Postgresql – How to get the amount of free space with Postgresql

You didn't specify why you wanted to use s_client.

If it is to interact with the database, any decent client will do. psql can be called with the sslmode=require option. See man psql.

If it is to check the SSL certificate (which is why I came across your question), it still doesn't work with s_client as Magnus pointed out 7 years ago. you can now do it with openssl s_client if you have a version >= 1.1.1, as pointed out in the answer of Adam Batkin. Use openssl version to check, or just directly try this command to see if it works.

echo "" | openssl s_client -starttls postgres -connect EXAMPLE.COM:5432 -showcerts

If you have an older version which doesn't support postgres, this python script can also retrieve the SSL certificate: https://github.com/thusoy/postgres-mitm/blob/master/postgres_get_server_cert.py

For example to check certificate dates:

postgres_get_server_cert.py example.com:5432 | openssl x509 -noout -dates

OK, got it from someone else. This query should do the trick:

select *
from pg_stat_activity
where datname = 'mydatabasename';