How can connect psql with normal user without specifying database name?

Question

I can connect mysql with username without specifying any database name,showkey is one of mysql user:

mysql -u showkey -p
Enter password: 
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 14
Server version: 10.3.31-MariaDB-0+deb10u1 Debian 10

Now i log into mysql without specifying any database name.showkey is also a normal user for my postgresql,test is a database name in my postgresql.

psql -d test -U showkey
Timing is on.
Pager usage is off.
Null display is "missing data".
psql (11.14 (Debian 11.14-0+deb10u1))
Type "help" for help.

test=# 

I conncet the test database with a normal user showkey,now want to connect postgresql with normal user without specifying database name such way as in mysql:

psql -U showkey
psql: FATAL:  database "showkey" does not exist

sudo -u postgres psql can connect postgresql with a super user instead of with normal user.