DataFusion - Allow Column Filters

My current use case for this is a table that is basically using a Single Table inheritance model. I want to join to another table, but the foreign key is only accurate if the discriminator column is a certain known value. If I could ensure that this column was equal to that value, then I would know that my join is valid.


I am not asking for the ability to perform multi-child polymorphic join here (I think that would require both this feature and in most practical cases a DataFusion feature supporting arbitrary column selection, in order to support achieving it through an "Add Rows" UNION fusion.) Rather, I think it would be nice to provide simple filters against literal values as part of a DataFusion. My discriminator check is a special case of this need.


Currently, this is achievable by prefiltering either in the initial DataSet or via a DataFlow. However, it seems that this should be possible via a non-materialized view, which is what I understand the DataFusion to represent.

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