Two queries differing only in having a DISTINCT clause produce the same result, demonstrated by EXCEPT:
=> SELECT DISTINCT ON ((string_agg(air_way_bills.number::text, ','::text)))
-> string_agg(air_way_bills.number::text, ','::text) AS number,
-> air_way_bills.order_id
-> FROM pt.air_way_bills
-> where air_way_bills.order_id = 2792
-> GROUP BY air_way_bills.order_id
-> except
-> select string_agg(air_way_bills.number::text, ','::text) AS number,
-> air_way_bills.order_id
-> FROM pt.air_way_bills
-> where air_way_bills.order_id = 2792
-> GROUP BY air_way_bills.order_id;
number | order_id
--------+----------
(0 rows)
but joining with them produces different results:
=> SELECT o.id,
-> a.number AS awb
-> FROM pt.orders o
-> LEFT JOIN (
(> SELECT
(> string_agg(air_way_bills.number::text, ','::text) AS number,
(> air_way_bills.order_id
(> FROM pt.air_way_bills
(> GROUP BY air_way_bills.order_id) a ON a.order_id = o.id
-> where o.id = 2792;
id | awb
------+----------
2792 | 91484540
(1 row)
=> SELECT o.id,
-> a.number AS awb
-> FROM pt.orders o
-> LEFT JOIN (
(> SELECT DISTINCT ON ((string_agg(air_way_bills.number::text, ','::text)))
(> string_agg(air_way_bills.number::text, ','::text) AS number,
(> air_way_bills.order_id
(> FROM pt.air_way_bills
(> GROUP BY air_way_bills.order_id) a ON a.order_id = o.id
-> where o.id = 2792
id | awb
------+-----
2792 |
(1 row)