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[GitHub] erankor commented on issue #5979: Kafka Indexing Service lagging every hour


erankor commented on issue #5979: Kafka Indexing Service lagging every hour
URL: https://github.com/apache/incubator-druid/issues/5979#issuecomment-404050959
 
 
   @jihoonson, a couple of additional questions on the MySQL tables - 
   1. Following this issue, we started tracking slow queries on the MySQL DB, and I'm seeing queries like the following repeat often -
   `SELECT payload FROM druid_segments WHERE used = true AND dataSource = 'player-events-historical' AND ((start <= '2018-07-11T04:00:00.000Z' AND `end` >= '2018-07-11T03:00:00.000Z'));`
     a. Is it possible to delete rows with used = false from this table? While I saw there is already an index for `used`, keeping the table small can probably help. In our case, only ~1% of the rows there have used=true, I'm guessing it's because KIS tasks create partitioned segments that we later merge.
     b. I'm thinking about replacing the existing (used) index with an index of (used, end), since it seems these queries always have end >= <some recent timestamp>, does that make sense?
   2. Going over the tables one by one, I saw that druid_tasks & druid_tasklogs also have rows being accumulated, does it make sense to clean those? from what I saw, there is no date on druid_tasklogs, but I can get a list of old tasks from druid_tasks, and then delete both the tasks and their logs by id.
   
   Thank you!
   
   Eran

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