When you install multiple PostgreSQL versions via apt-get on Ubuntu, the system defaults to using the older version's binaries. This occurs because:

• The package manager prioritizes stable repository packages
• Default PATH settings point to the original installation
• Symbolic links aren't automatically updated for newer versions

First verify which versions are actually installed:

dpkg -l | grep postgresql

And locate the binaries:

which psql
/usr/bin/psql --version

Ubuntu's update-alternatives system provides the cleanest solution:

sudo update-alternatives --config psql
sudo update-alternatives --config pg_dump
sudo update-alternatives --config createdb

You'll see output like:

There are 2 choices for the alternative psql (providing /usr/bin/psql).

  Selection    Path                 Priority   Status
------------------------------------------------------------
* 0            /usr/bin/psql-8.4     100       auto mode
  1            /usr/bin/psql-8.4     100       manual mode
  2            /usr/bin/psql-9.0     90        manual mode

Select the number corresponding to PostgreSQL 9.0.

For more control, prepend the newer version's bin directory to your PATH:

export PATH=/usr/lib/postgresql/9.0/bin:$PATH

Make this permanent by adding to ~/.bashrc:

echo 'export PATH=/usr/lib/postgresql/9.0/bin:$PATH' >> ~/.bashrc
source ~/.bashrc

Confirm the change took effect:

psql --version
createdb --version

Both should now report 9.0.x.

Remember that switching CLI tools doesn't affect running services. To manage services:

sudo service postgresql stop
sudo service postgresql start 9.0

If you encounter problems:

• Connection refused: Check pg_hba.conf and postgresql.conf
• Version mismatch: Ensure client and server versions match
• Port conflicts: Different versions should run on different ports

For clarity, here's a working 9.0 setup where 8.4 was default:

# In /etc/postgresql/9.0/main/postgresql.conf
port = 5433  # Different from 8.4's 5432

# In ~/.bash_profile
alias psql9='PGPORT=5433 psql -h localhost'

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When you install multiple PostgreSQL versions via apt-get on Ubuntu, the system defaults to using the older version's CLI tools (like createdb and psql) due to how Linux prioritizes paths in $PATH. Here's how Ubuntu typically structures the installations:

# Default 8.4 installation
/usr/lib/postgresql/8.4/bin/psql
# PPA-installed 9.0
/usr/lib/postgresql/9.0/bin/psql

The most straightforward solution is to use absolute paths:

/usr/lib/postgresql/9.0/bin/psql -U postgres
/usr/lib/postgresql/9.0/bin/createdb my_new_db

Update your .bashrc or .zshrc to prioritize PostgreSQL 9.0 binaries:

# Add this to ~/.bashrc
export PATH=/usr/lib/postgresql/9.0/bin:$PATH

Then reload your shell:

source ~/.bashrc

Create distinct aliases for each version in your shell configuration:

alias psql84='/usr/lib/postgresql/8.4/bin/psql'
alias psql90='/usr/lib/postgresql/9.0/bin/psql'
alias createdb90='/usr/lib/postgresql/9.0/bin/createdb'

After making changes, confirm which version is active:

psql --version
# Should return: psql (PostgreSQL) 9.0.x

Ubuntu's pg_wrapper system lets you handle multiple clusters:

# List available clusters
pg_lsclusters

# Start specific version cluster
sudo pg_ctlcluster 9.0 main start

# Connect to 9.0 cluster
psql --cluster 9.0/main

If you encounter port conflicts (both versions defaulting to 5432), modify postgresql.conf:

# For 9.0 installation
sudo nano /etc/postgresql/9.0/main/postgresql.conf
# Change port = 5433