viirya commented on code in PR #39248:
URL: https://github.com/apache/spark/pull/39248#discussion_r1058775725
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala:
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@@ -127,6 +125,54 @@ abstract class Expression extends TreeNode[Expression] {
def references: AttributeSet = _references
+ /**
+ * Returns true if the expression contains mutable state.
+ *
+ * A stateful expression should never be evaluated multiple times for a
single row. This should
+ * only be a problem for interpreted execution. This can be prevented by
creating fresh copies
+ * of the stateful expression before execution. A common example to trigger
this issue:
+ * {{{
+ * val rand = functions.rand()
+ * df.select(rand, rand) // These 2 rand should not share a state.
+ * }}}
+ */
+ def stateful: Boolean = false
+
+ /**
+ * Returns a copy of this expression where all stateful expressions are
replaced with fresh
+ * uninitialized copies. If the expression contains no stateful expressions
then the original
+ * expression is returned.
+ */
+ def freshCopyIfContainsStatefulExpression(): Expression = {
+ val childrenIndexedSeq: IndexedSeq[Expression] = children match {
+ case types: IndexedSeq[Expression] => types
+ case other => other.toIndexedSeq
+ }
+ val newChildren =
childrenIndexedSeq.map(_.freshCopyIfContainsStatefulExpression())
+ // A more efficient version of `children.zip(newChildren).exists(_ ne _)`
+ val anyChildChanged = {
+ val size = newChildren.length
+ var i = 0
+ var res: Boolean = false
+ while (!res && i < size) {
+ res |= (childrenIndexedSeq(i) ne newChildren(i))
+ i += 1
+ }
+ res
+ }
+ // If the children contain stateful expressions and get copied, or this
expression is stateful,
+ // copy this expression with the new children.
+ if (anyChildChanged || stateful) {
+ CurrentOrigin.withOrigin(origin) {
+ val res = withNewChildrenInternal(newChildren)
+ res.copyTagsFrom(this)
Review Comment:
Yea, you're correct. It is just a string for node name.
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