Earlier today I had a MySQL slave go down for a few hours, which wasn’t a big deal. When it was brought back up it wasn’t syncing properly:
1 Seconds_Behind_Master: NULL
Apparently there was an issue with a query that was showing up under LAST_ERROR; running
1 STOP SLAVE; 2 SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 1; 3 START SLAVE;
fixed the issue and then issued another SHOW SLAVE STATUS\G; and got the correct output:
1 Seconds_Behind_Master: 27269
About half an hour later the slave was all caught up and replication was working again.