Question

Difference between rebuild index and create reverse index on db level

Asked by: hans_vd

Hi,

On an oracle table i read 9 times the information.
Totale record in select:             81607

The avarage time reading
desc: 17,69877778
asc:  16,13633333
After i rebuild the index
The avarage time reading
desc: 17,077
asc:  14,01122222
so not a lot of time profit
After this i recreate the index with reverse
Then i recreate the index wihtout revers:
The avarage time reading
desc: 0,076777778
asc:  0,074555556

Can you explain me the difference between rebuild index en recreate index with reverse.  What happens putting on/off the reverse that after the reading goes a lot faster.

Thanks a lot

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Asked On
2006-08-07 at 23:28:22ID21946587
Tags

index

,

oracle

,

reverse

,

rebuild

Topic

Oracle Database

Participating Experts
3
Points
500
Comments
9

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Answers

 

by: gvsbnarayanaPosted on 2006-08-08 at 02:31:44ID: 17269860

Rebuild an Index --> Oracle create a new index and drops the existing one.. It is useful to reclaim the space utilized by the index after a mass deletion on the related table. When a large number of deletes occoured, then there migth be some "holes" in the index structure and you can reclaim the space as well as you can also decrease the depth of an index, which may result in performance.
Creating Reverse key index is a complete different matter. You are using a different mechanism to create your index.
Reverse Key indexes are more useful for a PK generated using a sequence.. Refer to oracle documentation for more details.
HTH
Regards,
Badri

 

by: schwertnerPosted on 2006-08-08 at 03:27:02ID: 17270118

By reversing the index you achive more balanced B-tree index.
This means the following:

LOOk at these key sequence (by insertion of records):

9872
9976
9651
9763

The B-tree will be disbalanced (all key points to one of the subtrees).
Oracle will need additional time to balance the tree.

When you use reverse keys this order will be automatically changed into

2789
6799
1569
3679

The keys will not disbalance the tree so fast and
there will be significantly less balancing activities.
This will spread the keys randomly in the tree

 

by: hans_vdPosted on 2006-08-08 at 04:19:18ID: 17270372

Hi,

thanks for the explanation, but i was more intrested if someone could explain me the time-difference.  If i rebuild the index the read-time does not improve a lot, but when is create a reverse index, and then remove the reverse index again, the read time is a lot faster.

best regards

 

by: markgeerPosted on 2006-08-08 at 06:35:39ID: 17271130

Are you sure that the index was used each time?  Maybe Oracle's query optimizer was using full-table scans part of the time.  Did you calculate statistics after rebuilding the index?  What else was happening in the database at the same time?  Maybe another big query caused the blocks of this table and/or index to be flushed out of the cache during your earlier tests, and not during your later tests?

 

by: schwertnerPosted on 2006-08-08 at 06:50:12ID: 17271256

I think you experience side effects of rebuilding the index.
When an index is created, rebuild all rows are read sequentially
in the db_buffer_cache. So they (or major part of them, or some part of them) will not be read from the disk physically. They will read from the RAM instead and this is much more faster.

 

by: hans_vdPosted on 2006-08-08 at 07:18:56ID: 17271487

The times showed are average values from 9 times reading for each result.  
So i read more then one time to calcuate the time.

 

by: markgeerPosted on 2006-08-08 at 07:24:00ID: 17271538

Right, I understood that you did the reads 9 times, but was anything else happening in the database at the same time?  Also, did you calculate the table and index statistics after the index rebuild?  What kind of statement did you use?  Are you sure that it used the index?

 

by: schwertnerPosted on 2006-08-08 at 23:19:50ID: 17276866

Be aware that without calculating the index and table statistics (also regularly) the CBO (Cost Based Optimizer) will not work and will not take in account the indexes on a table.
Building an index and not getting statistics is equal to not to have index at all.

 

by: hans_vdPosted on 2006-08-09 at 02:32:33ID: 17277506

Thanks,

i didn't do the index statistics.

20120131-EE-VQP-002

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