haoch opened a new pull request, #32619:
URL: https://github.com/apache/airflow/pull/32619
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**General problem**: `joinedload` on `TaskInstance.dag_run` is generally not
needed except for UI/API (list all task instances and related fields at once),
but the unnecessary `JOIN` is a killer of Airflow scheduling performance
especially when there are thousands of task instances in single DagRun.
**The real problem in some large scale Airflow clusters**:
We manage thousands of airflow executors (nodes) in production, and each
DagRun may have 10k task instances.
* In Scheduler side , the impact is very obvious as after making it lazy, we
do see 3+ times scheduling performance improvement.
* In Executor side, when tasks are queued and preparing to execute, it will
check if dependencies meet with all finished task instances, a large amount of
concurrent JOIN SQL will slow database performance. Especially when the config
content size of some DagRun is large, thousands of concurrent executors will
pull the same large DagRun at same time and it will also a huge network
throughput from database and make the whole cluster unhealthy.
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