resolve the collation conflict between "Latin1_General_CI_AI" and "Albanian_BIN" in the equal to operation.
There are two workarounds. One is to force the collation conversion in the string comparing:
SELECT A.* FROM dbo.Test1 A, #TestCollations B WHERE A.C1=B.[Name] collate database_default
However it's a little bit annoying if you have lots of string comparing in your database. A better solution is to explicitly give the collation for the string column in the temp table:
IF object_id('tempdb..#TestCollations') IS NOT NULL
DROP TABLE #TestCollations
GO
CREATE TABLE #TestCollations (
RID int identity not null,
[Name] nvarchar(100) collate database_default NOT NULL
PRIMARY KEY CLUSTERED(RID))
INSERT #TestCollations ([Name]) VALUES (N'R2')
INSERT #TestCollations ([Name]) VALUES (N'R3')
SELECT A.* FROM dbo.Test1 A, #TestCollations B WHERE A.C1=B.[Name]
GO
The "collate database_default" clause in the column definition forces SQL Server to take current database(CollateTest)'s collation instead of TempDB's for the temp table.
If a SQL Server hosts multiple databases, or you need to restore external databases to the server, you can easily run into the collation conflict issue if the user databases use temp tables and string columns do not declare collations properly.
Conclusion
If you use table variables, you are free to use user defined data types, user defined types (UDTs), and xml collections that are defined in your database. You don't need to worry about collation conflicts. If you have to use a temp table due to various reasons, please keep in mind the following best practices:
- Always attach "collate database_default" clause for string columns in a temp table.
- Always use un-typed xml for xml columns in a temp table.
- Always use the equivalent native data type for a user defined data type in a temp table.
- If you need to use UDTs in a temp table, you have to register the type in TempDB.
Appendix
Table Variable vs. Temp Table Summary:
|
Feature |
Table Variable |
Temp Table |
Note |
|
Table Name |
Max 128 characters |
Max 116 characters |
|
|
Data Storage |
In memory and TempDB |
TempDB |
|
|
|
In memory |
TempDB |
A table variable inherits current database settings and can use the registered UDTs, user defined data types, and xml collections in the database. A temp table inherits the settings of TempDB and cannot use the types created in the user database if the same types do not exist in the TempDB. |
|
Scope |
Current batch |
Current session |
Temp tables created in a stored procedure (SP) can be referenced by dynamic queries in the SP, sub SPs, triggers fired by the affected tables of the SP. |
|
Constraints |
Allowed |
Allowed |
For table variables, since no DDL is allowed, constraints can not be created in separate DDL statements. |
|
DDL |
Not allowed |
Allowed. |
E.g. create Index on the temp table. |
|
Concurrent |
Supported |
Supported |
Constraints and Indexes with explicit name in a temp table cause duplicate name error. |
|
Statistics |
Not supported |
Supported |
Estimated row number in execution plan for table variable is always 1 |
|
Parallel execution plan |
Supported only for select |
Supported |
Parallel query execution plans are not generated for queries that modify table variables. |
|
Transaction and Locking |
Not participated |
Participated |
Data in table variable is not affected if the transaction is rolled back |
|
Cause Recompile |
No |
Yes |
Temp Table creation causes SPs/batches to recompile |
|
SELECT INTO <t> |
Not supported |
Supported |
|
|
INSERT <t> EXEC |
Not supported |
Supported |
|
|
Use |
UDFs, Stored procedures, Triggers, Batches |
Stored procedures, Triggers, Batches |
Temp tables can't be used in UDFs. |
