MS-470 Drop index success, which table not created

Former-commit-id: e82f92b1a459a3cbf77d31387b00e198b605dccc

cpp/CHANGELOG.md

@@ -24,6 +24,7 @@ Please mark all change in change log and use the ticket from JIRA.
 - MS-462 - Run milvus server twices, should display error
 - MS-463 - Search timeout
 - MS-467 - mysql db test failed
+- MS-470 - Drop index success, which table not created
 - MS-471 - code coverage run failed
 
 ## Improvement

cpp/src/server/grpc_impl/GrpcRequestTask.cpp

@@ -960,8 +960,15 @@ DropIndexTask::OnExecute() {
             return SetError(res, "Invalid table name: " + table_name_);
         }
 
-        //step 2: check table existence
-        engine::Status stat = DBWrapper::DB()->DropIndex(table_name_);
+        //step 2:check index existence
+        engine::TableIndex index;
+        engine::Status stat = DBWrapper::DB()->DescribeIndex(table_name_, index);
+        if (index.engine_type_ == 1) {
+            return SetError(SERVER_UNEXPECTED_ERROR, "index not existed");
+        }
+
+        //step 3: check table existence
+        stat = DBWrapper::DB()->DropIndex(table_name_);
         if (!stat.ok()) {
             return SetError(DB_META_TRANSACTION_FAILED, stat.ToString());
         }