Re: [PR] [SPARK-58118][SDP] Allow user-specified AUTO CDC schema to omit the reserved metadata column [spark]

["HyukjinKwon (via GitHub)" <gi...@apache.org>]

HyukjinKwon commented on code in PR #57644:
URL: https://github.com/apache/spark/pull/57644#discussion_r3717403120


##########
sql/pipelines/src/main/scala/org/apache/spark/sql/pipelines/graph/DatasetManager.scala:
##########
@@ -284,9 +284,18 @@ object DatasetManager extends Logging {
     val (catalog, identifier) =
       PipelinesCatalogUtils.resolveTableCatalog(context.spark, table.identifier)
 
-    val outputSchema = table.specifiedSchema.getOrElse(
-      resolvedDataflowGraph.inferredSchema(table.identifier).asNullable
-    )
+    val outputSchema = table.specifiedSchema match {
+      case Some(ss) =>
+        // The user schema describes the logical table; the engine owns the reserved AUTO CDC
+        // metadata column(s). Append any that the incoming flows produce but the user omitted,
+        // so the created table matches what the AUTO CDC MERGE writes at runtime.
+        val omittedReservedFields = AutoCdcMergeFlow
+          .reservedFields(resolvedDataflowGraph.inferredSchema(table.identifier))
+          .filterNot(f => ss.fieldNames.contains(f.name))

Review Comment:
   Optional micro-nit: `ss.fieldNames` is recomputed (fresh Array) with a linear `contains` per reserved field. `val specifiedNames = ss.fieldNames.toSet` before the `filterNot` makes membership O(1). Materialization-time only, so take-it-or-leave-it.



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