What does "psql: /usr/pgsql-11/lib/libpq.so.5: no version information available (required by psql)" actually mean? I have a question regarding following output:
[nikola@192 ~] $ sudo su - postgres
[postgres@192 ~] $ psql dvdrental postgres
psql: /usr/pgsql-11/lib/libpq.so.5: no version information available (required by psql)
psql: /usr/pgsql-11/lib/libpq.so.5: no version information available (required by psql)
psql (11.7, server 11.8)
Type &quot;help&quot; for help.

dvdrental=# 

What do following two messages stand for and why? Messages I am asking about:

psql: /usr/pgsql-11/lib/libpq.so.5: no version information available (required by psql)
psql (11.7, server 11.8) - BTW, for this psql --version command I get following output:
psql (PostgreSQL) 11.8 (had to install PostgreSQL Server version 11.7 in order to perform command such as \dt and \l)
...and also this output:


postgres=# select * FROM version();
                                                version                                                 
--------------------------------------------------------------------------------------------------------
 PostgreSQL 11.8 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2), 64-bit
(1 row)


P.S. I'm running PostgreSQL 11.8 on Fedora 30.
UPDATE1:
As requested by @LaurenzAlbe output for rpm -qa|grep postgres command is:
[nikola@192 ~] $ rpm -qa|grep postgres
postgresql-11.7-2.fc30.x86_64
postgresql11-11.8-1PGDG.f30.x86_64
postgresql11-libs-11.8-1PGDG.f30.x86_64
postgresql11-server-11.8-1PGDG.f30.x86_64
postgresql-server-11.7-2.fc30.x86_64

UPDATE2:
I've ran dnf remove postgresql postgresql-server command -&gt; databases are still there and I can access them via pgAdmin4 and psql: /usr/pgsql-11/lib/libpq.so.5: no version information available (required by psql message is no longer showing up. But, there is now new issue -&gt; when I access postgres shell in terminal using sudo su - postgres command and then perform psql some-database-name postgres I get message

bash: psql: command not found... 
Install package 'postgresql' to provide command 'psql'? [N/y]

I presume this is asking me to install AGAIN Fedora package which in 1st place I am trying to get rid off (to not show psql: /usr/pgsql-11/lib/libpq.so.5: no version information available (required by psql) message).
Any suggestion how to solve this without installing Fedora package but from PostgreSQL Global Development group?
P.S. BTW, I've installed Postgres server by following this link which uses PGDG packages (but something didn't work right on my machine and I guess I've been offered by Fedora30 OS to install Fedora packages which is obviously what I did :) ).
You have installed a mixture of packets: some from the Fedora distribution, some from the PostgreSQL Global Development group.
These don't work well together. In particular, you have psql from the Fedora packages in the PATH, but libpq from the PGDG packages is on the library path.
That explains your question #2.
Ad question #1: that warning is printed if the library version of the installed library is lower than the one the software was linked with.
Remove the Fedora packages to get rid of the problem:
dnf remove postgresql postgresql-server

I hope that puts the correct psql on your PATH; if not, reinstall the postgresql11 package.