Re: Conflict between JSON_AGG and COPY

On 04/07/2018 03:44 AM, Đỗ Ngọc Trí Cường wrote:
> Dear all,
>
> I've found one case. I don't know this is a bug or I config/query some
> things wrong.
>
> Let I describe it. I have a table with structure and data is:
>
>  id | username |   fullname
> ----+-------------+---------------
>   1 | john        | John
>   2 | anna        | Anna
>   3 | sussi        | Sussi
>   4 | david      | David Beckham
>   5 | wayne     | Wayne Rooney
>
> I want to export it to a file in JSON format so I run the query as below:
>
> COPY (SELECT row_to_json(t) FROM (SELECT json_agg(t1) AS "RECORDS"
> FROM test_table t1) t) TO '/home/postgres/test1.json';
>
> But the result I got will include "\n" in the result:
>
> {"RECORDS":[{"id":1,"username":"john","fullname":"John"}, \n
> {"id":2,"username":"anna","fullname":"Anna"}, \n
> {"id":3,"username":"sussi","fullname":"Sussi"}, \n
> {"id":4,"username":"david","fullname":"David Beckham"}, \n
> {"id":5,"username":"wayne","fullname":"Wayne Rooney"}]}
>
> Then, I try to get the same data in the other way:
>
> COPY (WITH t2 AS (select row_to_json(t) as js from test_table t),
>         t1 AS (SELECT ARRAY_TO_JSON(array_agg(js)) as "RECORDS"
> FROM t2)
>     SELECT row_to_json(t1) FROM t1)
>     TO '/home/postgres/test2.json';

CREATE TABLE test_table (id integer, username varchar, fullname varchar);

INSERT INTO
test_table
VALUES
(1, 'john', 'John'),
(2, 'anna', 'Anna'),
(3, 'sussi', 'Sussi'),
(4, 'david', 'David Beckham'),
(5, 'wayne', 'Wayne Rooney');

This can be shortened to:

COPY
(select array_to_json(array_agg(row_to_json(t))) FROM test_table AS t)
TO '/home/postgres/test2.json';

>
> And the result I got is quite match what I expect.
>
> {"RECORDS":[{"id":1,"username":"john","fullname":"John"},{"id":2,"username":"anna","fullname":"Anna"},{"id":3,"username":"sussi","fullname":"Sussi"},{"id":4,"username":"david","fullname":"David
> Beckham"},{"id":5,"username":"wayne","fullname":"Wayne Rooney"}]}
>
>
> I think the COPY command does not the `\n` character for pretty in
> `json_agg` command.

Well in the first case you are starting by concatenating the 5 rows in
the table into a single row with the table rows separated by new lines:

SELECT json_agg(t1) AS "RECORDS" FROM test_table t1;
RECORDS
-----------------------------------------------------------
[{"id":1,"username":"john","fullname":"John"}, +
{"id":2,"username":"anna","fullname":"Anna"}, +
{"id":3,"username":"sussi","fullname":"Sussi"}, +
{"id":4,"username":"david","fullname":"David Beckham"}, +
{"id":5,"username":"wayne","fullname":"Wayne Rooney"}]
(1 row)

In the second case you start by maintaining the separate table rows:

select row_to_json(t) as js from test_table t;
js
--------------------------------------------------------
{"id":1,"username":"john","fullname":"John"}
{"id":2,"username":"anna","fullname":"Anna"}
{"id":3,"username":"sussi","fullname":"Sussi"}
{"id":4,"username":"david","fullname":"David Beckham"}
{"id":5,"username":"wayne","fullname":"Wayne Rooney"}
(5 rows)

and then keeping that as an array of arrays:

select array_agg(row_to_json(t)) from test_table t;

{"{\"id\":1,\"username\":\"john\",\"fullname\":\"John\"}","{\"id\":2,\"username\":\"anna\",\"fullname\":\"Anna\"}","{\"id\":3,\"username\":\"sussi\",\"fullname\":\"Sussi\"}","{\"id\":4,\"username\":\"david\",\"fullname\":\"David
Beckham\"}","{\"id\":5,\"username\":\"wayne\",\"fullname\":\"Wayne
Rooney\"}"}
(1 row)

which then gets turned back into JSON:

select ARRAY_TO_JSON(array_agg(row_to_json(t))) from test_table t;

[{"id":1,"username":"john","fullname":"John"},{"id":2,"username":"anna","fullname":"Anna"},{"id":3,"username":"sussi","fullname":"Sussi"},{"id":4,"username":"david","fullname":"David
Beckham"},{"id":5,"username":"wayne","fullname":"Wayne Rooney"}]

>
> Please help me give me your idea. Am I wrong or this is really a bug?
>
> Thank you and best regards,
>
> Đỗ Ngọc Trí*Cường*(Mr.)
>
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--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com