Github user gatorsmile commented on a diff in the pull request: https://github.com/apache/spark/pull/23248#discussion_r240079120 --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/python/ExtractPythonUDFs.scala --- @@ -131,8 +131,20 @@ object ExtractPythonUDFs extends Rule[LogicalPlan] with PredicateHelper { expressions.flatMap(collectEvaluableUDFs) } - def apply(plan: LogicalPlan): LogicalPlan = plan transformUp { - case plan: LogicalPlan => extract(plan) + def apply(plan: LogicalPlan): LogicalPlan = plan match { + // SPARK-26293: A subquery will be rewritten into join later, and will go through this rule + // eventually. Here we skip subquery, as Python UDF only needs to be extracted once. + case _: Subquery => plan --- End diff -- Basically, we want to ensure this rule is running once and only once. In the future, if we have another rule/function that calls Optimizer.this.execute(plan), this rule needs to be fixed again... We have a very strong hidden assumption in the implementation. This looks risky in the long term. The current fix is fine for backporting to 2.4.
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