Temporary Tables and Table Variables in .NET

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7 Temporary Tables and Table Variables
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CREATE TABLE ##Globals ( varname sysname NOT NULL PRIMARY KEY, val SQL_VARIANT NULL ); GO EXEC dbo.sp_procoption 'dbo.CreateGlobals', 'startup', 'true';
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After restarting SQL Server, the global temporary table will be created automatically and persist until someone explicitly drops it . To test the procedure, restart SQL Server and then run the following code:
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SET NOCOUNT ON; INSERT INTO ##Globals VALUES('var1', CAST('abc' AS VARCHAR(10))); SELECT * FROM ##Globals;
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You probably guessed already that ##Globals is a shared global temporary table where you can logically maintain cross-session global variables . This can be useful, for example, when you need to maintain temporary counters or other variables that are globally accessible by all sessions . The preceding code creates a new global variable called var1, initializes it with the character string abc , and queries the table . Here s the output of this code:
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varname val ----------- ----------var1 abc
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When you re done, run the following code for cleanup:
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USE master; DROP PROC dbo.CreateGlobals; DROP TABLE ##Globals;
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Table Variables
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Table variables are probably among the least understood T-SQL elements . Many myths and misconceptions surround them, and these are embraced even by experienced T-SQL programmers . One widespread myth is that table variables are memory-resident only, without physical representation . Another myth is that table variables are always preferable to temporary tables . In this section, I ll dispel these myths and explain the scenarios in which table variables are preferable to temporary tables as well as scenarios in which they aren t preferable . I ll do so by first going through the fundamentals of table variables, just as I did with temporary tables, and follow with tangible examples .
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Inside Microsoft SQL Server 2008: T-SQL Programming
You create a table variable using a DECLARE statement, followed by the variable name and the table definition . You then refer to it as you do with permanent tables . Here s a very basic example:
DECLARE @T1 TABLE(col1 INT); INSERT @T1 VALUES(1); SELECT * FROM @T1;
Note that the table-valued parameters that were added in SQL Server 2008 are implemented internally like table variables . So the performance discussions in this section regarding table variables apply to table-valued parameters as well . Table-valued parameters were discussed earlier in the book in 3, Stored Procedures .
Limitations
Many limitations apply to table variables but not to temporary tables . In this section, I ll describe some of them, whereas others will be described in dedicated sections .
You cannot create explicit indexes on table variables, only PRIMARY KEY and UNIQUE constraints, which create unique indexes underneath the covers . You cannot create non-unique indexes . If you need an index on a non-unique column, you must add attributes that make the combination unique and create a PRIMARY KEY or UNIQUE constraint on the combination . You cannot alter the definition of a table variable once it is declared . This means that everything you need in the table definition must be included in the original DECLARE statement . This fact is limiting on one hand, but it also results in fewer recompilations . Remember that one of the triggers of recompilations is DDL changes . You cannot issue SELECT INTO against a table variable, rather you have to use INSERT SELECT instead . Prior to SQL Server 2008 this limitation put table variables at a disadvantage compared to temporary tables because SELECT INTO could be done as a minimally logged operation, though INSERT SELECT couldn t . SQL Server 2008 introduces improvements in minimally logged operations, including the ability to process INSERT SELECT with minimal logging . I ll demonstrate this capability later in the chapter . You cannot qualify a column name with a nondelimited table variable name (as in @T1 .col1) . This is especially an issue when referring to a table variable s column in correlated subqueries with column name ambiguity . To circumvent this limitation, you have to delimit the table variable name (as in [@T1] .col1, or @T1 .col1) . In queries that modify table variables, parallel plans will not be used . Queries that only read from table variables can be parallelized .
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