Re: SELECT DISTINCT ON removes results - Mailing list pgsql-general

From Guyren Howe
Subject Re: SELECT DISTINCT ON removes results
Date
Msg-id 1413574A-19A4-4A0B-A420-9151FFC3C448@gmail.com
Whole thread Raw
In response to Re: SELECT DISTINCT ON removes results  ("David G. Johnston" <david.g.johnston@gmail.com>)
List pgsql-general

On Oct 28, 2016, at 13:50 , David G. Johnston <david.g.johnston@gmail.com> wrote:

On Fri, Oct 28, 2016 at 1:39 PM, Guyren Howe <guyren@gmail.com> wrote:
Using 9.5, this query:

Unless I'm missing something, this ought to be impossible.

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)

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