# Thread: When is a Dimension Shared?

1. Junior Member
Join Date
Jul 2007
Posts
21

## When is a Dimension Shared?

Hi All
I'm wondering about the creation of virtual cubes and how shared dimensions are expected to be defined in the XML Schema document.
With two source cubes, it is easy, a dimension is shared if it is in both cubes; but what happens when there are three cubes?
Is it possible to make a virtual cube from three source cubes? If so, is a dimension shared when it appears in every cube, or just two of them?

Say I have:
Cube1: Dim_A, Dim_B
Cube2: Dim_A, Dim_C, Dim_D
Cube3: Dim_A, Dim_B, Dim_C

Can I make a Virtual cube as follows
VirtualCube: Dim_A, Dim_B, Dim_C, Dim_D

<VirtualCube>
<VirtualCubeDimension name="Dim_A"/>
<VirtualCubeDimension name="Dim_B"/>
<VirtualCubeDimension name="Dim_C"/>
<VirtualCubeDimension cubeName="Cube3" name="Dim_D"/>
</VirtualCube>

For dimensions Dim_B and Dim_C, neither appear in all cubes, but they appear in more than one - should they have the cubeName attribute, and if so, which cube should they be specified as coming from (or does it not actually matter)?

I hope this doesn't make you head hurt as much as mine

TIA
Alan

Join Date
Nov 1999
Posts
1,618
This ought to work - but we've not tested beyond two. Things which haven't been tested don't usually work, but it's worth a try.

Virtual cube dimensions have to come from shared dimensions. If a cube doesn't have a dimension based on the shared dimension that's fine - the dimension will just float for that cube.

What we don't have is the ability to disambiguate if a cube uses a shared dimension more than once. But I don't think you need that.

(Will - you've studied this more than me. Please chime in if I'm talking out of my ass!)

Julian

3. Junior Member
Join Date
Jul 2007
Posts
21
Thanks Julian,
So, if I understand you correctly, theoretically, if I create all my dimensions as shared then I can use these dimensions in one or more cubes; these cubes being either "real" or virtual. In these cases, I don't need to qualify which cube the dimension is to come from.
If I have a dimension that is specific to one cube, I need to qualify it with the cube that it has come from when using it in a virtual cube.
I understand if you haven't tested more than 2 yet, are there plans to go beyond 2 in the testing or will you wait to hear what we come back with?

Cheers
Alan

Join Date
Nov 1999
Posts
1,618
We will be improving virtual dimension support in mondrian-3.0. It's in the roadmap, and Will Gorman will be doing the work.

If encounter issues, please log bugs. Hint: We will be much more likely to fix the bugs if the bugs include testcases against the foodmart schema.

Julian

5. wgorman Guest
Hi Alan,

As Julian mentioned, I'm working on the plumbing for better support of shared dimensions. I'm about to begin work on the virtual cube portion of the functionality, so your thoughts are appreciated. And as Julian pointed out, any test cases based on the foodmart data schema are also greatly appreciated!

Will

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