[GitHub] [spark] bersprockets commented on a change in pull request #29737: [SPARK-32864][SQL] Support ORC forced positional evolution

[GitBox <gi...@apache.org>]

bersprockets commented on a change in pull request #29737:
URL: https://github.com/apache/spark/pull/29737#discussion_r490289663



##########
File path: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/orc/OrcUtils.scala
##########
@@ -142,13 +142,17 @@ object OrcUtils extends Logging {
       reader: Reader,
       conf: Configuration): Option[(Array[Int], Boolean)] = {
     val orcFieldNames = reader.getSchema.getFieldNames.asScala
+    val forcePositionalEvolution = OrcConf.FORCE_POSITIONAL_EVOLUTION.getBoolean(conf)
     if (orcFieldNames.isEmpty) {
       // SPARK-8501: Some old empty ORC files always have an empty schema stored in their footer.
       None
     } else {
-      if (orcFieldNames.forall(_.startsWith("_col"))) {
-        // This is a ORC file written by Hive, no field names in the physical schema, assume the
-        // physical schema maps to the data scheme by index.
+      if (forcePositionalEvolution || orcFieldNames.forall(_.startsWith("_col"))) {
+        // This is either an ORC file written by an old versions of Hive and there are no field
+        // names in the physical schema, or a file written by a newer versions of Hive where
+        // `orc.force.positional.evolution=true` and columns were renamed so the physical schema

Review comment:
       Super tiny nit (feel free to ignore if it doesn't make sense to you). I suggest:
   
   `orc.force.positional.evolution=true` (possibly because columns were renamed so the physical schema doesn't match the data schema).
   
   Your current version of the comment implies orc.force.positional.evolution=true will happen when the columns are renamed. But you can set orc.force.positional.evolution=true anytime, and you can rename the columns and neglect to set orc.force.positional.evolution, correct?

##########
File path: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/orc/OrcUtils.scala
##########
@@ -142,13 +142,17 @@ object OrcUtils extends Logging {
       reader: Reader,
       conf: Configuration): Option[(Array[Int], Boolean)] = {
     val orcFieldNames = reader.getSchema.getFieldNames.asScala
+    val forcePositionalEvolution = OrcConf.FORCE_POSITIONAL_EVOLUTION.getBoolean(conf)
     if (orcFieldNames.isEmpty) {
       // SPARK-8501: Some old empty ORC files always have an empty schema stored in their footer.
       None
     } else {
-      if (orcFieldNames.forall(_.startsWith("_col"))) {
-        // This is a ORC file written by Hive, no field names in the physical schema, assume the
-        // physical schema maps to the data scheme by index.
+      if (forcePositionalEvolution || orcFieldNames.forall(_.startsWith("_col"))) {
+        // This is either an ORC file written by an old versions of Hive and there are no field

Review comment:
       Super tiny nit: "written by an old versions of Hive" should be
   written by an old version of Hive
   or
   written by old versions of Hive




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