diff --git a/rest-api-spec/src/main/resources/rest-api-spec/test/index/100_partial_flat_object.yml b/rest-api-spec/src/main/resources/rest-api-spec/test/index/100_partial_flat_object.yml
index 6fc2654bcfc8f..0c00ff6e4a6a8 100644
--- a/rest-api-spec/src/main/resources/rest-api-spec/test/index/100_partial_flat_object.yml
+++ b/rest-api-spec/src/main/resources/rest-api-spec/test/index/100_partial_flat_object.yml
@@ -506,20 +506,6 @@ teardown:
 
   - length: { hits.hits: 2 }
 
-  # Exists Query with nested dot path, use the flat_object_field_name.last_key
-  - do:
-      search:
-        body: {
-          _source: true,
-          query: {
-            "exists": {
-              "field": issue.labels.type
-            }
-          }
-        }
-
-  - length: { hits.hits: 3 }
-
   # Exists Query without dot path for the flat_object_field_name
   - do:
       search:
@@ -613,3 +599,22 @@ teardown:
 
   - length: { hits.hits: 1 }
   - match: { hits.hits.0._source.issue.labels.comment: [ [ "Doe","Shipped" ],[ "John","Approved" ] ] }
+
+---
+"Exists query for sub field":
+  - skip:
+      version: " - 2.99.99"
+      reason: "exists query for sub field of flat_object field has bug before 3.0.0"
+
+  - do:
+      search:
+        body: {
+          _source: true,
+          query: {
+            "exists": {
+              "field": issue.labels.category.type
+            }
+          }
+        }
+
+  - length: { hits.hits: 3 }
diff --git a/rest-api-spec/src/main/resources/rest-api-spec/test/index/105_partial_flat_object_nested.yml b/rest-api-spec/src/main/resources/rest-api-spec/test/index/105_partial_flat_object_nested.yml
index 549ddbdde7bab..ccb9c3e3fc23d 100644
--- a/rest-api-spec/src/main/resources/rest-api-spec/test/index/105_partial_flat_object_nested.yml
+++ b/rest-api-spec/src/main/resources/rest-api-spec/test/index/105_partial_flat_object_nested.yml
@@ -506,23 +506,6 @@ teardown:
 
   - length: { hits.hits: 2 }
 
-  # Exists Query with nested dot path, use the flat_object_field_name.last_key
-  - do:
-      search:
-        body: {
-          _source: true,
-          query: {
-            nested: {
-              path: "issue",
-              query: {
-                "exists": {
-                  "field": issue.labels.type
-                } } }
-          }
-        }
-
-  - length: { hits.hits: 2 }
-
   # Exists Query without dot path for the flat_object_field_name
   - do:
       search:
@@ -634,3 +617,27 @@ teardown:
 
   - length: { hits.hits: 1 }
   - match: { hits.hits.0._source.issue.0.labels.comment: [ [ "Doe","Shipped" ],[ "John","Approved" ] ] }
+
+---
+"Exists query for sub field":
+  - skip:
+      version: " - 2.99.99"
+      reason: "exists query for sub field of flat_object field has bug before 3.0.0"
+
+  - do:
+      search:
+        body: {
+          _source: true,
+          query: {
+            nested: {
+              path: "issue",
+              query: {
+                "exists": {
+                  "field": issue.labels.category.type
+                }
+              }
+            }
+          }
+        }
+
+  - length: { hits.hits: 2 }
diff --git a/rest-api-spec/src/main/resources/rest-api-spec/test/index/91_flat_object_null_value.yml b/rest-api-spec/src/main/resources/rest-api-spec/test/index/91_flat_object_null_value.yml
index 716b6fb51cb43..455eb4fb91a43 100644
--- a/rest-api-spec/src/main/resources/rest-api-spec/test/index/91_flat_object_null_value.yml
+++ b/rest-api-spec/src/main/resources/rest-api-spec/test/index/91_flat_object_null_value.yml
@@ -373,7 +373,7 @@ teardown:
         body: {
           _source: true,
           query: {
-            exists: { "field": "record.d" }
+            exists: { "field": "record.name.d.name" }
           },
           sort: [{ order: asc}]
         }
diff --git a/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java b/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java
index 4425e4e5b0b39..7b1b2615f996d 100644
--- a/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java
+++ b/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java
@@ -478,8 +478,7 @@ public Query existsQuery(QueryShardContext context) {
             String searchKey;
             String searchField;
             if (isSubField()) {
-                searchKey = this.rootFieldName;
-                searchField = name();
+                return rangeQuery(null, null, true, true, context);
             } else {
                 if (hasDocValues()) {
                     return new FieldExistsQuery(name());
diff --git a/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldTypeTests.java b/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldTypeTests.java
index 600f04edcd552..2ab1ad689a0b9 100644
--- a/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldTypeTests.java
+++ b/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldTypeTests.java
@@ -418,8 +418,12 @@ public void testExistsQuery() {
                 ft.getValueFieldType(),
                 ft.getValueAndPathFieldType()
             );
-            assertEquals(new TermQuery(new Term("field", "field.bar")), dynamicMappedFieldType.existsQuery(null));
-
+            Automaton termAutomaton = PrefixQuery.toAutomaton(new BytesRef("field.bar="));
+            Automaton dvAutomaton = PrefixQuery.toAutomaton(new BytesRef("field.field.bar="));
+            Query indexQuery = new AutomatonQuery(new Term("field" + VALUE_AND_PATH_SUFFIX), termAutomaton, true);
+            Query dvQuery = new AutomatonQuery(new Term("field" + VALUE_AND_PATH_SUFFIX), dvAutomaton, true, DOC_VALUES_REWRITE);
+            Query expected = new IndexOrDocValuesQuery(indexQuery, dvQuery);
+            assertEquals(expected, dynamicMappedFieldType.existsQuery(MOCK_QSC_ENABLE_INDEX_DOC_VALUES));
         }
 
         {
@@ -1176,8 +1180,8 @@ public void testRangeQuery() {
                         );
                         continue;
                     }
-                    boolean nullLowerTerm = true;// randomBoolean();
-                    boolean nullUpperTerm = true;// nullLowerTerm == false || randomBoolean();
+                    boolean nullLowerTerm = randomBoolean();
+                    boolean nullUpperTerm = nullLowerTerm == false || randomBoolean();
 
                     Automaton a1 = PrefixQuery.toAutomaton(new BytesRef("field.field1="));
                     Automaton a2 = TermRangeQuery.toAutomaton(