// TODO: move to validation class?
        /// <summary>
        /// Submit request to query engine for execution.
        /// </summary>
        private static Dictionary <string, string> ExecuteQuery(
            string operation,
            RequestManager request)
        {
            return(operation switch
            {
                // TODO: (switch) move literal string to const
                "InsertKey" => QueryProvider.InsertKey(request),
                "InsertTag" => QueryProvider.InsertTag(request),

                "SelectKey" => QueryProvider.SelectKey(request),
                "SelectTags" => QueryProvider.SelectTags(request),
                "SelectTagCount" => QueryProvider.SelectTagCount(request),
                "SelectKeyList" => QueryProvider.SelectKeyList(request),
                "SelectKeyTags" => QueryProvider.SelectKeyTags(request),

                "UpdateKey" => QueryProvider.UpdateKey(request),
                "UpdateData" => QueryProvider.UpdateData(request),
                "UpdateTag" => QueryProvider.UpdateTag(request),
                "UpdateKeyTag" => QueryProvider.UpdateKeyTag(request),

                "DeleteKey" => QueryProvider.DeleteKey(request),
                "DeleteTag" => QueryProvider.DeleteTag(request),
                "DeleteKeyTags" => QueryProvider.DeleteKeyTags(request),
                "DeleteKeyTag" => QueryProvider.DeleteKeyTag(request),

                _ => new Dictionary <string, string>(),
            });