revert to hashmap

Signed-off-by: xinyual <xinyual@amazon.com>

Author: xinyual

opensearch/src/main/java/org/opensearch/sql/opensearch/request/system/OpenSearchDescribeIndexRequest.java

@@ -10,6 +10,7 @@
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
@@ -98,7 +99,7 @@ public List<ExprValue> search() {
    */
   // TODO possible collision if two indices have fields with the same name and different mappings
   public Map<String, OpenSearchDataType> getFieldTypes() {
-    Map<String, OpenSearchDataType> fieldTypes = new LinkedHashMap<>();
+    Map<String, OpenSearchDataType> fieldTypes = new HashMap<>();
     Map<String, IndexMapping> indexMappings =
         client.getIndexMappings(getLocalIndexNames(indexName.getIndexNames()));
     for (IndexMapping indexMapping : indexMappings.values()) {