Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/22957#discussion_r238028577
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
---
@@ -223,14 +223,35 @@ abstract class Expression extends
TreeNode[Expression] {
}
/**
- * Returns true when two expressions will always compute the same
result, even if they differ
+ * Returns true when two expressions will always compute the same
output, even if they differ
* cosmetically (i.e. capitalization of names in attributes may be
different).
*
* See [[Canonicalize]] for more details.
+ *
+ * This method should be used (instead of `sameResult`) when comparing
if 2 expressions are the
+ * same and one can replace the other (eg. in Optimizer/Analyzer rules
where we want to replace
+ * equivalent expressions). It should not be used (and `sameResult`
should be used instead) when
+ * comparing if 2 expressions produce the same results (in this case
`semanticEquals` can be too
+ * strict).
*/
def semanticEquals(other: Expression): Boolean =
deterministic && other.deterministic && canonicalized ==
other.canonicalized
+ /**
+ * Returns true when two expressions will always compute the same
result, even if the output from
+ * plan perspective may be different, because of different names or
similar differences.
+ * Usually this means that their canonicalized form equals, but it may
also not be the case, as
+ * different output expressions can evaluate to the same result as well
(eg. when an expression
+ * is aliased).
+ *
+ * This method should be used (instead of `semanticEquals`) when
checking if 2 expressions
+ * produce the same results (eg. as in the case we are interested to
check if the ordering is the
+ * same). It should not be used (and `semanticEquals` should be used
instead) when comparing if 2
+ * expressions are the same and one can replace the other.
+ */
+ final def sameResult(other: Expression): Boolean =
--- End diff --
Add unit test cases to SubexpressionEliminationSuite?
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