# UNDF: UNDF-2026-000000174
diff --git a/src/main/java/org/apache/ibatis/mapping/ResultMapping.java b/src/main/java/org/apache/ibatis/mapping/ResultMapping.java
--- a/src/main/java/org/apache/ibatis/mapping/ResultMapping.java
+++ b/src/main/java/org/apache/ibatis/mapping/ResultMapping.java
@@ -18,8 +18,9 @@
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.EnumSet;
 import java.util.List;
 import java.util.Set;

 import org.apache.ibatis.session.Configuration;
 import org.apache.ibatis.type.JdbcType;
 import org.apache.ibatis.type.TypeHandler;
@@ -42,7 +43,7 @@ public class ResultMapping {
   private Set<String> notNullColumns;
   private String columnPrefix;
-  private List<ResultFlag> flags;
+  private Set<ResultFlag> flags;
   private List<ResultMapping> composites;
   private String resultSet;
   private String foreignColumn;
@@ -66,7 +67,7 @@ public class ResultMapping {
     public Builder(Configuration configuration, String property) {
       resultMapping.configuration = configuration;
       resultMapping.property = property;
-      resultMapping.flags = new ArrayList<>();
+      resultMapping.flags = EnumSet.noneOf(ResultFlag.class);
       resultMapping.composites = new ArrayList<>();
       resultMapping.lazy = configuration.isLazyLoadingEnabled();
     }
@@ -133,7 +134,11 @@ public class ResultMapping {

     public Builder flags(List<ResultFlag> flags) {
-      resultMapping.flags = flags;
+      // Convert to EnumSet for O(1) contains() — ArrayList.contains() is O(F) per call
+      EnumSet<ResultFlag> enumFlags = EnumSet.noneOf(ResultFlag.class);
+      if (flags != null && !flags.isEmpty()) {
+        enumFlags.addAll(flags);
+      }
+      resultMapping.flags = enumFlags;
       return this;
     }

@@ -153,7 +158,7 @@ public class ResultMapping {

     public ResultMapping build() {
       // lock down collections
-      resultMapping.flags = Collections.unmodifiableList(resultMapping.flags);
+      resultMapping.flags = Collections.unmodifiableSet(resultMapping.flags);
       resultMapping.composites = Collections.unmodifiableList(resultMapping.composites);
       validate();
       return resultMapping;
@@ -230,7 +235,7 @@ public class ResultMapping {
       return columnPrefix;
     }

-  public List<ResultFlag> getFlags() {
+  public Set<ResultFlag> getFlags() {
     return flags;
   }