Commits on Jun 3, 2019

1. sql: implement EXPLODE and generators …

   This pull request implements the EXPLODE expressions and generators,
   which are used together to provide generation of rows based on a
   column or expression that can yield multiple values.
   
   For this, there have been some quite core changes:
   - SQL method of sql.Type now returns the sqltypes.Value and an error.
     This avoids panics in this method. Since generators can yield errors
     it's better to just return an error and not just kill the server.
   - Array type can now internally handle either arrays or generators,
     making it transparent for the user. If it's used in an EXPLODE
     expression, the generator will be used. Otherwise, it will be
     converted automatically to an array and used without the user even
     knowing what happened behind the scenes. That way, we don't need
     yet another type to represent generators.
   
   Aside from that, there are some new additions:
   - New EXPLODE function, which is just a placeholder. EXPLODE type is the
     underlying type of its argument for resolution purposes. During analysis
     Explode nodes will be replaced by Generate functions and a Generate node.
   - New Generate function, which is the non-placeholder version of EXPLODE
     and the one that goes into the final execution tree. This one returns
     the same type as its argument and let's the Generate plan node be the
     one that returns the underlying type once the values are generated.
   - Generate node, which wraps a Project in which there is one and only one
     explode expression.
   - resolve_generators analysis rule, which will turn projects with an
     Explode expression into a Generate node with the project as a children,
     replacing the Explode expressions with Generate expressions.
   - validate_explode_usage validation rule, which will ensure explode is
     not used outside a Project node.
   
   Signed-off-by: Miguel Molina <miguel@erizocosmi.co>

   

   erizocosmico committed Jun 3, 2019
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