[GitHub] [spark] mgaido91 commented on a change in pull request #24831: [SPARK-19799][SQL] Better support for WITH clause

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mgaido91 commented on a change in pull request #24831: [SPARK-19799][SQL] Better support for WITH clause
URL: https://github.com/apache/spark/pull/24831#discussion_r299054885
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CTESubstitution.scala
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.analysis
+
+import org.apache.spark.sql.catalyst.expressions.SubqueryExpression
+import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, With}
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.internal.SQLConf
+
+/**
+ * Analyze WITH nodes and substitute child plan with CTE definitions.
+ */
+object CTESubstitution extends Rule[LogicalPlan] {
+  def apply(plan: LogicalPlan): LogicalPlan = if (SQLConf.get.legacyCTESubstitutionEnabled) {
+    legacyTraverseAndsubstituteCTE(plan)
+  } else {
+    traverseAndSubstituteCTE(plan, false)
+  }
+
+  def legacyTraverseAndsubstituteCTE(plan: LogicalPlan): LogicalPlan = plan.resolveOperatorsUp {
+    case With(child, relations) =>
+      // substitute CTE expressions right-to-left to resolve references to previous CTEs:
+      // with a as (select * from t), b as (select * from a) select * from b
+      relations.foldRight(child) {
+        case ((cteName, ctePlan), currentPlan) => substituteCTE(currentPlan, cteName, ctePlan)
+      }
+    case other => other
+  }
+
+  /**
+   * Traverse the plan and expression nodes as a tree and replace matching references to CTE
+   * definitions.
+   * - If the rule encounters a WITH node then it substitutes the child of the node with CTE
+   *   definitions of the node right-to-left order as a definition can reference to a previous
+   *   one.
+   *   For example the following query is valid:
+   *   WITH
+   *     t AS (SELECT 1),
+   *     t2 AS (SELECT * FROM t)
+   *   SELECT * FROM t2
+   * - If a CTE definition contains an inner WITH node then substitution of inner should take
+   *   precedence because it can shadow an outer CTE definition.
+   *   For example the following query should return 2:
+   *   WITH
+   *     t AS (SELECT 1),
+   *     t2 AS (
+   *       WITH t AS (SELECT 2)
+   *       SELECT * FROM t
+   *     )
+   *   SELECT * FROM t2
+   * - If a CTE definition contains a subquery that contains an inner WITH node then substitution
+   *   of inner should take precedence because it can shadow an outer CTE definition.
+   *   For example the following query should return 2:
+   *   WITH t AS (SELECT 1 AS c)
+   *   SELECT max(c) FROM (
+   *     WITH t AS (SELECT 2 AS c)
+   *     SELECT * FROM t
+   *   )
+   * - If a CTE definition contains a subquery expression that contains an inner WITH node then
+   *   substitution of inner should take precedence because it can shadow an outer CTE
+   *   definition.
+   *   For example the following query should return 2:
+   *   WITH t AS (SELECT 1)
+   *   SELECT (
+   *     WITH t AS (SELECT 2)
+   *     SELECT * FROM t
+   *   )
+   * @param plan  the plan to be traversed
+   * @param inTraverse whether the current traverse is called from another traverse, only in this
+   *                   can name collision occur
+   * @return then plan where CTE substitution is applied
+   */
+  private def traverseAndSubstituteCTE(plan: LogicalPlan, inTraverse: Boolean): LogicalPlan =
+    plan.resolveOperatorsUp {
+      case With(child: LogicalPlan, relations) =>
+        val traversedChild = child transformExpressions {
+          case e: SubqueryExpression => e.withNewPlan(traverseAndSubstituteCTE(e.plan, true))
+        }
+
+        relations.foldRight(traversedChild) {
+          case ((cteName, ctePlan), currentPlan) =>
+            substituteCTE(currentPlan, cteName, traverseAndSubstituteCTE(ctePlan, true))
+        }
+
+      case o if inTraverse =>
+        o.transformExpressions {
+          case e: SubqueryExpression => e.withNewPlan(traverseAndSubstituteCTE(e.plan, inTraverse))
+        }
+    }
+
+  // A CTE definition might not be used at all so ctePlan is call by name.
+  private def substituteCTE(
+      plan: LogicalPlan,
+      cteName: String,
+      ctePlan: => LogicalPlan): LogicalPlan = {
+    lazy val lazyCTEPlan = ctePlan
 
 Review comment:
   why do we need this?

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