Postgres 13

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CREATE TABLE visitors_log (visitor_id int, day timestamptz, source text);
    
INSERT INTO visitors_log VALUES
  (9, '2019-12-30 12:10:10-05', 'Twitter')
, (7, '2019-12-14 22:10:26-04', 'Netflix')
, (5, '2019-12-13 15:21:04-05', 'Netflix')
, (9, '2019-12-22 23:34:47-05', 'Twitter')
, (7, '2019-12-22 00:10:26-04', 'Netflix')
, (9, '2019-12-22 13:20:42-04', 'Twitter')
, (7, '2019-12-22 10:10:26-04', 'Netflix') -- added a row, so test case makes sense!
;
7 rows affected

SELECT count(*) FILTER (WHERE source = 'Netflix') * 100.0
     / count(*) AS visitors_percentage
FROM   visitors_log -- visits.visitors_log
WHERE  day >= timestamp '2019-12-22' AT TIME ZONE 'PST'
AND    day <  timestamp '2019-12-23' AT TIME ZONE 'PST';


visitors_percentage


33.3333333333333333


CREATE OR REPLACE FUNCTION my_func(_source text, _day date, _tz text)
  RETURNS numeric
  LANGUAGE sql IMMUTABLE PARALLEL SAFE AS
$func$
SELECT round(count(*) FILTER (WHERE source = _source) * 100.0 / count(*), 2) AS visitors_percentage
FROM   visitors_log  -- visits.visitors_log
WHERE  day >= _day::timestamp AT TIME ZONE _tz
AND    day < (_day + 1)::timestamp AT TIME ZONE _tz;
$func$;
SELECT my_func('Netflix', '2019-12-22', 'PST');


my_func


33.33