How to fix 01P01: deprecated_feature in PostgreSQL

Check your PostgreSQL logs to see the full warning message:

# View recent PostgreSQL logs
sudo tail -n 100 /var/log/postgresql/postgresql-*.log | grep "01P01\|deprecated"

The warning message will specify which feature triggered it. Common examples:
- "MD5 password support is deprecated"
- "Configuration parameter 'X' has been deprecated"

You can also query for MD5 passwords specifically:

-- Find roles still using MD5 passwords
SELECT rolname
FROM pg_authid
WHERE rolcanlogin = true
  AND rolpassword LIKE 'md5%';

Check the official PostgreSQL documentation for your version:

1. Visit the [Deprecated Features wiki page](https://wiki.postgresql.org/wiki/Deprecated_Features)
2. Check the [Obsolete or Renamed Features appendix](https://www.postgresql.org/docs/current/appendix-obsolete.html)
3. Review release notes for your PostgreSQL version

The documentation will indicate:
- Why the feature was deprecated
- What the recommended alternative is
- Timeline for removal (if specified)

For MD5 password deprecation (most common case):

-- Update postgresql.conf
-- Set password_encryption = 'scram-sha-256'

-- Then update each user's password
ALTER USER username WITH PASSWORD 'new_password';
-- This will now use SCRAM-SHA-256 automatically

Update pg_hba.conf to use scram-sha-256:

# Change from:
host    all    all    0.0.0.0/0    md5

# To:
host    all    all    0.0.0.0/0    scram-sha-256

Reload PostgreSQL configuration:

sudo systemctl reload postgresql
# Or via SQL:
SELECT pg_reload_conf();

Verify that applications can still connect with the new authentication method:

# Test connection with psql
psql -h localhost -U username -d database_name

# Check for any deprecation warnings
psql -U postgres -c "SHOW password_encryption;"

If using connection poolers (PgBouncer, pgpool), ensure they support SCRAM-SHA-256 authentication. Most modern versions do, but older versions may need updating.

Test all application connections to ensure compatibility:
- Web applications
- Background workers
- Backup scripts
- Monitoring tools

Once you've confirmed the new approach works:

1. Update all environments (dev, staging, production)
2. Update connection strings in application configuration
3. Document the changes for team members
4. Update deployment scripts and infrastructure-as-code

For CI/CD pipelines:

# GitHub Actions example
- name: Setup PostgreSQL
  run: |
    echo "password_encryption = scram-sha-256" >> postgresql.conf
    psql -c "ALTER USER testuser WITH PASSWORD 'testpass';"