Github user yhuai commented on a diff in the pull request: https://github.com/apache/spark/pull/13084#discussion_r63133598 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala --- @@ -222,6 +222,33 @@ trait Unevaluable extends Expression { /** + * An expression that gets replaced at runtime (currently by the optimizer) into a different + * expression for evaluation. This is mainly used to provide compatibility with other databases. + * For example, we use this to support "nvl" by replacing it with "coalesce". + */ +trait RuntimeReplaceable extends Unevaluable { --- End diff -- This is very cool!
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