title: Power BI should we have thick or thin branches in star schema model
body: <p><em>This question has no satisfactory answer. Please feel encouraged to answer it or comment.</em> </p>
  
  <p>Let's consider the following data model. We have three dimensions in our model. If you need to name them, be it (A) product, (B) brand, (C) region. B is a container for A, so it is a hierarchy. Many products in one brand. The tables denoted as A, B, C, AB, ABC are bridge tables which contain only unique values. </p>
  
  <p>Now the questions:</p>
  
  <ol>
  <li><p>Is the AB bridge table necessary in the following model? Couldn't we
  connect A and B tables directly to ABC.</p></li>
  <li><p>Is it a good idea to create Cartesian product for all the dimensions
  in the model as a <strong>Central Bridge Table</strong>?</p></li>
  <li><p>Should we plug Budget table with AB dimensions to bridge AB or to
  bridge ABC? Depending on what is the answer to first question.</p></li>
  <li><p>How should we plug Advertizing table to the model? To bridge ABC or to especially created bridge table BC and that one connect to ABC?</p></li>
  </ol>
  
  <p>Now the schema:</p>
  
  <pre><code>   +-------+
     |       |
     |  A    +-----+
     |       |     |
     +-------+     |
                   |
                   v
     +-------+  +--+----+      +--------+      +------------+
     |       |  |       |      |        |      | Sales      |
     |  B    +--&gt;  AB   +-----&gt;|  ABC   +-----&gt;|   ABC      |
     |       |  |       |      |        |      |            |
     +-------+  +--+----+      +---+----+      +------------+
                                   ^
                                   |
     +-------+                     |           +------------+
     |       |                     |           | Budget     |
     |  C    +---------------------+           |   AB       |
     |       |                                 |            |
     +-------+                                 +------------+
  
  
                                               +------------+
                                               | Advertizing|
                                               |   BC       |
                                               |            |
                                               +------------+
  </code></pre>
  
  <p><strong>DAX bridging.</strong></p>
  
  <p>I love to construct bridge tables in DAX, not in M. There a few reasons for that. First, it is done with simple code. Second, it introduces sort of tidiness to Query Editor because I see there only source tables (not bridges). </p>
  
  <p><a href="https://stackoverflow.com/q/57074718/1903793">Bridge tables - DAX or M?</a></p>
  
  <p>So creating a Bridge for A dimension looks like this:</p>
  
  <pre><code>#A =
  DISTINCT (
      UNION (
          TOPN ( 0, ROW ( "A", "Apple" ) ),
          DISTINCT ( Sales[A] ),
          DISTINCT ( Budget[A] ),
          DISTINCT ( Advertizing[A] )
      )
  )
  </code></pre>
  
  <p>Bridge for AB would be a Cartesian product of so created A and B:</p>
  
  <pre><code>AB =
  CROSSJOIN (
      DISTINCT ( '#A'[A] ),
      DISTINCT ( '#B'[B] ),
      "A@B", COMBINEVALUES("@",'#A'[A], '#B'[B])
  )
  </code></pre>
  
  <p><strong>Update after receiving the first answer.</strong></p>
  
  <p>I don't want to edit the content of my question once the bounty has started. After the first answer, I realized that hierarchies came to my question by accident and distract you from what I would like to find out. You can forget about hierarchies and treat dimensions A, B, C as independent dimensions.</p>
  
  <p>I would like to focus on how to build a star schema in case we have many independent dimensions and some tables, say dictionaries with joined dimensions. For example, we can have a sales budget defined by region and brand, and an advertising budget defined by product_color. Should we build a central bridge table that has all dimensions (Cartesian product of ABC)? Or alternatively, should the central bridge table have thickbranches with many dimensions? In the second case we would have [AB] -> [ABC] &lt;- [BC].</p>