Faculty of Engineering and Mathematical Sciences 
Not logged in (login)

help3401


This forum is provided to promote discussion amongst students enrolled in Data Warehousing (CITS3401).
 
Options:
RSS cloud
Jump to:

Lecture 4: 60,000 possible dimension table rows

3 of 239 articles shown, currently 1 other person reading this forum.
photo
From: Edward A.
Date: Tue 24th Mar, 9:57pm
Actions: 
        Login-to-reply
Hi Zeyi ,

I was just wondering how you calculated that each dimension table in scenario C for the example you ran through in lecture 4 would have 60,000 possible rows? 

Wouldn't it be possible for a dimension to have 100 million unique values if there are 100 million facts in the fact table? How can you be certain there would be only 60,000 possible values for each dimension, or is that simply an assumption?

Lecture 4: 60,000 possible dimension table rows

photo
From: Edward A.  O.P.
Date: Sun 29th Mar, 11:44am
Actions: 
        Login-to-reply
"Edward Atkinson"                               wrote:

> I was just wondering how you calculated that each dimension table in scenario C for the example you ran through in lecture 4 would have 60,000 possible rows? 
> 

To answer my own question in the hope it might help someone else. The 60,000 unique values for each dimension limitation is a product of the fact that a 2 byte key is used to index rows in the dimension tables. This means that 2^16 - 1 rows can be stored, 65536 rows, therefore giving us our 60,000 unique value limit. 

> Wouldn't it be possible for a dimension to have 100 million unique values if there are 100 million facts in the fact table? How can you be certain there would be only 60,000 possible values for each dimension, or is that simply an assumption?

In the context of scenario A the assumption that there are only 60,000 unique values for each dimension also still makes sense as the likelihood of two rows in a dimension table of  100 million rows having the same values for 100 attributes with 60,000 possible levels is practically 0. 

Lecture 4: 60,000 possible dimension table rows

photo
From: Zeyi W.
Date: Sun 29th Mar, 8:35pm
Actions: 
        Login-to-reply
Good answers. They are extreme/simplified examples. Based on your answers, I believe you have got the idea :)
This Page


Program written by: [email protected]
Feedback welcome
Last modified:  8:27am May 24 2020