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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