[GitHub] [spark] HyukjinKwon commented on a change in pull request #28050: [MINOR][DOC] Refine comments of QueryPlan regarding subquery

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HyukjinKwon commented on a change in pull request #28050: [MINOR][DOC] Refine comments of QueryPlan regarding subquery
URL: https://github.com/apache/spark/pull/28050#discussion_r399265273
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
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 @@ -23,6 +23,16 @@ import org.apache.spark.sql.catalyst.trees.{CurrentOrigin, TreeNode, TreeNodeTag
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types.{DataType, StructType}
 
+/**
+ * An abstraction of the Spark SQL query plan tree, which can be logical or physical. This class
+ * defines some basic properties of a query plan node, as well as some new transform APIs to
+ * transform the expressions of the plan node.
+ *
+ * Note that, the query plan is a mutually recursive structure:
+ *   QueryPlan -> Expression (subquery) -> QueryPlan
+ * The tree traverse APIs like `transform`, `foreach`, `collect`, etc. that are
 
 Review comment:
   Can we clarify which APIs can go through query plans inside subqueries too?

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