why max_parallel_workers_per_gather != max_parallel_workers

user_8523137 0
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I saw a lot of posts and papers that set the following values:

max_parallel_workers - set the number as the number of cores.

max_parallel_workers_per_gather - default set to 2 or set as max_parallel_workers / 2

when using parallel query the benfit and speed improvment is based on max_parallel_workers_per_gather

  • why I can't find posts that recommend to set max_parallel_workers_per_gather = max_parallel_workers ?
  • if we set max_parallel_workers_per_gather = max_parallel_workers what traps can be ?
parallel-processing postgresql
1 Answers

max_parallel_workers defines a pool of workers. Each query that is run, can take workers out of that pool up to the amount of max_parallel_workers_per_gather

Now if you make both values the same, that means that a query can potentially use all available workers and other queries that run at the same time can not make use of parallel query, even if it did benefit from it.

If you never run more than one statement at a time, then you can set them both to the same value. But if you have a (busy) system where you expect many concurrent queries and want multiple of them to benefit from using multiple workers, it is better to keep max_parallel_workers_per_gather lower than the maximum.

user_3303150
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