docs/attachments/LUCENE-8041/LUCENE-8041.patch

diff --git a/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java
index 0ef21292c0..742accece4 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java
@@ -21,9 +21,10 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
-import java.util.TreeMap;
+import java.util.Map;
 
 import org.apache.lucene.codecs.CodecUtil;
 import org.apache.lucene.codecs.FieldsProducer;
@@ -119,7 +120,7 @@ public final class BlockTreeTermsReader extends FieldsProducer {
   // produce DocsEnum on demand
   final PostingsReaderBase postingsReader;
 
-  private final TreeMap<String,FieldReader> fields = new TreeMap<>();
+  private final Map<String,FieldReader> fields = new HashMap<>();
 
   final String segment;
   
@@ -257,7 +258,9 @@ public final class BlockTreeTermsReader extends FieldsProducer {
 
   @Override
   public Iterator<String> iterator() {
-    return Collections.unmodifiableSet(fields.keySet()).iterator();
+    List<String> fieldNames = new ArrayList<>(fields.keySet());
+    Collections.sort(fieldNames);
+    return Collections.unmodifiableList(fieldNames).iterator();
   }
 
   @Override
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/perfield/PerFieldPostingsFormat.java b/lucene/core/src/java/org/apache/lucene/codecs/perfield/PerFieldPostingsFormat.java
index 36f035854d..429ab6599d 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/perfield/PerFieldPostingsFormat.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/perfield/PerFieldPostingsFormat.java
@@ -29,7 +29,6 @@ import java.util.List;
 import java.util.Map;
 import java.util.ServiceLoader; // javadocs
 import java.util.Set;
-import java.util.TreeMap;
 import java.util.TreeSet;
 
 import org.apache.lucene.codecs.FieldsConsumer;
@@ -247,7 +246,7 @@ public abstract class PerFieldPostingsFormat extends PostingsFormat {
 
     private static final long BASE_RAM_BYTES_USED = RamUsageEstimator.shallowSizeOfInstance(FieldsReader.class);
 
-    private final Map<String,FieldsProducer> fields = new TreeMap<>();
+    private final Map<String,FieldsProducer> fields = new HashMap<>();
     private final Map<String,FieldsProducer> formats = new HashMap<>();
     private final String segment;
     
@@ -308,7 +307,9 @@ public abstract class PerFieldPostingsFormat extends PostingsFormat {
 
     @Override
     public Iterator<String> iterator() {
-      return Collections.unmodifiableSet(fields.keySet()).iterator();
+      List<String> fieldNames = new ArrayList<>(fields.keySet());
+      Collections.sort(fieldNames);
+      return Collections.unmodifiableList(fieldNames).iterator();
     }
 
     @Override