What is a bitmap in sqlserver ?what does bitmap actually contain.

Question

hi,

     I have read in some article that, null values are maintained using bitmap.but i cant get what exactly is bitmap and what is purpose?

Please do reply.

 

Regards

Chaithu

Answer 1

Just a series of bits, each can be 0 or 1. Each bit represents a column in the table and 0 means the column isn't null, where 1 means the column is null.

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Tibor Karaszi, SQL Server MVP | web | blog

Answer 2

Hi tibork,

        What is scope of the bit map.Like will it on the table or will it be for each row?

 

regards

Chaithu

         

Answer 3

Check out this similar post on bitmap indexing & filtering: http://social.msdn.microsoft.com/Forums/en-US/sqldatawarehousing/thread/3b7fc3a0-32e2-4c73-99c1-1357ebcc4753

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~manoj | email: http://scr.im/m22g
http://sqlwithmanoj.wordpress.com
MCCA 2011

Answer 4

The null bitmap exists for each record or row.  Paul Randal has some great blog posts that explain this:

http://www.sqlskills.com/BLOGS/PAUL/post/Search-Engine-QA-27-How-does-the-storage-engine-find-variable-length-columns.aspx
http://www.sqlskills.com/BLOGS/PAUL/post/A-SQL-Server-DBA-myth-a-day-(630)-three-null-bitmap-myths.aspx
http://www.sqlskills.com/BLOGS/PAUL/post/Misconceptions-around-null-bitmap-size.aspx
http://www.sqlskills.com/BLOGS/PAUL/post/Inside-the-Storage-Engine-Anatomy-of-a-record.aspx

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Jonathan Kehayias | Senior Consultant, SQLSkills.com
SQL Server MVP | Microsoft Certified Master: SQL Server 2008
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