feat: Add logging to explain reasons for Comet not being able to run a query stage natively

common/src/main/scala/org/apache/comet/CometConf.scala

@@ -276,6 +276,14 @@ object CometConf {
       .booleanConf
       .createWithDefault(false)
 
+  val COMET_EXPLAIN_FALLBACK_ENABLED: ConfigEntry[Boolean] =
+    conf("spark.comet.explainFallback.enabled")
+      .doc(
+        "When this setting is enabled, Comet will provide logging explaining the reason(s) " +
+          "why a query stage cannot be executed natively.")
+      .booleanConf
+      .createWithDefault(false)
+
   val COMET_BATCH_SIZE: ConfigEntry[Int] = conf("spark.comet.batchSize")
     .doc("The columnar batch size, i.e., the maximum number of rows that a batch can contain.")
     .intConf

docs/source/user-guide/configs.md

@@ -41,6 +41,7 @@ Comet provides the following configuration settings.
 | spark.comet.exec.memoryFraction | The fraction of memory from Comet memory overhead that the native memory manager can use for execution. The purpose of this config is to set aside memory for untracked data structures, as well as imprecise size estimation during memory acquisition. Default value is 0.7. | 0.7 |
 | spark.comet.exec.shuffle.codec | The codec of Comet native shuffle used to compress shuffle data. Only zstd is supported. | zstd |
 | spark.comet.exec.shuffle.enabled | Whether to enable Comet native shuffle. By default, this config is false. Note that this requires setting 'spark.shuffle.manager' to 'org.apache.spark.sql.comet.execution.shuffle.CometShuffleManager'. 'spark.shuffle.manager' must be set before starting the Spark application and cannot be changed during the application. | false |
+| spark.comet.explainFallback.enabled | When this setting is enabled, Comet will provide logging explaining the reason(s) why a query stage cannot be executed natively. | false |
 | spark.comet.memory.overhead.factor | Fraction of executor memory to be allocated as additional non-heap memory per executor process for Comet. Default value is 0.2. | 0.2 |
 | spark.comet.memory.overhead.min | Minimum amount of additional memory to be allocated per executor process for Comet, in MiB. | 402653184b |
 | spark.comet.nativeLoadRequired | Whether to require Comet native library to load successfully when Comet is enabled. If not, Comet will silently fallback to Spark when it fails to load the native lib. Otherwise, an error will be thrown and the Spark job will be aborted. | false |

docs/source/user-guide/installation.md

@@ -67,7 +67,8 @@ $SPARK_HOME/bin/spark-shell \
     --conf spark.sql.extensions=org.apache.comet.CometSparkSessionExtensions \
     --conf spark.comet.enabled=true \
     --conf spark.comet.exec.enabled=true \
-    --conf spark.comet.exec.all.enabled=true
+    --conf spark.comet.exec.all.enabled=true \
+    --conf spark.comet.explainFallback.enabled=true
 ```
 
 ### Verify Comet enabled for Spark SQL query
@@ -95,6 +96,16 @@ INFO src/lib.rs: Comet native library initialized
              PushedFilters: [IsNotNull(a), GreaterThan(a,5)], ReadSchema: struct<a:int>
 ```
 
+With the configuration `spark.comet.explainFallback.enabled=true`, Comet will log any reasons that prevent a plan from
+being executed natively.
+
+```scala
+scala> spark.sql("select * from t1 where cast(cast(a as double) as decimal(10,2)) > 5").show
+WARN CometSparkSessionExtensions$CometExecRule: Comet cannot execute this plan natively because:
+	- Comet does not guarantee correct results for cast from DoubleType to DecimalType(10,2) with timezone Some(America/Denver) and evalMode LEGACY (No overflow check). To enable all incompatible casts, set spark.comet.cast.allowIncompatible=true
+	- CollectLimit is not supported
+```
+
 ### Enable Comet shuffle
 
 Comet shuffle feature is disabled by default. To enable it, please add related configs:

spark/src/main/scala/org/apache/comet/CometSparkSessionExtensions.scala

@@ -734,6 +734,22 @@ class CometSparkSessionExtensions
       } else {
         var newPlan = transform(plan)
 
+        // if the plan cannot be run natively then explain why (when appropriate config is enabled)
+        if (CometConf.COMET_EXPLAIN_FALLBACK_ENABLED.get() && !isCometNative(newPlan)) {
+          new ExtendedExplainInfo().extensionInfo(newPlan) match {
+            case reasons if reasons.isEmpty =>
+              logWarning(
+                "Comet cannot execute this plan natively, but no reason is given. " +
+                  "This is likely a bug.")
+            case reasons if reasons.size == 1 =>
+              logWarning(s"Comet cannot execute this plan natively because ${reasons.head}")
+            case reasons =>
+              logWarning(
+                "Comet cannot execute this plan natively because:\n\t- " +
+                  s"${reasons.mkString("\n\t- ")}")
+          }
+        }
+
         // Remove placeholders
         newPlan = newPlan.transform {
           case CometSinkPlaceHolder(_, _, s) => s

spark/src/main/scala/org/apache/comet/ExtendedExplainInfo.scala

@@ -45,7 +45,7 @@ class ExtendedExplainInfo extends ExtendedExplainGenerator {
     }
   }
 
-  private def extensionInfo(node: TreeNode[_]): Set[String] = {
+  def extensionInfo(node: TreeNode[_]): Set[String] = {
     var info = mutable.Seq[String]()
     val sorted = sortup(node)
     sorted.foreach { p =>