postgres COPY (...) TO PROGRAM on Windows giving error - windows

Question

I am trying to dump PostgreSQL tables using a COPY statement. I am using COPY instead of pg_dump because I need to perform a transformation on one of the columns as it comes out.
The statement I would like to execute is essentially this
COPY ( SELECT 'punt' ) TO PROGRAM '7z a -si C:/Users/Public/Documents/punt';
but I am getting the generic error
ERROR: program "7z a -si C:/Users/Public/Documents/punt" failed
DETAIL: child process exited with exit code 1
I am trying to pass the COPY output through 7zip similar to the fourth example in the COPY documentation in order to compress the output as the database is about 130 GB and I'm ideally working with SSDs. The following command works fine:
COPY ( SELECT 'punt' ) TO 'C:/Users/Public/Documents/punt';
with result COPY 1. I have executed the following command on the psql prompt and it also works which makes me fairly confident about syntax and permissions:
\! echo 'punt' | 7z a -si C:/Users/Public/Documents/punt
Am I missing something about the cmd pipeline or the 7zip executable permissions?
EDIT: Postgres version is 9.3.3
I'm still getting the same error after changing the dir permissions to NETWORK SERVICE based on Craig Ringer's answer. This, however, did not fix the problem and the error is identical. Even after changing the target directory's permissions giving full control to Everyone, I still got the same error. I also tried changing the permission on the 7zip executable and the 7zip Program Files Directory to no avail.
In experimentation, I ran the following, taking the input from copy out of the equation:
COPY ( select '1' ) TO PROGRAM 'echo punt | 7z a -si C:/Users/Public/Documents/punt';
and curiously got the failure message, but instead with exit code 255, which for 7zip has the meaning "User stopped the process". Can't seem to find a reason for this myself.

Answer 1

It'll be a permissions issue.
The PostgreSQL server runs as NETWORKSERVICE for 9.2 and above, or user postgres for 9.1 and below, by default. COPY runs server-side as the user PostgreSQL its self runs as.
Neither of these are likely to have the right to write to C:\Users\Public\Documents.
You'll need to grant the PostgreSQL service user the right to write there using the Windows security properties dialog, or choose another location it can write to.
It works with \! because psql is running as your user, which will have permissions to write there, and \! runs the command client-side as the user running psql.

Answer 2

Strikethrough the 7zip bit in my edit, the 255 error code is actually from cmd not finding the 7zip exe. I plopped 7zip.exe on C:/ (couldn't figure out escaping the space in Program Files) and ran
COPY ( SELECT 'punt' ) TO PROGRAM 'C:/7z a -si C:/Users/Public/Documents/punt';
and this worked. I'm guessing the NETWORKSERVICE user doesn't have the path environment variable loaded, but I'm not sure about that.
I set the permissions on C:/Users/Public/Documents back to its default and the COPY ... TO PROGRAM still worked so file permission wasn't an issue in this specific case. I would still look to Craig Ringer's suggestion first if I encountered a problem like this again.

Answer 3

CMD doesn't recognize a path with spaces.
COPY FROM PROGRAM doesn't have access to PATH variables.
Just wrap your PROGRAM_NAME like RESOURCE_NAME in double apostrophes and you are done:
COPY ( SELECT 'punt' ) TO PROGRAM '"C:/Program Files/bla/bla/7z.exe" a -si "C:/Users/Public/Documents/punt"';