Monday, January 11, 2010

SQL Server Myths Debunked (Part 1)

MythBusters Today I wanted to talk about some of the common misconceptions or myths that I have encountered over the years.  A lot of these myths are so wide spread because of the sheer amount of misinformation available.  Myths are born in a number of ways, but typically SQL Server myths are brought to life by propagated misinformation from blogs, articles, unreliable sources etc. that spreads through the community like a wild fire.

 

Below is a list of myths that I have encountered and will be discussing in this series.

  • Table Variables do not exist in TempDB
  • Table variables cannot have indexes
  • Putting a db in simple recovery stops the log from growing
  • Shrinking a log breaks the log chain 
  • Backing up the transaction log frees up OS storage
  • Table data can be stored separately of the clustered index
  • Columns just need to exist in the index to be used
  • While loops are faster than cursors
  • sp_executesql is always better than exec
  • tables have a guaranteed sort order

    Table Variables do not exist in TempDB

    Lets start with one of my favorite myths.  There is a very common misconception that table variables do not exist in TempDB, but only in memory.  This is absolutely false.  Table variables exist in memory, when they are small enough to fit, but they always consume storage in TempDB and make entries into the TempDB  log.

    The example below demonstrates how you can view the table makeup in sys.tables.

    --Make the master db the current db
    USE [master]
    GO
     
    --Declare a table variable
    DECLARE @t TABLE(
    Id INT,
    Col CHAR(1)
    );
     
    --Get Table Variable Definition
    SELECT t.NAME,c.name,ty.name, [c].[max_length]
    FROM tempdb.sys.tables t 
    INNER JOIN tempdb.sys.columns c
        ON [t].[object_id] = [c].[object_id]
    INNER JOIN tempdb.sys.types ty
        ON [c].[system_type_id] = [ty].[system_type_id]
    WHERE t.name LIKE '#%'

    Table variables cannot have indexes

    The next myth also deals with table variables.  It is often thought that table variables cannot have indexes, which is absolutely false.  Table variables can have indexes, if you specify them at the time of creation.  The only restriction is that you cannot create non-unique indexes.  It is important to remember that even though an index exists on the table variable, the optimizer still does not maintain statistics on table variables.  This means the optimizer will still assume one row, even if a scan is done.

    DECLARE @t TABLE(
    Id INT PRIMARY KEY CLUSTERED,
    Col CHAR(1) UNIQUE
    );
     
    --Get Table Variable Definition
    SELECT i.name, i.type_desc,i.is_unique,i.is_primary_key,i.[is_unique_constraint]
    FROM tempdb.sys.tables t 
    INNER JOIN tempdb.sys.indexes i
        ON i.[object_id] = t.[object_id]
    WHERE t.name LIKE '#%'

    Results:

    image

    Putting a db in simple recovery stops the log from growing

    One of the most misunderstood myths in the SQL Server realm is how the simple recovery model actually works.  Some believe that by changing the recovery model to simple the transaction log will not grow.  This is wrong on so many levels.  If the transaction log were not able to grow, how would any transactions be rolled back?  When the recovery model is set to simple the transaction log still has to grow to accommodate large transactions. Let’s have a look at this in action.

    SET NOCOUNT ON
    GO
     
    USE [master]
    GO
     
    IF db_id('TestSimpleRecovery') IS NOT NULL
    BEGIN
        DROP DATABASE TestSimpleRecovery;
    END
    GO
     
    --Create DB
    CREATE DATABASE TestSimpleRecovery;
    GO
     
    --Change Recovery Model To Simple
    ALTER DATABASE TestSimpleRecovery SET RECOVERY SIMPLE
    GO
     
    --Change the FileGrowth To 100MB
    ALTER DATABASE TestSimpleRecovery 
    MODIFY FILE(NAME = TestSimpleRecovery_Log,FILEGROWTH=100MB);
    GO
     
    --Switch DB Context
    USE [TestSimpleRecovery]
    GO
     
    --Get Current Log Size (before XACT)
    SELECT ((size * 8) / 1024.) / 1024. AS SizeMBsBeforeTransaction
    FROM sys.[master_files]
    WHERE type = 1 AND [database_id] = db_id('TestSimpleRecovery')
    GO
     
    --Drop Sample Table
    IF EXISTS(SELECT 1 FROM sys.tables WHERE NAME = 'TestData')
    BEGIN
        DROP TABLE dbo.[TestData];
    END
    GO
     
    CREATE TABLE dbo.TestData(
    RowNum INT,
    SomeId INT,
    SomeCode CHAR(2)
    );
    GO
     
    DECLARE @i INT
    SET @i = 1
     
    BEGIN TRANSACTION
     
    WHILE @i < 100
    BEGIN
     
    INSERT INTO dbo.TestData
    SELECT TOP 1000
        ROW_NUMBER() OVER (ORDER BY t1.NAME) AS RowNumber,
        ABS(CHECKSUM(NEWID()))%2500+1 AS SomeId, 
        CHAR(ABS(CHECKSUM(NEWID()))%26+65)
        + CHAR(ABS(CHECKSUM(NEWID()))%26+65) AS SomeCode
    FROM 
        Master.dbo.SysColumns t1,
        Master.dbo.SysColumns t2
        
        SET @i = @i + 1 
    END
     
    COMMIT TRANSACTION
    GO
     
    --Get Current Log Size (After XACT)
    SELECT ((size * 8) / 1024.) / 1024. AS SizeMBsAfterTransaction
    FROM sys.[master_files]
    WHERE type = 1 AND [database_id] = db_id('TestSimpleRecovery')
    GO
     
    /*
    SizeMBsBeforeTransaction
    ---------------------------------------
    0.00054931640
     
    SizeMBsAfterTransaction
    ---------------------------------------
    0.09820556640
    */

    Shrinking a log breaks the log chain 

    The next myth I will be discussing is shrinking the transaction log.  It is often purveyed that shrinking the transaction log breaks the log chain.  This is absolutely false!  This is a prime example of how confusion can cause misinformation.  It is true that backing up the log with the truncate only option breaks the log chain, but this is completely different than shrinking the log.  Shrinking the log is used to empty space from a data or log file.  Shrinking the log file does not affect the active portion of the log at all, which means the backup chain is fully intact.  For more information, you should read this article,

    http://msdn.microsoft.com/en-us/library/ms178037.aspx.
    IF NOT EXISTS(SELECT 1 FROM sys.databases WHERE name = 'ShrinkLog_Test')
    BEGIN
        CREATE DATABASE [ShrinkLog_Test];
    END
    GO
     
    BACKUP DATABASE [ShrinkLog_Test]
    TO DISK = 'c:\ShrinkLog_Test.bak' WITH INIT, STATS=10
    GO
     
    BACKUP LOG [ShrinkLog_Test]
    TO DISK = 'c:\ShrinkLog_Test.trn' WITH INIT, STATS=10
    GO
     
    SELECT TOP 1 b.type, b.first_lsn, b.last_lsn
    FROM msdb..backupset b
    WHERE b.[database_name] = 'ShrinkLog_Test'
    ORDER BY [backup_start_date] DESC
     
    /*
    type first_lsn                               last_lsn
    ---- --------------------------------------- ---------------------------------------
    L    41000000005400064                       41000000009000001
    */
     
    USE [ShrinkLog_Test]
    GO
     
    DBCC SHRINKFILE('ShrinkLog_Test_log',1,TRUNCATEONLY)
    GO
     
    SELECT TOP 1 b.type, b.first_lsn, b.last_lsn
    FROM msdb..backupset b
    WHERE b.[database_name] = 'ShrinkLog_Test'
    ORDER BY [backup_start_date] DESC
     
    /*
    type first_lsn                               last_lsn
    ---- --------------------------------------- ---------------------------------------
    L    41000000005400064                       41000000009000001
    */
     
    USE master
    GO
     
    RESTORE DATABASE ShrinkLog_Test FROM DISK = 'c:\ShrinkLog_Test.bak' WITH norecovery, REPLACE
    GO
    RESTORE log ShrinkLog_Test FROM DISK = 'c:\ShrinkLog_Test.trn' WITH recovery, REPLACE
    GO
     
    /*
    Processed 168 pages for database 'ShrinkLog_Test', file 'ShrinkLog_Test' on file 1.
    Processed 3 pages for database 'ShrinkLog_Test', file 'ShrinkLog_Test_log' on file 1.
    RESTORE DATABASE successfully processed 171 pages in 0.061 seconds (21.780 MB/sec).
    Processed 0 pages for database 'ShrinkLog_Test', file 'ShrinkLog_Test' on file 1.
    Processed 5 pages for database 'ShrinkLog_Test', file 'ShrinkLog_Test_log' on file 1.
    RESTORE LOG successfully processed 5 pages in 0.043 seconds (0.794 MB/sec).
    */

    Backing up the transaction log frees up OS storage>

    The next myth is one of the most problematic ones because those who believe this become bedazzled when they backup their transaction log and it does not give the space back to the OS.  I would like to point out that I do not recommend shrinking data files unless you absolutely crunched for space.  Shrinking the log is the only way to return storage back to the OS.

    SET NOCOUNT ON
    GO
     
    USE [master]
    GO
     
    IF db_id('TestSimpleRecovery') IS NOT NULL
    BEGIN
        DROP DATABASE TestSimpleRecovery;
    END
    GO
     
    --Create DB
    CREATE DATABASE TestSimpleRecovery;
    GO
     
    --Change Recovery Model To Simple
    ALTER DATABASE TestSimpleRecovery SET RECOVERY FULL
    GO
     
    --Change the FileGrowth To 100MB
    ALTER DATABASE TestSimpleRecovery 
    MODIFY FILE(NAME = TestSimpleRecovery_Log,FILEGROWTH=100MB);
    GO
     
    BACKUP DATABASE TestSimpleRecovery TO DISK = 'C:\Test.bak'
     
    --Switch DB Context
    USE [TestSimpleRecovery]
    GO
     
    --Get Current Log Size (before XACT)
    SELECT ((size * 8) / 1024.) / 1024. AS SizeMBsBeforeTransaction
    FROM sys.[master_files]
    WHERE type = 1 AND [database_id] = db_id('TestSimpleRecovery')
    GO
     
    --Drop Sample Table
    IF EXISTS(SELECT 1 FROM sys.tables WHERE NAME = 'TestData')
    BEGIN
        DROP TABLE dbo.[TestData];
    END
    GO
     
    CREATE TABLE dbo.TestData(
    RowNum INT,
    SomeId INT,
    SomeCode CHAR(2)
    );
    GO
     
    DECLARE @i INT
    SET @i = 1
     
    BEGIN TRANSACTION
     
    WHILE @i < 100
    BEGIN
     
    INSERT INTO dbo.TestData
    SELECT TOP 1000
        ROW_NUMBER() OVER (ORDER BY t1.NAME) AS RowNumber,
        ABS(CHECKSUM(NEWID()))%2500+1 AS SomeId, 
        CHAR(ABS(CHECKSUM(NEWID()))%26+65)
        + CHAR(ABS(CHECKSUM(NEWID()))%26+65) AS SomeCode
    FROM 
        Master.dbo.SysColumns t1,
        Master.dbo.SysColumns t2
        
        SET @i = @i + 1 
    END
     
    COMMIT TRANSACTION
    GO
     
    --Get Current Log Size (After XACT)
    SELECT ((size * 8) / 1024.) / 1024. AS SizeMBsAfterTransaction
    FROM sys.[master_files]
    WHERE type = 1 AND [database_id] = db_id('TestSimpleRecovery')
    GO
     
    BACKUP log TestSimpleRecovery TO DISK = 'c:\test.trn'
    GO
     
    --Get Current Log Size (After Log Backup)
    SELECT ((size * 8) / 1024.) / 1024. AS SizeMBsAfterBackup
    FROM sys.[master_files]
    WHERE type = 1 AND [database_id] = db_id('TestSimpleRecovery')
    GO
     
    /*
    Processed 168 pages for database 'TestSimpleRecovery', file 'TestSimpleRecovery' on file 1.
    Processed 2 pages for database 'TestSimpleRecovery', file 'TestSimpleRecovery_log' on file 1.
    BACKUP DATABASE successfully processed 170 pages in 0.361 seconds (3.677 MB/sec).
    SizeMBsBeforeTransaction
    ---------------------------------------
    0.00054931640
     
    SizeMBsAfterTransaction
    ---------------------------------------
    0.09820556640
     
    Processed 1407 pages for database 'TestSimpleRecovery', file 'TestSimpleRecovery_log' on file 1.
    BACKUP LOG successfully processed 1407 pages in 3.002 seconds (3.659 MB/sec).
    SizeMBsAfterBackup
    ---------------------------------------
    0.09820556640
    */

    I am trying to keep this series manageable, so I will stop here and pickup the remaining items in part 2, of this series.  If you have any myths that you would like to see busted or have myths you would like to share, please feel free to leave me comments.  Stay tuned part 2!

    Until next time happy coding.

  • 7 comments:

    Brian Tkatch said...

    Good stuff.

    Typo: "some many" should be "so many"

    Adam Haines said...

    Thanks Brian. I dont know how I could have missed that one.

    Thanks,
    Adam

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    Pinal Dave said...

    Awesome!

    Can not wait to read the part 2.

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