cloud-fan commented on code in PR #36455:
URL: https://github.com/apache/spark/pull/36455#discussion_r866056903
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala:
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@@ -454,6 +454,33 @@ trait Nondeterministic extends Expression {
protected def evalInternal(input: InternalRow): Any
}
+/**
+ * An expression that contains predicate expression branch, so not all branch
will be hit
+ * at runtime. All optimization should be careful with the evaluation order.
+ */
+trait ConditionalExpression extends Expression {
+ /**
+ * Return the expression which can always be hit at runtime, For example:
+ * 1. If: common subexpressions will always be evaluated at the beginning,
but the true and
+ * false expressions in `If` may not get accessed, according to the
predicate
+ * expression. We should only return the predicate expression.
+ * 2. CaseWhen: like `If`, the children of `CaseWhen` only get accessed in a
certain
+ * condition. We should only return the first condition
expression as it
+ * will always get accessed.
+ * 3. Coalesce: it's also a conditional expression, we should only return
the first child,
+ * because others may not get accessed.
+ * 4. NaNvl: we can only guarantee the left child can be always accessed.
+ * And if we hit the left child, the right will not be accessed.
+ */
+ def head: Expression = children.head
Review Comment:
Since we are doing abstraction here, let's make it more general. I think
there are 2 things we need to care about for a condition expression:
1. inputs that will always be evaluated. Today all conditional expressions
only have one input that will always be evaluated, but I don't see why it can't
be a `Seq`. How about `def alwaysEvaluatedInputs: Seq[Expression]`
2. groups of branches. For each group, at least one branch will be hit at
runtime, so that we can eagerly evaluate the common expressions of a group. How
about `def branchGroups: Seq[Seq[Expression]]`
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