Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/11647#discussion_r55958696
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
---
@@ -598,3 +601,61 @@ abstract class TernaryExpression extends Expression {
}
}
}
+
+/**
+ * Rewrites an expression using rules that are guaranteed preserve the
result while attempting
+ * to remove cosmetic variations. Deterministic expressions that are
`equal` after canonicalization
+ * will always return the same answer given the same input (i.e. false
positives should not be
+ * possible). However, it is possible that two canonical expressions that
are not equal will in fact
+ * return the same answer given any input (i.e. false negatives are
possible).
+ *
+ * The following rules are applied:
+ * - Names and nullability hints for
[[org.apache.spark.sql.types.DataType]]s are stripped.
+ * - Commutative and associative operations ([[Add]] and [[Multiply]])
have their children ordered
+ * by `hashCode`.
+* - [[EqualTo]] and [[EqualNullSafe]] are reordered by `hashCode`.
+ * - Other comparisons ([[GreaterThan]], [[LessThan]]) are reversed by
`hashCode`.
+ */
+object Canonicalize extends {
--- End diff --
fwiw, if you are keeping this class, i think we should just have it in its
own file. The Expression.scala file is getting pretty long.
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