Postgres Parallel Query Statement Queuing

query_large_table_func.sql

/**
 * MIT License
 *
 * Copyright (c) 2024-2025 Brian Pace
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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 * SOFTWARE.
 */
 
CREATE OR REPLACE FUNCTION public.query_large_table_func(filter_value numeric)
 RETURNS TABLE(id integer, value double precision)
 LANGUAGE plpgsql
AS $function$
DECLARE
    max_workers numeric := current_setting('max_parallel_workers')::numeric;
    running_workers numeric;
    pct_avail int;
    trash text;
    attempt int := 0;
BEGIN

    -- Fetch the initial count of running parallel workers
    SELECT COUNT(*)::numeric INTO running_workers 
    FROM pg_stat_activity 
    WHERE backend_type = 'parallel worker';


    -- Calculate initial percentage of available workers
    pct_avail := round(((max_workers - running_workers) / max_workers) * 100);


    -- Check for availability and loop if below 50% availability, with a limit on attempts
    WHILE pct_avail < 50 AND attempt <= 10 LOOP
        RAISE LOG 'Waiting for available parallel processes... Max workers: %, Available workers: %', 
                  max_workers, max_workers - running_workers;

        -- Sleep for 2 seconds and re-check
        PERFORM pg_sleep(2);
        attempt := attempt + 1;

        -- Recalculate the number of running workers and availability percentage
        SELECT COUNT(*)::numeric INTO running_workers 
        FROM pg_stat_activity 
        WHERE backend_type = 'parallel worker';

        pct_avail := round(((max_workers - running_workers) / max_workers) * 100);
    END LOOP;


    -- Error if Max Attempt was reached
    IF attempt > 10
    THEN
        RAISE EXCEPTION 'Insufficient parallel workers: Max: %, Running: %, Available: %', 
                        max_workers, running_workers, max_workers - running_workers
        USING ERRCODE = '57014', 
              HINT = 'Try again later when more parallel workers are available.'; 
    END IF;

    -- Execute the query with parallelism
    RETURN QUERY 
        SELECT lt.id, lt.value
        FROM large_table lt
        WHERE lt.value > filter_value
        ORDER BY lt.id DESC;
END;
$function$
;