HashGrowBatchesDecide
Waiting to elect a Parallel Hash participant to decide on future batch growth.
Additional Information​
In parallel hash queries, a number of hash tables are used and both sides of the join are split into batches. PostgreSQL tries to calculate the number of batches before loading. This wait is seen while electing a parallel hash participant to do the calculation.
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.
Additional Links​
PostgreSQL documentation - Parallel Query
PostgreSQL documentation - The Statistics Collector
EDB - Parallel Hash Joins in PostgreSQL Explained
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