PostgreSQL connection strings embedded in your application can take two different forms: the key-value notation or the postgresql:// URI scheme. When it comes to using psql though, another form of connection string is introduced, with command line options -h -p -U and environment variable support.

In this short article you will learn that you can use either of the three different forms in psql and thus easily copy & paste you application connection string right at the console to test it!

When using psql to connect to your Postgres database, you might be used to using the following options:

$ psql --help
psql is the PostgreSQL interactive terminal.


General options:
  -c, --command=COMMAND    run only single command (SQL or internal) and exit
  -d, --dbname=DBNAME      database name to connect to (default: "dim")

Connection options:
  -h, --host=HOSTNAME      database server host or socket directory (default: "local socket")
  -p, --port=PORT          database server port (default: "5432")
  -U, --username=USERNAME  database user name (default: "dim")
  -w, --no-password        never prompt for password
  -W, --password           force password prompt (should happen automatically)

The following trick is not apparent in this help message, and maybe easy to miss when reading the manual page for psql. You can actually use a whole connection string for the dbname parameter.

Here’s three different ways to establish a connection to the same database, the one I’m using in my book The Art of PostgreSQL of course:

$ psql -Atx -U taop -d taop -h localhost -p 5432 -c 'select current_date'

$ psql -Atx postgresql://taop@localhost:5432/taop -c 'select current_date'

$ psql -Atx "host=localhost port=5432 dbname=taop user=taop" -c 'select current_date' 

What happens is that the first comamnd line argument is used by psql as the dbname here, and the connection string parsing provided by libpq is done on that parameter. So that for the -d dbname parameter, you can actually pass in a whole connection string:

$ psql -d "host=localhost port=5432 dbname=taop user=taop"
psql (12devel, server 10.10)
Type "help" for help.

taop> select current_date;
│ current_date │
│ 2019-09-04   │
(1 row)

So if your application code is using a driver based on libpq or a compatible connection string scheme, re-using the application connection string on your terminal with psql is a simple copy-paste away.

import sys
import psycopg2
import psycopg2.extras
from calendar import Calendar

CONNSTRING = "dbname=taop application_name=factbook"

I hope you’ll find this simple trick useful in your daily usage of psql and PostgreSQL! For more about psql, you can also read my article Setting up psql, the PostgreSQL CLI where we dive in my current (at the time) setup for it. I recently changed my ~/.psqlrc file to integrate pspg as the pager, and will probably do write-up about that later.

Meanwhile, check-out my book The Art of PostgreSQL, you can register to get a free sample with many other good SQL techniques to improve your developer skills!