PgSQL - How to import database dump only when database completely empty? - postgresql

Question

The use-case actually to automate this with ansible. I want to import database dump only when database is completely empty (no tables inside). Of course there's always a way to execute sql statement, but this is last resort, I believe there should be more elegant solution for this.
pg_restore manual doesn't provide this option as far as I see.
Here's how I'm planning to do this with ansible:
- name: db_restore | Receive latest DB backup
shell: s3cmd --skip-existing get `s3cmd ls s3://{{ aws_bucket }}/ | grep sentry | tail -1 | awk '{print $4}'` sql.latest.tgz
args:
chdir: /root/
creates: sql.latest.tgz
- name: db_restore | Check if file exists
stat: path=/root/sql.latest.tgz
register: sql_latest
- name: db_restore | Restore latest DB backup if backup file found
shell: PGPASSWORD={{ dbpassword }} tar -xzOf /root/sentry*.tgz db.sql | psql -U{{ dbuser }} -h{{ pgsql_server }} --set ON_ERROR_STOP=on {{ dbname }}
when: sql_latest.stat.exists
ignore_errors: True
Ideally this should check if DB empty. No ansible module exist for this purpose. Google is also in silence.. Current solution actually also works, this will give error when import will fail, and I can just ignore error, but it's a bit painful to see a false alarm.

Answer 1

There's not really any such thing as "empty" as such; it generally has the built-in types, the default PL/PgSQL language, etc, even if you create from template0. If you create from a different template there could be a whole lot more in there.
PostgreSQL doesn't keep a record of the first non-template write to a DB, so you can't say "changed since created" either.
That's why there's no --if-empty option to pg_restore. It doesn't really make sense.
By far and away the best option is to execute psql to query the information_schema and determine if there are any tables in the public schema. Or, even better, query for the presence of specific tables and types you know will be created by the dump.
e.g.
psql -qAt mydbname -c "select 1 from information_schema.tables where table_schema = 'public' and table_name = 'testtable';"
You can then test for zero/nonzero rows returned on stdout. Or wrap it in SELECT EXISTS(...) to get a boolean from psql. Or use a DO block that ERRORs if the table exists if you need a zero/nonzero exit status from psql.

Answer 2

To regard the database as empty, we must know there nothing has been added from the point of creation. As postgres does not track this (as already mentioned by #Craig Ringer) I recommend a different approach with regards to ansible.
So, just use a handler mechanism like:
- name: Create zabbbix postgres DB
postgresql_db: name="{{zabbix_db_name}}"
notify:
- Init zabbix database

Answer 3

Since it is hard to tell, if a database is "empty", as explained by others, it is much easier to check, if the database exists, then create and restore in one step. I'm doing it like this:
- name: Check my_database database already exists
become: yes
become_user: postgres
shell: psql -l | grep my_database
ignore_errors: true
register: my_database_db_existence
- debug: var=my_database_db_existence
- name: Copy backup of the my-database database
shell: your-s3-command here
when: my_database_db_existence | failed
- name: Restore my_database database on first run
become_user: postgres
shell: createdb -O my_user my_database && psql -d my_database -f /path/to/my_dump.sql
when: my_database_db_existence | failed
P.S. Also written a detailed blog post explaining each ansible task in the implementation.

Answer 4

In my Ansible continuous deployment I prefer don't check empty DB or not. I run container with default properties and create DB if it not exist, after that I restore DB (create schemes, tables and etc):
- hosts: all
vars:
database_name: "maindb"
pg_admin_name: "postgres"
pg_admin_password: "postgres"
pghost: "localhost"
pg_user_name: "vr_user"
pg_user_password: "ChanGeMe2021"
tasks:
- name: Check if database is exist
community.postgresql.postgresql_info:
login_host: "{{ pghost }}"
login_user: "{{ pg_admin_name }}"
login_password: "{{ pg_admin_password }}"
filter:
- "databases"
register: pg_info
- name: Create database if not exist
block:
- name: Say status
ansible.builtin.debug:
msg: "Database is not exist!"
- name: Copy dadabase shchema
ansible.builtin.copy:
src: "./files/maindb.sql"
dest: "/tmp/maindb.sql"
- name: Create database
community.postgresql.postgresql_db:
login_host: "{{ pghost }}"
login_user: "{{ pg_admin_name }}"
login_password: "{{ pg_admin_password }}"
name: "{{ database_name }}"
encoding: UTF-8
# lc_collate: ru_RU.utf8
# lc_ctype: ru_RU.utf8
- name: Create role
community.postgresql.postgresql_user:
login_host: "{{ pghost }}"
login_user: "{{ pg_admin_name }}"
login_password: "{{ pg_admin_password }}"
name: "{{ pg_user_name }}"
password: "{{ pg_user_password }}"
- name: Restore database
community.postgresql.postgresql_db:
login_host: "{{ pghost }}"
login_user: "{{ pg_admin_name }}"
login_password: "{{ pg_admin_password }}"
name: "{{ database_name }}"
state: restore
target: "/tmp/maindb.sql"
register: pg_restore_result
failed_when: "'ERROR' in pg_restore_result.stderr"
- name: Print restore result
ansible.builtin.debug:
msg: "{{ pg_restore_result }}"
rescue:
- name: Rollback database
community.postgresql.postgresql_db:
login_host: "{{ pghost }}"
login_user: "{{ pg_admin_name }}"
login_password: "{{ pg_admin_password }}"
name: "{{ database_name }}"
state: absent
- name: Print when errors
ansible.builtin.debug:
msg: "Restore failed, because: {{ pg_restore_result.stderr_lines[1] }}"
when: pg_info.databases[database_name] is not defined
This code you can find here