wombatu-kun commented on code in PR #19458:
URL: https://github.com/apache/hudi/pull/19458#discussion_r3701801763


##########
hudi-spark-datasource/hudi-spark/src/test/scala/org/apache/spark/sql/hudi/common/TestNestedSchemaPruningOptimization.scala:
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@@ -58,26 +64,34 @@ class TestNestedSchemaPruningOptimization extends 
HoodieSparkSqlTestBase {
     }
   }
 
-  test("Test nested schema pruning with DefaultHoodieRecordPayload") {
+  test("Test nested schema pruning with a projection-incompatible custom 
payload") {
     withTempDir { tmp =>
       val tableName = generateTableName
       val tablePath = s"${tmp.getCanonicalPath}/$tableName"
 
-      // NOTE: On the file-group-reader based read path the payload class does 
not affect nested
-      //       schema pruning, so the read schema is pruned the same way as 
with the default payload
+      // NOTE: A payload class outside the well-known set puts the table in 
CUSTOM merge mode, whose
+      //       merger is not projection compatible, so the file group reader 
merges on the full
+      //       table schema internally 
(FileGroupReaderSchemaHandler#generateRequiredSchema) and
+      //       projects the merged rows back down to the pruned read schema 
afterwards
       createTableWithNestedStructSchema("mor", tableName, tablePath,
-        Map(HoodieWriteConfig.WRITE_PAYLOAD_CLASS_NAME.key -> 
"org.apache.hudi.common.model.DefaultHoodieRecordPayload"))
+        Map(HoodieWriteConfig.WRITE_PAYLOAD_CLASS_NAME.key -> 
classOf[CustomPayloadForTesting].getName),
+        populateMetaFields = true)
+
+      // The update writes a log file, so the pruned reads below actually 
merge through that gate
+      spark.sql(s"UPDATE $tableName SET ts = 123457 WHERE id = 1")
 
       val selectDF = spark.sql(s"SELECT id, item.name FROM $tableName")
 
+      // Spark still prunes the scan schema; the full-schema requirement is 
internal to the reader
       val expectedSchema = StructType(Seq(
         StructField("id", IntegerType, nullable = true),
         StructField("item", StructType(Seq(StructField("name", StringType, 
nullable = false))), nullable = true)
       ))
-
       assertPrunedReadSchema(selectDF, tableName, expectedSchema)
 
       checkAnswer(s"SELECT id, item.name FROM $tableName")(Seq(1, "a1"))
+      // The merged row keeps nested leaves that the pruned read schema dropped

Review Comment:
   This says the merged row keeps leaves the pruned read schema dropped, but 
the checkAnswer below issues its own query that Spark prunes to (id, 
item.price, ts), so nothing the earlier read schema dropped is observed. Could 
you reword it to what the assertion actually pins - that a second, 
differently-pruned query still returns correctly merged values?



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