ParallelBitmapScan
Waiting for parallel bitmap scan to become initialized.
Additional Information​
ParallelBitmapScan is an Inter Process Communication (IPC) wait and is seen when the planner selects a Parallel Bitmap Scan execution path. Scan processes need to be acquired and a dynamic shared memory (DSM) segment created.
Although Highgo’s blog - Parallel Foreign Scan of PostgreSQL covers the (distributed) foreign scan, the introduction covers the steps involved in the normal parallel bitmap scan.
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​
High Go Blog - Parallel Foreign Scan of PostgreSQL
PostgreSQL documentation - Parallel Query
PostgreSQL documentation - The Statistics Collector
EDB - Parallel Hash Joins in PostgreSQL Explained
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