[SPARK-32945][SQL] Avoid collapsing projects if reaching max allowed common exprs

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -732,10 +732,12 @@ object ColumnPruning extends Rule[LogicalPlan] {
  *    `GlobalLimit(LocalLimit)` pattern is also considered.
  */
 object CollapseProject extends Rule[LogicalPlan] {
-
   def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
     case p1 @ Project(_, p2: Project) =>
-      if (haveCommonNonDeterministicOutput(p1.projectList, p2.projectList)) {
+      val maxCommonExprs = SQLConf.get.maxCommonExprsInCollapseProject
+
+      if (haveCommonNonDeterministicOutput(p1.projectList, p2.projectList) ||
+        getLargestNumOfCommonOutput(p1.projectList, p2.projectList) >= maxCommonExprs) {
         p1
       } else {
         p2.copy(projectList = buildCleanedProjectList(p1.projectList, p2.projectList))
@@ -766,6 +768,23 @@ object CollapseProject extends Rule[LogicalPlan] {
     })
   }
 
+  // Counts for the largest times common outputs from lower operator are used in upper operators.
+  private def getLargestNumOfCommonOutput(
+      upper: Seq[NamedExpression], lower: Seq[NamedExpression]): Int = {
+    val aliases = collectAliases(lower)
+    val exprMap = mutable.HashMap.empty[Attribute, Int]
+
+    upper.foreach(_.collect {
+      case a: Attribute if aliases.contains(a) => exprMap.update(a, exprMap.getOrElse(a, 0) + 1)
+    })
+
+    if (exprMap.size > 0) {
+      exprMap.maxBy(_._2)._2
+    } else {
+      0
+    }
+  }
+
   private def haveCommonNonDeterministicOutput(
       upper: Seq[NamedExpression], lower: Seq[NamedExpression]): Boolean = {
     // Create a map of Aliases to their values from the lower projection.

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -1926,6 +1926,19 @@ object SQLConf {
     .booleanConf
     .createWithDefault(true)
 
+  val MAX_COMMON_EXPRS_IN_COLLAPSE_PROJECT =
+    buildConf("spark.sql.optimizer.maxCommonExprsInCollapseProject")
+      .doc("An integer number indicates the maximum allowed number of a common expression " +
+        "can be collapsed into upper Project from lower Project by optimizer rule " +
+        "`CollapseProject`. Normally `CollapseProject` will collapse adjacent Project " +
+        "and merge expressions. But in some edge cases, expensive expressions might be " +
+        "duplicated many times in merged Project by this optimization. This config sets " +
+        "a maximum number. Once an expression is duplicated equal to or more than this number " +
+        "if merging two Project, Spark SQL will skip the merging.")
+      .version("3.1.0")
+      .intConf
+      .createWithDefault(20)
+
   val DECIMAL_OPERATIONS_ALLOW_PREC_LOSS =
     buildConf("spark.sql.decimalOperations.allowPrecisionLoss")
       .internal()
@@ -3289,6 +3302,8 @@ class SQLConf extends Serializable with Logging {
 
   def replaceExceptWithFilter: Boolean = getConf(REPLACE_EXCEPT_WITH_FILTER)
 
+  def maxCommonExprsInCollapseProject: Int = getConf(MAX_COMMON_EXPRS_IN_COLLAPSE_PROJECT)
+
   def decimalOperationsAllowPrecisionLoss: Boolean = getConf(DECIMAL_OPERATIONS_ALLOW_PREC_LOSS)
 
   def literalPickMinimumPrecision: Boolean = getConf(LITERAL_PICK_MINIMUM_PRECISION)

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/CollapseProjectSuite.scala

@@ -20,11 +20,12 @@ package org.apache.spark.sql.catalyst.optimizer
 import org.apache.spark.sql.catalyst.analysis.EliminateSubqueryAliases
 import org.apache.spark.sql.catalyst.dsl.expressions._
 import org.apache.spark.sql.catalyst.dsl.plans._
-import org.apache.spark.sql.catalyst.expressions.{Alias, Rand}
+import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.PlanTest
 import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
-import org.apache.spark.sql.types.MetadataBuilder
+import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.types.{MetadataBuilder, StructType}
 
 class CollapseProjectSuite extends PlanTest {
   object Optimize extends RuleExecutor[LogicalPlan] {
@@ -170,4 +171,34 @@ class CollapseProjectSuite extends PlanTest {
     val expected = Sample(0.0, 0.6, false, 11L, relation.select('a as 'c)).analyze
     comparePlans(optimized, expected)
   }
+
+  test("SPARK-32945: avoid collapsing projects if reaching max allowed common exprs") {
+    val options = Map.empty[String, String]
+    val schema = StructType.fromDDL("a int, b int, c string, d long")
+
+    Seq("1", "2", "3", "4").foreach { maxCommonExprs =>
+      withSQLConf(SQLConf.MAX_COMMON_EXPRS_IN_COLLAPSE_PROJECT.key -> maxCommonExprs) {
+        // If we collapse two Projects, `JsonToStructs` will be repeated three times.
+        val relation = LocalRelation('json.string)
+        val query = relation.select(
+          JsonToStructs(schema, options, 'json).as("struct"))
+          .select(
+            GetStructField('struct, 0).as("a"),
+            GetStructField('struct, 1).as("b"),
+            GetStructField('struct, 2).as("c")).analyze
+        val optimized = Optimize.execute(query)
+
+        if (maxCommonExprs.toInt <= 3) {
+          val expected = query
+          comparePlans(optimized, expected)
+        } else {
+          val expected = relation.select(
+            GetStructField(JsonToStructs(schema, options, 'json), 0).as("a"),
+            GetStructField(JsonToStructs(schema, options, 'json), 1).as("b"),
+            GetStructField(JsonToStructs(schema, options, 'json), 2).as("c")).analyze
+          comparePlans(optimized, expected)
+        }
+      }
+    }
+  }
 }