Dependent Origination

COPY command of Postgres

Posted on: January 24, 2014

COPY command of Postgres is very useful, convenient and super fast! However, in using it, I have discovered a few considerations that have to go in when you actually write the code.

“COPY table_name FROM /localdisk/path_to_file”

You are going to get an error message saying only superuse can copy from a local file. However, any user can copy from stdin, or copy to stdout. Now you have four workarounds.

1. Use “\COPY” instead. “\”, in postgres, tells the command to treat the “FROM” as if from stdin. That way, any user can copy from stdin, and (educated guess starts:) the implementation probably redirected the give file to stdin. So COPY FROM a local file can work for you, even if you are not a super user of the database.

2. However, that ‘\’ cannot really work in Python. So in Python, you can use psycopg2’s cursor class to do the copy from. A sample code is like this:

import psycopg2

conn = psycopg2.connect(“dbname=db user=user password=password host=host port=port”)

cursor = conn.cursor()

cursor.copy_from(file, “table_name)

conn.commit()

3. You can also use psql command line to copy from.

psql -c “COPY table FROM stdin” and redirect your file into the stdin. Or use subprocess in Python to do that. See here for example.

4. Should you have permission to change the database permissions, you can make yourself a superuser.

ALTER USER myuser WITH SUPERUSER

See here for more details.

I have personally tried #1 and #2 and they all worked. #3 and #4 I haven’t tried but they are listed here for future reference.

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