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IDS Forum

Re: B-tree height

Posted By: Khaled Bentebal
Date: Wednesday, 11 January 2012, at 6:12 p.m.

In Response To: Re: B-tree height (FRANK)

Frank,

Well indexing a column of type varchar(255) is not the greatest thing
but if you have too ...

Before you go ahead and move the index to another dbspace based on a 4K,
8K or 16K page, try to copy the source table to a raw table with the
same structure, create the same index on the raw table. This will give
you an idea how many levels you really need and how many pages it occupies.

By the way, when you create the idex, do not gorget to set PSORT_NPROCS
et PDQPRIORITY to 100 if you use the Ultimate edition; otherwise you do
not have access to PDQ nor fragmentation. This will make the index
creation faster.

You can also consider fragenting the index to lower the levels since the
levels are per fragment.

On the side, I would recommand to have a char instead of a varchar for
the column, and you have to use the varchar, specify a max and also a
min value (ex: col1 varchar(255,30)) with the min value being the most
common min value . This will lake updates faster since no movement of
rows will take place within a page unless the varchar groes beyong the
min value.

Cordialement, Regards,

Khaled Bentebal
Directeur Général - ConsultiX
Président UGIF - User Group Informix France
IIUG - Board of Directors
Tél: 33 (0) 1 39 12 18 00
Fax: 33 (0) 1 39 12 18 18
Mobile: 33 (0) 6 07 78 41 97
Email: khaled.bentebal@consult-ix.fr
Site Web: www.consult-ix.fr

Le 11/01/12 23:27, FRANK a écrit :
> Thanks lot, Khaled!
>
> We have a table, it has only 5.5 millions rows, You know what, its index
> tree level is 7! Close to break your record!
>
> Why?The reason is: we have 2k page size, and the idex key size is
> varchar(255)!
>
> I am going to move this index to a larger page size dbspace....
>
> Thanks,
> Frank
>
> On Wed, Jan 11, 2012 at 5:03 PM, Khaled Bentebal<
> khaled.bentebal@consult-ix.fr> wrote:
>
>> HI Frank,
>>
>> The B-TREE index max level is 20. This limit is in the source code of
>> the product. You cannot play around with it. It is not a parameter in a
>> config file.
>>
>> Remember, the BTREE index level is exponential. The lower the level the
>> better. That is why , we suggest to drop and recreate the indexes that
>> are related to very volatile tables. evry so often to make them more
>> compact and faster.
>>
>> There is no mark of bad or good for indexes as far as index levels. The
>> lower the better. The BTREE (Balanced TREE) is a tree that has to be
>> balanced at all times. That is why BTREEs can go down a level; done
>> implicitely but the engine for you when you do an insert, delete or eben
>> an update. That is why a insert can very fast sometimes when no
>> rebalancing is needed and can be slower if you happen to be at the time
>> when the engine has to rebalance the tree.
>>
>> For your info (depending on the size of the key), an index for table
>> with a 300 million rows goes down to 6 levels. To go to 7 levels, your
>> table has to be huge. To go to 8 levels, it is even bigger. I do not
>> think that people using Informix have seen indexes with 9 levels. If so,
>> we would like to know.
>>
>> If you run oncheck -pT on a table you can see how pages at an index
>> level are filled. That light give you an idea if it needs to be
>> recreated or not.
>>
>> Cordialement, Regards,
>>
>> Khaled Bentebal
>> Directeur Général - ConsultiX
>> Président UGIF - User Group Informix France
>> IIUG - Board of Directors
>> Tél: 33 (0) 1 39 12 18 00
>> Fax: 33 (0) 1 39 12 18 18
>> Mobile: 33 (0) 6 07 78 41 97
>> Email: khaled.bentebal@consult-ix.fr
>> Site Web: www.consult-ix.fr
>>
>> Le 11/01/12 20:25, FRANK a écrit :
>>> Folks,
>>>
>>> IDS11.50FC8, Linux.
>>>
>>> What is the limitation of B-tree index levels? Any recommendation on the
>>> level mark of BAD?
>>>
>>> Thanks,
>>> Frank
>>>
>>> --f46d043c089a8768d004b64596d9
>>>
>>>
>>>
>>
> *******************************************************************************
>>> Forum Note: Use "Reply" to post a response in the discussion forum.
>>>
>>>
>>
>>
>>
> *******************************************************************************
>> Forum Note: Use "Reply" to post a response in the discussion forum.
>>
>>
> --f46d0444e9f127adbc04b648218c
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

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