Why there is a Limitations on Index size (900 Bytes)?

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Why there is a Limitations on Index size (900 Bytes)?

SQL*

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Points: 6638
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August 1, 2012 at 3:46 am

#259587

Hi,
Is it correct that all the pages, index/data/iam..etc pages does have the 8KB in size? if yes, why we have this limitation. if not what is the size of the each and every page by tyep. i.e., what is the size of the Index page? What is the max size of the Data Page?
Thank you,
🙂

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Gail Shaw SSC Guru Points: 1004504 More actions · Answer 1

Yes, all pages are 8kb in size.

Limitation probably to keep the size of the index tree down. Wouldn't make any sense to have 8kb index key, the index tree would be incredibly deep and very inefficient.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SQL* SSCertifiable Points: 6638 More actions · Answer 2

GilaMonster (8/1/2012)
Yes, all pages are 8kb in size.
Limitation probably to keep the size of the index tree down. Wouldn't make any sense to have 8kb index key, the index tree would be incredibly deep and very inefficient.

What about the remaining free space? Around 7 Kb of space will be empty or will it contain any information?

Is the statement is correct that you have provided?

Thanks,

🙂

Lowell SSC Guru Points: 323518 More actions · Answer 3

it's not a one to one relationship. more than one index value will be able to fit in each 8k page;

so an index will be made up of as many pages it needs to hold all it's data.

i think you were thinking each index key goes on it's own page, and that's not how the space would be used.

Lowell

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Gail Shaw SSC Guru Points: 1004504 More actions · Answer 4

SQL* (8/1/2012)
GilaMonster (8/1/2012)
Yes, all pages are 8kb in size.
Limitation probably to keep the size of the index tree down. Wouldn't make any sense to have 8kb index key, the index tree would be incredibly deep and very inefficient.
What about the remaining free space? Around 7 Kb of space will be empty or will it contain any information?

What happens to a data page when you create a row of only 900 bytes? Same thing with an index page.

SQL doesn't put one row per page, it fills the page with as many rows as possible. In the case of 900 byte rows, fitting 9 rows to the page.

I think you may want to revise basic index architecture.

http://www.sqlservercentral.com/articles/Indexing/68439/

http://www.sqlservercentral.com/articles/Indexing/68563/

http://www.sqlservercentral.com/articles/Indexing/68636/

Is the statement is correct that you have provided?

Err... yes.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SQL* SSCertifiable Points: 6638 More actions · Answer 5

Thank you all (GilaMonster), for your answers.

🙂