rdblue commented on code in PR #12836:
URL: https://github.com/apache/iceberg/pull/12836#discussion_r2051039187


##########
parquet/src/main/java/org/apache/iceberg/data/parquet/BaseParquetReaders.java:
##########
@@ -237,55 +236,37 @@ public ParquetValueReader<?> struct(
           int fieldD = type.getMaxDefinitionLevel(path(fieldType.getName())) - 
1;
           int id = fieldType.getId().intValue();
           readersById.put(id, ParquetValueReaders.option(fieldType, fieldD, 
fieldReader));
-          typesById.put(id, fieldType);
-          if (idToConstant.containsKey(id)) {
-            maxDefinitionLevelsById.put(id, fieldD);
-          }
         }
       }
 
-      List<Types.NestedField> expectedFields =
-          expected != null ? expected.fields() : ImmutableList.of();
+      int constantDefinitionLevel = type.getMaxDefinitionLevel(currentPath());
+      List<Types.NestedField> expectedFields = expected.fields();
       List<ParquetValueReader<?>> reorderedFields =
           Lists.newArrayListWithExpectedSize(expectedFields.size());
-      List<Type> types = 
Lists.newArrayListWithExpectedSize(expectedFields.size());
-      // Defaulting to parent max definition level
-      int defaultMaxDefinitionLevel = 
type.getMaxDefinitionLevel(currentPath());
+
       for (Types.NestedField field : expectedFields) {
         int id = field.fieldId();
-        ParquetValueReader<?> reader = readersById.get(id);
-        if (idToConstant.containsKey(id)) {
-          // containsKey is used because the constant may be null
-          int fieldMaxDefinitionLevel =
-              maxDefinitionLevelsById.getOrDefault(id, 
defaultMaxDefinitionLevel);
-          reorderedFields.add(
-              ParquetValueReaders.constant(idToConstant.get(id), 
fieldMaxDefinitionLevel));
-          types.add(null);
-        } else if (id == MetadataColumns.ROW_POSITION.fieldId()) {
-          reorderedFields.add(ParquetValueReaders.position());
-          types.add(null);
-        } else if (id == MetadataColumns.IS_DELETED.fieldId()) {
-          reorderedFields.add(ParquetValueReaders.constant(false));
-          types.add(null);
-        } else if (reader != null) {
-          reorderedFields.add(reader);
-          types.add(typesById.get(id));
-        } else if (field.initialDefault() != null) {
-          reorderedFields.add(
-              ParquetValueReaders.constant(
-                  convertConstant(field.type(), field.initialDefault()),
-                  maxDefinitionLevelsById.getOrDefault(id, 
defaultMaxDefinitionLevel)));
-          types.add(typesById.get(id));
-        } else if (field.isOptional()) {
-          reorderedFields.add(ParquetValueReaders.nulls());
-          types.add(null);
-        } else {
-          throw new IllegalArgumentException(
-              String.format("Missing required field: %s", field.name()));
-        }
+        ParquetValueReader<?> reader =
+            ParquetValueReaders.replaceWithMetadataReader(
+                id, readersById.get(id), idToConstant, 
constantDefinitionLevel);
+        reorderedFields.add(defaultReader(field, reader, 
constantDefinitionLevel));

Review Comment:
   The changes here were needed to reduce the complexity of the `struct` method 
that was failing checkstyle. I refactored into 2 components:
   * `ParquetValueReaders.replaceWithMetadataReader` handles metadata columns 
and constants
   * `defaultReader` handles defaults based on the schema; this requires 
`convertConstant` so it is not shared in `ParquetValueReaders`
   
   I also simplified the constant handling that was introduced in #4627. That 
PR introduced a map with the max definition level for columns that are replaced 
with constants and defaults the definition level to the parent struct's value 
when there is no underlying column (for instance, `_pos`). This was 
over-complicated because the fields don't need a column-specific DL. If the 
parent struct is non-null then the constant should be added to it, so the DL 
can always be the parent's DL.



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