Re: Conflict between JSON_AGG and COPY

Dear Arian Klaver,

I think there is a misunderstood here.

I think that I quite understand how is the second query run.

The question I asked here is why exporting data, which is generated by a query "JSON_AGG" , with command "COPY". The data contain "\n" as 2 characters instead of "new line" character.

The second query is for the old version of PostgreSQL (9.3 and previous) cause of they don't have JSON_AGG aggregate function. Since 9.4, we have "JSON_AGG" already. So I want to rewrite and reduce the length of the query. But it is don't work as I expected with command COPY.

Thank you and best regards,

Đỗ Ngọc Trí Cường (Mr.) | Software Development Department | +84 28 3715 5325

From: "Adrian Klaver" <adrian(dot)klaver(at)aklaver(dot)com>
To: "Đỗ Ngọc Trí Cường" <dntcuong(at)digi-texx(dot)vn>, "pgsql-general" <pgsql-general(at)lists(dot)postgresql(dot)org>
Sent: Monday, April 9, 2018 12:59:44 AM
Subject: 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';

BQ_BEGIN

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.

BQ_END

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"}]

BQ_BEGIN

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.)

*Software Development Dept.*

Mobile: +84 9 3800 3394 <tel:+84917220706>

Phone: +84 28 3715 6322 <callto:+84%208%203715%205325>

Email: dntcuong(at)digi-texx(dot)vn

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--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com