Postgres 11

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DO
$$
BEGIN
   EXECUTE format(
      'SET mycustom.var = %L'
     , ARRAY(SELECT *
         FROM   generate_series(1, 10) 
         ORDER  BY random()
         LIMIT  6
         )::text
      );
END
$$;

SELECT set_config('mycustom.var2'
                , ARRAY(SELECT *
                     FROM   generate_series(1, 10) 
                     ORDER  BY random()
                     LIMIT  6
                     )::text
                , false)
                


set_config


{10,4,2,8,1,5}


SELECT current_setting('mycustom.var')::int[];


current_setting


{10,1,9,3,7,8}


SELECT current_setting('mycustom.var2')::int[];


current_setting


{10,4,2,8,1,5}