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Minimally Logged Inserts
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As mentioned earlier, you can use SELECT INTO with temporary tables but not with table variables . With table variables you have to use INSERT SELECT instead . Prior to SQL Server 2008, INSERT SELECT involved more logging than SELECT INTO . This was true even with the reduced logging that happens with inserts against objects in tempdb . SQL Server 2008 adds the INSERT SELECT statement to the list of insertion methods that can be performed in a minimally logged mode, just like SELECT INTO .
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Inside Microsoft SQL Server 2008: T-SQL Programming
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I ll demonstrate this capability through an example . I ll insert data into the temporary object using SELECT INTO and INSERT SELECT in both SQL Server 2005 and SQL Server 2008 . To figure out the amount of logging involved with the operation, I ll query the undocumented fn_dblog function before and after the operation, and calculate the differences in terms of number of log records, and total record lengths, like so:
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CHECKPOINT; GO DECLARE @numrecords AS INT, @size AS BIGINT; SELECT @numrecords = COUNT(*), @size = COALESCE(SUM([Log Record Length]), 0) FROM fn_dblog(NULL, NULL) AS D; -- <operation> SELECT COUNT(*) - @numrecords AS numrecords, CAST((COALESCE(SUM([Log Record Length]), 0) - @size) / 1024. / 1024. AS NUMERIC(12, 2)) AS size_mb FROM fn_dblog(NULL, NULL) AS D;
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The first test is with the SELECT INTO statement that is processed with minimal logging in both SQL Server 2005 and SQL Server 2008, provided that the recovery model of the database is not set to FULL . As a reminder, tempdb s recovery model is SIMPLE and cannot be changed . Here s the code I used for this test:
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USE tempdb; CHECKPOINT; GO DECLARE @numrecords AS INT, @size AS BIGINT; SELECT @numrecords = COUNT(*), @size = COALESCE(SUM([Log Record Length]), 0) FROM fn_dblog(NULL, NULL) AS D; SELECT n, CAST('a' AS CHAR(2000)) AS filler INTO #TestLogging FROM dbo.Nums WHERE n <= 100000; SELECT COUNT(*) - @numrecords AS numrecords, CAST((COALESCE(SUM([Log Record Length]), 0) - @size) / 1024. / 1024. AS NUMERIC(12, 2)) AS size_mb FROM fn_dblog(NULL, NULL) AS D; GO DROP TABLE #TestLogging;
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As you can see, the operation is a SELECT INTO statement populating the temporary table #TestLogging with 100,000 rows by querying the Nums table . The output I got in SQL Server 2005 and SQL Server 2008 was similar:
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numrecords size_mb ----------- -------9560 0.63
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The number of log records is far lower than the number of rows inserted because only changes in allocation bitmaps (GAM, SGAM, PFS, IAM) were recorded in the log . Also, the total size recorded in the log is very small . Next, I used the following code to test an INSERT SELECT against a table variable populating it with the same sample data used in the SELECT INTO test:
USE tempdb; CHECKPOINT; GO DECLARE @numrecords AS INT, @size AS BIGINT; SELECT @numrecords = COUNT(*), @size = COALESCE(SUM([Log Record Length]), 0) FROM fn_dblog(NULL, NULL) AS D; DECLARE @TestLogging AS TABLE(n INT, filler CHAR(2000)); INSERT INTO @TestLogging(n, filler) SELECT n, CAST('a' AS CHAR(2000)) FROM dbo.Nums WHERE n <= 100000; SELECT COUNT(*) - @numrecords AS numrecords, CAST((COALESCE(SUM([Log Record Length]), 0) - @size) / 1024. / 1024. AS NUMERIC(12, 2)) AS size_mb FROM fn_dblog(NULL, NULL) AS D; GO
Here s the output I got in SQL Server 2005, indicating more logging activity than the corresponding SELECT INTO method:
numrecords size_mb ----------- -------184394 12.92
In SQL Server 2008 the output of the INSERT SELECT method was similar to the output I got for the corresponding SELECT INTO test, indicating minimal logging in both cases:
numrecords size_mb ----------- -------9539 0.63
Inside Microsoft SQL Server 2008: T-SQL Programming
This improvement in SQL Server 2008 means that temporary tables don t have an advantage over table variables in terms of amount of logging of SELECT INTO versus INSERT SELECT .
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