PostgreSQL starten

In this lesson you run a PostgreSQL database in a container. You write a row of data, read it back, and then learn the difference between a container that loses its data and one that keeps it in a named volume.

PostgreSQL is an official image. This lesson uses the tag 17, the PostgreSQL 17 release line. Its tags are listed on the official PostgreSQL image page on Docker Hub, and the project’s documentation is at postgresql.org.

Warning: the password in this lesson, learner-only-change-me, is for practice on your own computer. Choose your own password if you like, and never reuse a password that you use for anything else.

Step 1: Start the database

Type this command and press Enter:

docker run -d --name my-postgres -e POSTGRES_PASSWORD=learner-only-change-me postgres:17

The new parts of the command are:

• -e POSTGRES_PASSWORD=... sets an environment variable. The PostgreSQL image reads it to set the password of the database administrator, whose name is postgres.
• The database does not get a published port in this lesson. Only containers on the same network can reach it, and you reach it from inside the container instead.

Wait about 10 seconds, then type this command:

docker logs my-postgres

When the database is ready, the logs contain the line database system is ready to accept connections. If the logs do not show it yet, wait a few seconds and run the command again.

Step 2: Write a row and read it back

Open the PostgreSQL command line inside the container. Type this and press Enter:

docker exec -it my-postgres psql -U postgres

The prompt changes to postgres=#. This is psql, the PostgreSQL command line. It does not ask for a password here, because you are already inside the container. Type each line below, press Enter, and finish each SQL statement with a semicolon.

CREATE TABLE notes (id serial PRIMARY KEY, message text);
INSERT INTO notes (message) VALUES ('hello from a container');
SELECT * FROM notes;

• CREATE TABLE makes a table called notes with two columns.
• INSERT writes one row into the table.
• SELECT * FROM notes; reads every row back. The result looks like this:

 id |        message
----+------------------------
  1 | hello from a container
(1 row)

Type \q and press Enter to leave psql. You are back at your normal terminal prompt, and the database keeps running in the background.

Step 3: Remove the container and see what is lost

Type these two commands, one after the other:

docker stop my-postgres
docker rm my-postgres

Now start a new container from the same image, with no volume:

docker run -d --name my-postgres -e POSTGRES_PASSWORD=learner-only-change-me postgres:17

Wait about 10 seconds, then ask the new database for the row:

docker exec -it my-postgres psql -U postgres -c "SELECT * FROM notes;"

The answer is an error:

ERROR:  relation "notes" does not exist

The table was stored inside the first container. When you removed that container, its data went with it. Stop and remove this container before you continue:

docker stop my-postgres
docker rm my-postgres

Step 4: Keep the data in a named volume

A volume is storage that Docker keeps outside any container. The -v option attaches it to the folder where PostgreSQL stores its data. Start a container with a volume called pgdata:

docker run -d --name my-postgres -e POSTGRES_PASSWORD=learner-only-change-me -v pgdata:/var/lib/postgresql/data postgres:17

Wait about 10 seconds, then create the table and the row again, as in Step 2. Run the same docker exec -it my-postgres psql -U postgres command, then the three SQL lines, then \q.

Now remove the container, exactly as before:

docker stop my-postgres
docker rm my-postgres

Start a new container with the same volume:

docker run -d --name my-postgres -e POSTGRES_PASSWORD=learner-only-change-me -v pgdata:/var/lib/postgresql/data postgres:17

Wait about 10 seconds, then read the row again:

docker exec -it my-postgres psql -U postgres -c "SELECT * FROM notes;"

The row is still there, with the same message. The data survived because the new container used the same volume.

Step 5: Clean up the container

Stop and remove the container. Keep the volume, because the next lessons do not need it but you may want to read the data again later:

docker stop my-postgres
docker rm my-postgres

The volume pgdata stays on your computer until you delete it. The Clean up and next steps lesson shows how to remove it, and it warns that deleting a volume deletes the data inside it.

What you learned

• A container’s own files disappear when the container is removed.
• A named volume keeps data after the container is removed.
• docker exec runs a command inside a running container.
• psql is the PostgreSQL command line. Type \q to quit it.

What comes next

Go on to Run Node.js, where a small program runs in a container without Node.js installed on your computer.