PostgreSQL Parallel GUC Documentation

gistfile1.txt

    max_parallel_degree 0

         Sets the maximum degree of parallelism for an individual parallel
         operation.  Note that the requested number of workers may not actually
         be available at runtime.  Parallel workers are taken from the pool
         of processes established by `max-worker-processes`

    parallel_tuple_cost 0.1

        Sets the planner's estimate of the cost of transferring a tuple
        from a parallel worker process to another process.
        The default is 0.1.

    parallel_setup_cost 1000

        Sets the planner's estimate of the cost of launching parallel worker
        processes.
        The default is 1000.

    force_parallel_mode off|on

        Allows the use of parallel queries for testing purposes even in cases
        where no performance benefit is expected.
        The allowed values of `force_parallel_mode` are
        "off" (use parallel mode only when it is expected to improve
        performance), "on" (force parallel query for all queries
        for which it is thought to be safe), and "regress" (like
        on, but with additional behavior changes to facilitate automated
        regression testing).
		
        More specifically, setting this value to "on" will add
        a "Gather" node to the top of any query plan for which this
        appears to be safe, so that the query runs inside of a parallel worker.
        Even when a parallel worker is not available or cannot be used,
        operations such as starting a subtransaction that would be prohibited
        in a parallel query context will be prohibited unless the planner
        believes that this will cause the query to fail.  If failures or
        unexpected results occur when this option is set, some functions used
        by the query may need to be marked `PARALLEL UNSAFE`
        (or, possibly, `PARALLEL RESTRICTED`).
		
        Setting this value to "regress" has all of the same effects
        as setting it to "on" plus some additional effect that are
        intended to facilitate automated regression testing.  Normally,
        messages from a parallel worker are prefixed with a context line,
        but a setting of "regress" suppresses this to guarantee
        reproducible results.  Also, the "Gather" nodes added to
        plans by this setting are hidden from the `EXPLAIN` output
        so that the output matches what would be obtained if this setting
        were turned "off".