Citus cluster response times function

citus-cluster-latency.sql

CREATE OR REPLACE FUNCTION latency(nodename text, nodeport int)
RETURNS double precision
AS $$
DECLARE
    start_time timestamptz;
    end_time timestamptz;
BEGIN
    /* warm up connection cache */
    perform master_run_on_worker(array[nodename], array[nodeport], array['select 1'], false);

    /* measure response time of a simple query */
    start_time := clock_timestamp();
    perform master_run_on_worker(array[nodename], array[nodeport], array['select 1'], false);
    end_time := clock_timestamp();

    /* return response time in milliseconds */
    return 1000. * (extract(epoch from end_time) - extract(epoch from start_time));
END;
$$ LANGUAGE plpgsql;
select create_distributed_function('latency(text,int)');

-- for each node n: run `select latency(n)` on all workers
select n.nodename AS fromhost, n.nodeport AS fromport, r.nodename AS tohost, r.nodeport AS toport, success, result::double precision
from pg_dist_node n, run_command_on_workers(format('select latency(%L, %L)', nodename, nodeport)) r;