Wrong Seconds_Behind_Master when only starting the SQL_Thread.

Hi,

When I run replication normally, I get a value in "Seconds_Behind_Master" from "SHOW SLAVE STATUS". However, when I run only the SQL Thread (on local relay logs that have been downloaded previously), I have "NULL" in "Seconds_Behind_Master". I would expect to have a numeric value.

Also, note that the manual ([1]) does not document a "NULL" value in "Seconds_Behind_Master", even if "NULL" is shown when the SQL Thread is not running. This might be a different bug thought and is documentation related.

[1]: https://mariadb.com/kb/en/library/show-slave-status/

mysql <<< "select version();"

version()

10.2.12-MariaDB-log

mysql <<< "start slave; do sleep(1); show slave status\G stop slave" | grep Seconds_Behind_Master

        Seconds_Behind_Master: 130422

mysql <<< "start slave sql_thread; do sleep(1); show slave status\G stop slave" | grep Seconds_Behind_Master

        Seconds_Behind_Master: NULL

mysql <<< "stop slave; do sleep(1); show slave status\G" | grep Seconds_Behind_Master

        Seconds_Behind_Master: NULL

Many thanks for looking into that,

JFG