When dealing with multiple PostgreSQL versions on Ubuntu, it's crucial to understand how the init system manages different versions. The postgresql service in /etc/init.d/ is actually a master script that controls all installed versions through symbolic links.

ls -l /etc/init.d/postgresql*
# Typical output:
# /etc/init.d/postgresql -> ../lib/postgresql-common/init.d-functions
# /etc/init.d/postgresql-9.1
# /etc/init.d/postgresql-9.3

To prevent PostgreSQL 9.1 from starting automatically while keeping it installed, use the update-rc.d command:

sudo update-rc.d postgresql-9.1 disable
# This creates these symlinks:
# /etc/rc*.d/KXXpostgresql-9.1 -> ../init.d/postgresql-9.1

If you're using systemd (Ubuntu 15.04+), the command would be:

sudo systemctl disable postgresql@9.1-main

After making changes, verify the new behavior:

# Check current status
sudo service postgresql-9.1 status

# Test the new boot configuration
sudo shutdown -r now
# After reboot:
pg_lsclusters

You can manually control specific clusters when needed:

# Start only when needed
sudo pg_ctlcluster 9.1 main start

# Stop when finished
sudo pg_ctlcluster 9.1 main stop

Unlike killing processes post-boot, this method:

• Prevents the service from starting at all
• Maintains clean service management
• Allows for proper dependency handling during startup
• Preserves the ability to manually start when needed

If you want to completely prevent accidental starts, you can also edit the PostgreSQL configuration:

# In /etc/postgresql/9.1/main/start.conf
# Change from 'auto' to:
manual

---

After upgrading from PostgreSQL 9.1 to 9.3 on Ubuntu 12.04, I found both versions automatically starting on boot. Here's what pg_lsclusters shows:

Ver Cluster Port Status Owner    Data directory               Log file
9.1 main    5433 online postgres /var/lib/postgresql/9.1/main /var/log/postgresql/postgresql-9.1-main.log
9.3 main    5432 online postgres /var/lib/postgresql/9.3/main /var/log/postgresql/postgresql-9.3-main.log

PostgreSQL on Ubuntu uses init.d scripts for service management. The main control script is /etc/init.d/postgresql, which handles all installed versions. The script reads configuration from /etc/postgresql-common/ to determine which clusters to start.

There are two clean ways to prevent PostgreSQL 9.1 from auto-starting:

Method 1: Using pg_ctlcluster

The recommended approach is to disable the specific cluster:

sudo pg_ctlcluster 9.1 main stop
sudo update-rc.d postgresql disable 9.1

This modifies the symlinks in /etc/rc*.d/ to prevent auto-start while keeping the installation intact.

Method 2: Manual Configuration Edit

Alternatively, edit the auto-start configuration:

sudo nano /etc/postgresql/9.1/main/start.conf

Change the line to read:

auto = false

After reboot, verify with:

pg_lsclusters

You should see 9.1 as "down" while 9.3 remains "online". To manually start 9.1 when needed:

sudo pg_ctlcluster 9.1 main start

For newer Ubuntu versions using systemd:

sudo systemctl disable postgresql@9.1-main
sudo systemctl stop postgresql@9.1-main

Unlike complete removal or kill scripts, this approach:

• Preserves all 9.1 data and configuration
• Maintains the ability to start 9.1 manually when needed
• Properly integrates with Ubuntu's service management
• Consumes zero resources when 9.1 isn't running