Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/18931#discussion_r136741637
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala
---
@@ -149,14 +149,146 @@ trait CodegenSupport extends SparkPlan {
ctx.freshNamePrefix = parent.variablePrefix
val evaluated = evaluateRequiredVariables(output, inputVars,
parent.usedInputs)
+
+ // Under certain conditions, we can put the logic to consume the rows
of this operator into
+ // another function. So we can prevent a generated function too long
to be optimized by JIT.
+ // The conditions:
+ // 1. The parent uses all variables in output. we can't defer variable
evaluation when consume
--- End diff --
E.g., a `ProjectExec` node doesn't necessarily evaluate all its output
variables before continuing `doConsume` of its parent node. It can defer the
evaluation until the variables are needed in the parent node's consume logic.
Once a variable's evaluation is deferred, and if we create a consume
function, the variable will be evaluated in the function. But now the
references of this variable is out of scope.
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