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);

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

This can be shortened to:

(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.

    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;
 [{"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;
 {"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.)

*Software Development Dept.*


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