Following instructions on this page I am trying to create a Data Connector to be used as a base class for a cube with update support.
That page suggests that putting an SQL query inside XData block is not suitable for a Data Connector that supports updates.
Later it also suggests that to enable updates your SQL query must include 

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I was having trouble with some dimension based calculated members defined in Analyzer.  They would work and then not work.   At one point I had two identical calculated measures, same code, same specification and one would work and one would not.  I was trying to set them up in "shared storage", that checkbox in the calculated member dialog that should allow the measures to be used across multiple pivots. 

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