Avoid copying the field alias lookup structure unnecessarily. (elastic#39726)

The field alias map is now only copied if a new alias is added, or if an alias
definition has changed. This follows the same behavior as we have for concrete
field mappings and avoids unnecessary object creation.

Author: jtibshirani

server/src/main/java/org/elasticsearch/index/mapper/FieldTypeLookup.java

@@ -68,15 +68,19 @@ public FieldTypeLookup copyAndAddAll(String type,
             MappedFieldType fieldType = fieldMapper.fieldType();
             MappedFieldType fullNameFieldType = fullName.get(fieldType.name());
 
-            if (!Objects.equals(fieldType, fullNameFieldType)) {
+            if (Objects.equals(fieldType, fullNameFieldType) == false) {
                 fullName = fullName.copyAndPut(fieldType.name(), fieldType);
             }
         }
 
         for (FieldAliasMapper fieldAliasMapper : fieldAliasMappers) {
             String aliasName = fieldAliasMapper.name();
             String path = fieldAliasMapper.path();
-            aliases = aliases.copyAndPut(aliasName, path);
+
+            String existingPath = aliases.get(aliasName);
+            if (Objects.equals(path, existingPath) == false) {
+                aliases = aliases.copyAndPut(aliasName, path);
+            }
         }
 
         return new FieldTypeLookup(fullName, aliases);