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<p class="MsoNormal"><span style="color:#1F497D">Very interested in shared degenerate dimensions. Besides the BFT use case, I’ve seen examples in a conventional star schema, where users wanted to treat a degenerate dimension that occurred on >1 fact table as
the same dimension.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">I’ve seen both these cases:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">At least one fact table can be treated as “complete”, listing all members of the shared dimension.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">The set of members is a union across all fact tables.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">It would be nice if the mondrian schema could define that the members of a degenerate dimension could be obtained from:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">-<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Distinct values in the fact table<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">-<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Distinct values across multiple fact tables<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">-<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Lookup from a table of distinct values created by ETL<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">That last option could even help performance of member queries for non-shared degenerate dimensions.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">--jeff<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> mondrian-bounces@pentaho.org [mailto:mondrian-bounces@pentaho.org]
<b>On Behalf Of </b>Matt Campbell<br>
<b>Sent:</b> Monday, October 20, 2014 12:16 PM<br>
<b>To:</b> Mondrian developer mailing list<br>
<b>Subject:</b> [Mondrian] Shared degenerate dimensions<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Data models with “Big Flat Tables” instead of star schemas seem to be getting more common, driven in part by columnar databases where use of separate dimension tables has little or no performance/storage benefit. Similarly, with document
stores like mongodb it often makes sense to store dimensional data together with facts in the same collection. Use of the Thin Kettle JDBC with Mondrian also results in BFT models, since Thin Kettle can only expose a single table with no ability to join.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This data model is far simpler, but loses important power and flexibility. Dimensions in cubes built on a BFT are degenerate and are un-sharable in Mondrian. But even with BFTs users will still want to combine data with overlapping but
non-identical dimensionalities. I.e. they still have motivations to use Virtual Cubes (or multiple measure groups in M4). I’m wondering what options might be worth considering to support this use case. Should there be a new dimension link type to allow
sharing degenerate dimensions? The idea of a shared degen dimension feels a bit like a CopyLink to me. It also seems like there would still need to be a single “official” source for the full set of members (<i>level.</i>members needs to return a consistent
set).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Curious if others have thoughts.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph"><o:p> </o:p></p>
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