[Mondrian] Shared degenerate dimensions

Matt Campbell mcampbell at pentaho.com
Mon Oct 20 12:16:17 EDT 2014


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.

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 (level.members needs to return a consistent set).

Curious if others have thoughts.

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