Re: Emitting JSON to file using COPY TO

Andrew Dunstan wrote:

> IMNSHO, we should produce either a single JSON
> document (the ARRAY case) or a series of JSON documents, one per row
> (the LINES case).

"COPY Operations" in the doc says:

" The backend sends a CopyOutResponse message to the frontend, followed
by zero or more CopyData messages (always one per row), followed by
CopyDone".

In the ARRAY case, the first messages with the copyjsontest
regression test look like this (tshark output):

PostgreSQL
Type: CopyOut response
Length: 13
Format: Text (0)
Columns: 3
Format: Text (0)
PostgreSQL
Type: Copy data
Length: 6
Copy data: 5b0a
PostgreSQL
Type: Copy data
Length: 76
Copy data:
207b226964223a312c226631223a226c696e652077697468205c2220696e2069743a2031…

The first Copy data message with contents "5b0a" does not qualify
as a row of data with 3 columns as advertised in the CopyOut
message. Isn't that a problem?

At least the json non-ARRAY case ("json lines") doesn't have
this issue, since every CopyData message corresponds effectively
to a row in the table.

[1] https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-COPY

Best regards,
--
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite