Re: [PR] [SPARK-58313][SDP] Validate SCD2 track-history columns at AutoCDC flow construction [spark]

["jose-torres (via GitHub)" <gi...@apache.org>]

jose-torres commented on code in PR #57490:
URL: https://github.com/apache/spark/pull/57490#discussion_r3648940929


##########
sql/pipelines/src/main/scala/org/apache/spark/sql/pipelines/autocdc/Scd2BatchProcessor.scala:
##########
@@ -1410,6 +1395,44 @@ object Scd2BatchProcessor {
     AutoCdcReservedNames.cdcMetadataColName
   )
 
+  /**
+   * Resolve [[ChangeArgs.trackHistorySelection]] against `schema` and return the field names of
+   * the history-tracking columns: the eligible user-data columns (those that are neither
+   * [[ChangeArgs.keys]] nor framework reserved columns) filtered through the selection.
+   *
+   * This is the single source of truth for which columns define an SCD2 run. It is called both
+   * per-microbatch (against the reconciled dataframe's schema) and at
+   * [[org.apache.spark.sql.pipelines.graph.AutoCdcMergeFlow]] construction time (against the
+   * user-selected source schema), so an unresolvable or ineligible selection fails fast with a
+   * user-actionable [[org.apache.spark.sql.AnalysisException]] instead of surfacing mid-stream
+   * (SPARK-58313).

Review Comment:
   nit: spark ticket comment as per other PR



##########
sql/pipelines/src/main/scala/org/apache/spark/sql/pipelines/autocdc/Scd2BatchProcessor.scala:
##########
@@ -922,28 +923,12 @@ case class Scd2BatchProcessor(
    * the eligible user-data columns (those not in [[ChangeArgs.keys]] or the framework
    * reserved set) filtered through [[ChangeArgs.trackHistorySelection]].
    */
-  private def computeTrackedHistoryColumns(df: DataFrame): Seq[String] = {
-    val conf = df.sparkSession.sessionState.conf
-    val resolver = conf.resolver
-
-    val keyColNames = changeArgs.keys.map(_.name)
-    val reservedColNames = Scd2BatchProcessor.reservedFrameworkColNames
-
-    val eligibleSchema = StructType(df.schema.fields.filterNot { field =>
-      reservedColNames.exists(resolver(_, field.name)) ||
-        keyColNames.exists(resolver(_, field.name))
-    })
-
-    ColumnSelection
-      .applyToSchema(
-        schemaName = "trackHistorySelection",
-        schema = eligibleSchema,
-        columnSelection = changeArgs.trackHistorySelection,
-        caseSensitive = conf.caseSensitiveAnalysis
-      )
-      .fieldNames
-      .toImmutableArraySeq
-  }
+  private def computeTrackedHistoryColumns(df: DataFrame): Seq[String] =
+    Scd2BatchProcessor.computeTrackedHistoryColumns(
+      schema = df.schema,
+      changeArgs = changeArgs,
+      caseSensitive = df.sparkSession.sessionState.conf.caseSensitiveAnalysis

Review Comment:
   I guess I'm OK with this, it's unlikely we'll introduce some third resolution mode all of the sudden.



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