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From Peter Figliozzi <pete.figlio...@gmail.com>
Subject code questions, sql.functions.scala
Date Fri, 30 Sep 2016 15:40:02 GMT
Taking isnan as a simple example, I'd like to understand what happens
downstream of sql.functions.scala.

1. The withExpr { } construct.  How does that work?  I see it refers to the
IsNan case class, which gets passed not the column e but e.expr, which is
an Expression.   I also see that withExpr is a private function of the
Column class.

2.  The IsNaN case class.  How does that work together with the call to
isnan?  I notice it only has to handle DoubleType and IntegerType, as
defined in its inputTypes override.  So what happens (or what has happened)
to any strings that were encountered in the call to .isnan?

3.  Expressions.  I see that values and datatypes are extracted from
Expressions.  But this happens in eval(), which is somehow passed an
Internal row.  It seems this is pretty central to understanding how data
gets to a sql.function to be acted on.  Can someone summarize how this
works?

Thanks much,

Pete

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