PostgreSQL Query To Create A Directory - sql

Question

Files are being written to a directory using the COPY query:
Copy (SELECT * FROM animals) To '/var/lib/postgresql/data/backups/2020-01-01/animals.sql' With CSV DELIMITER ',';
However if the directory 2020-01-01 does not exist, we get the error
could not open file "/var/lib/postgresql/data/backups/2020-01-01/animals.sql" for writing: No such file or directory
PostgeSQL server is running inside a Docker container with the volume mapping /mnt/backups:/var/lib/postgresql/data/backups
The Copy query is being sent from a Node.js app outside of the Docker container.
The mapped host directory /mnt/backups was created by Docker Compose and is owned by root, so the Node.js app sending the COPY query is unable to create the missing directories due to insufficient permissions.
The backup file is meant to be transferred out of the Docker container to the Docker host.
Question: Is it possible to use an SQL query to ask PostgreSQL 11.2 to create a directory if it does not exist? If not, how will you recommend the directory creation be done?
Using Node.js 12.14.1 on Ubuntu 18.04 host. Using PostgreSQL 11.2 inside container, Docker 19.03.5

Answer 1

An easy way to solve it is to create the file directly into the client machine. Using STDOUT from COPY you can let the query output be redirected to the client standard output, which you can catch and save in a file. For instance, using psql in the client machine:
$ psql -U your_user -d your_db -c "COPY (SELECT * FROM animals) TO STDOUT WITH CSV DELIMITER ','" > file.csv
Creating an output directoy in case it does not exist:
$ mkdir -p /mnt/backups/2020-01/ && psql -U your_user -d your_db -c "COPY (SELECT * FROM animals) TO STDOUT WITH CSV DELIMITER ','" > /mnt/backups/2020-01/file.csv
On a side note: try to avoid exporting files into the database server. Although it is possible, I consider it a bad practice. Doing so you will either write a file into the postgres system directories or give the postgres user permission to write somewhere else, and it is something you shouldn't be comfortable with. Export data directly to the client either using COPY as I mentioned or follow the advice from #Schwern. Good luck!

Answer 2

Postgres has its own backup and restore utilities which are likely to be a better choice than rolling your own.
When used with one of the archive file formats and combined with pg_restore, pg_dump provides a flexible archival and transfer mechanism. pg_dump can be used to backup an entire database, then pg_restore can be used to examine the archive and/or select which parts of the database are to be restored. The most flexible output file formats are the “custom” format (-Fc) and the “directory” format (-Fd). They allow for selection and reordering of all archived items, support parallel restoration, and are compressed by default. The “directory” format is the only format that supports parallel dumps.
A simple backup rotation script might look like this:
#!/bin/sh
table='animals'
url='postgres://username#host:port/database_name'
date=`date -Idate`
file="/path/to/your/backups/$date/$table.sql"
mkdir -p `dirname $file`
pg_dump $url -w -Fc --table=$table -f $file
To avoid hard coding the database password, -w means it will not prompt for a password and instead look for a password file. Or you can use any of many Postgres authentication options.