INDEX REBUILD

Question

INDEX REBUILD

mw_sql_developer

SSCoach

Points: 19445
More actions
June 14, 2010 at 2:00 pm

#387152

Here is what I did
1. Ran the following command
ALTER INDEX ALL ON APPOINTMENTS Rebuild;
2. Ran the following query
select index_id, avg_page_space_used_in_percent,
avg_fragmentation_in_percent
from
sys.dm_db_index_physical_stats( DB_ID('NGDEVL'), OBJECT_ID ('appointments'),NULL, NULL, 'DETAILED' )
WHERE
index_id <> 0
3. The results seems find except for one little problem ( Please see attached index.jpg)
Why is Sql server telling me that the avg_fragmentation_in_percent for index 1 is 75 ?
I thought after doing a rebuild it should say 0 for all indexes ?

Viewing 15 posts - 1 through 15 (of 27 total)

You must be logged in to reply to this topic. Login to reply

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

How big is the index in question? How many pages?

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

mw_sql_developer SSCoach Points: 19445 More actions · Answer 2

You will be surprised. This table does not have more than 100 rows.

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 3

Not surprised in the slightest, that's exactly what I expected.

For reasons that have to do with the way the first few pages are allocated, there's virtually no point in rebuilding an index that occupies less than 24 pages. It's generally recommended not to worry about fragmentation for indexes with less than 1000 pages.

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

mw_sql_developer SSCoach Points: 19445 More actions · Answer 4

What would be the command to use to drop a particular index and then recreate it ?

What I mean is I'd like to build the index but not have to worry about the details ( how it was originally defined ).

Is there a general command available ?

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 5

Drop/create?

Create index ... with drop existing?

Alter index ... disable/rebuild?

What's the point? What are you trying to achieve?

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

Bhuvnesh SSC Guru Points: 59351 More actions · Answer 6

mw112009 (6/14/2010)
What I mean is I'd like to build the index but not have to worry about the details

what does it mean ?

-------Bhuvnesh----------
I work only to learn Sql Server...though my company pays me for getting their stuff done;-)

mw_sql_developer SSCoach Points: 19445 More actions · Answer 7

OK here is my point.

Lets go to the begining of this thread.

Here is what I did

1. Ran the following command

ALTER INDEX ALL ON APPOINTMENTS Rebuild;

2. Ran the following query

select index_id, avg_page_space_used_in_percent,

avg_fragmentation_in_percent

from

sys.dm_db_index_physical_stats( DB_ID('NGDEVL'), OBJECT_ID ('appointments'),NULL, NULL, 'DETAILED' )

WHERE

index_id <> 0

3. The query says that the fragmentation rate is 75 on index 1 ( I mean even after doing a rebuild why do a I get a fragmentation rate of 75 only on one particular index ? )

4. I also mentioned that the table had less than 100 rows.

5. So then why is SQl server returning wrong information ?

6. How else can we rebuild the indexes so that we get a fragmentation rate of 0 for all indexes. Is there another command out there that I don't know ( besides the Alter Index... Rebuild command )

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 8

mw112009 (6/15/2010)
3. The query says that the fragmentation rate is 75 on index 1 ( I mean even after doing a rebuild why do a I get a fragmentation rate of 75 only on one particular index ? )

Because the index is too small for rebuild to have much if any effect.

4. I also mentioned that the table had less than 100 rows.

And i mentioned that it is not worth worrying about fragmentation for indexes under about 1000 pages

5. So then why is SQl server returning wrong information ?

It's not, it's perfectly correct, the fragmentation of that index is 75%

6. How else can we rebuild the indexes so that we get a fragmentation rate of 0 for all indexes. Is there another command out there that I don't know ( besides the Alter Index... Rebuild command )

There's no guarantee that any index rebuild of any form will leave you with 0% fragmentation. Don't worry about small amounts of fragmentation and don't worry about fragmentation on very small indexes. I'm willing to bet this index is 4 pages in size, well under the size where you should start worrying.

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

mw_sql_developer SSCoach Points: 19445 More actions · Answer 9

So is there a command out there that I can use to count the size of the index in pages ?

You mentioned the size of the above index may be less than 4 pages ? How do we find that out ?

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 10

It's in the same DMV that you're using to find fragmentation - sys.dm_db_index_physical_stats. The column is called page_count.

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

mw_sql_developer SSCoach Points: 19445 More actions · Answer 11

Please ignore my question above. I found the answer. There is a field by the name "page_count"

that is returned by the query.

Question: So you are saying if a index has a page_count > 4 and if the fragmentation percentage

is above 60 ( or 75 ) it is worth doing a rebuild.

mw_sql_developer SSCoach Points: 19445 More actions · Answer 12

In one of the posts above you mentioned the following:

There's no guarantee that any index rebuild of any form will leave you with 0% fragmentation. Don't worry about small amounts of fragmentation and don't worry about fragmentation on very small indexes. I'm willing to bet this index is 4 pages in size, well under the size where you should start worrying.

So then at what page count should we consider rebuilding an index ?

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 13

Will you please read what I'm saying...

GilaMonster (6/14/2010)
For reasons that have to do with the way the first few pages are allocated, there's virtually no point in rebuilding an index that occupies less than 24 pages. It's generally recommended not to worry about fragmentation for indexes with less than 1000 pages.

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

mw_sql_developer SSCoach Points: 19445 More actions · Answer 14

Just out of curiosity, is there a bulk command that I can issue that will take care of all the indexes

in the database ( I am just wondering how those dba's do this when they have about 2000 tables in their database )