ParallelAppend
Waiting to choose the next subplan during Parallel Append plan execution.
Additional Information
Parallel Append plans can be seen during Union All and when scanning a partitioned table. The planner splits execution into a number of subplans where the results have to be appended together to complete that stage’s output. This wait is seen whilst waiting to pick the next subplan.
How to reduce this wait
Parallel queries are excellent at scanning large amounts of data the quickly returning results. Run in isolation or in a considered manner they are a useful tool.
Do not allow concurrent parallel queries from many users to flood system CPU or IO resources.
Adjust max_worker_process , max_parallel_workers and max_parallel_workers_per_gather to control response of your query and overall impact to the system.
This execution plan can be switched off by use of the enable_parallel_append planner setting.
Additional Links
PostgreSQL documentation - Parallel Query
PostgreSQL documentation - The Statistics Collector
EDB - Parallel Hash Joins in PostgreSQL Explained
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