ubermen commented on issue #3840: [AIRFLOW-3001] Add task_instance table index 'ti_dag_date' URL: https://github.com/apache/incubator-airflow/pull/3840#issuecomment-419279096 There was no index composed of dag_id and execution_date. So, when scheduler find all tis of dagrun like this "select * from task_instance where dag_id = '~' and execution_date = '~' ", this query will be using ti_dag_state index (I was testing it in mysql workbench). Perhaps there's no problem when range of execution_date is small (under 1000 dagrun), but I had experienced slow allocation of tis when the dag had 1000+ accumulative dagrun. So, now I was using airflow with adding new index (dag_id, execution_date) on task_instance table. I have attached result of my test  
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