Пересылка: Which index is fastest

["Shulzhenko Vasyl" <vasilis@softline.kiev.ua>]

-----Исходное сообщение-----
От: Art S. Kagel <kagel@bloomberg.net>
Группы: comp.databases.informix
Кому: Sadagopan, Sathish, CFCTR <sadagop0458@att.com>
Дата: 11 марта 1999 г. 0:07
Тема: Re: Which index is fastest


>Sadagopan, Sathish, CFCTR wrote:
>>
>> table A(col1 serial, col2 char(1));
>> inserted 100,000 rows with col2="A"
>> inserted 100,000 rows with col2="B"
>>
>> I looked into sysindexes after an upd. stat.
>> index_A  (col1,col2)  has levels=3 , leaves=1552 ,nunique=200,000 ,
>> clust=893
>> index_b  (col2,col1)  has levels=3 , leaves=1551 , nunique=2 , clust=1786
>> Why does the number of leaves differ ?
>> What does clust denote ? Pl. explain.
>
>The field nunique is only updated when you run UPDATE STATISTICS LOW or
>HIGH (without the DISTRIBUTIONS ONLY option so it includes LOW).  The
>field clust is only relevant to a clustered index and is supposed to
>indicate the degree to which the table is still clustered as new rows
>are added it is also maintained by UPDATE STATISTICS LOW.  If you have
>not updated stats lately these values are meaningless.
>
>Art S. Kagel