55a64ab8_PHP__Statistics___Manual_mysqlnd_statistics__Result_Set_Description
[Statistic] bad_index_used [Scope] Connection [Notes] If you want these queries to be reported you can use mysqli_report(MYSQLI_REPORT_INDEX) to make ext/mysqli throw an exception. If you prefer a warning instead of an exception use mysqli_report(MYSQLI_REPORT_INDEX ^ MYSQLI_REPORT_STRICT) [Description] 
Number of queries that have generated a result set and did not use a good index (see also mysqld start option –log-slow-queries).
PHP: Statistics - Manual
Description