Create a simple function below by running the code below in the Query Tool
CREATE OR REPLACE FUNCTION general.myfunction(a integer, b integer)
RETURNS integer AS
$$
BEGIN
return a*b;
END;
$$
LANGUAGE plpgsql;

Note that a schema name is not required - if it is not supplied, the function will be created in the public schema

Call the function using a SELECT statement in the Query Tool
select general.myfunction(4,3)

If you have time, make a change to the function and call it again - note that to update the function, you can select it in pgAdmin then right-click > Scripts > CREATE Script, then amend the function definition run the statement

Create a test table by running the SQL below
CREATE TABLE test_table( col1 text, col2 text, col3 date);

Create a trigger function which sets col3 to today’s date when the trigger is executed - this is the action the trigger will generated
CREATE OR REPLACE FUNCTION test_table_trig() RETURNS TRIGGER as
$$
BEGIN
new.col3 = current_date;
return new;
END;
$$
LANGUAGE plpgsql;

Now create the trigger which will fire at the time of execution
CREATE TRIGGER test_table_trigger
BEFORE INSERT
ON test_table
FOR EACH ROW
EXECUTE PROCEDURE test_table_trig();

Test the trigger by using INSERT to add a row to the table
INSERT INTO test_table(col1,col2) VALUES('1','2')

Look at the table data, and check that today’s date has been added as a value for column 3

Open QGIS, and go to Processing > Graphical Modeler

In the Algorithms panel, search for Postgres, and double-click on PostgreSQL Execute SQL to include the process in your model

Double-click on the process in the model window to open it

In the dialog:

1. Set the Description to Add column

2. Set the Database (connection name) to the connection you created earlier

3. In the SQL query field, add
   ALTER TABLE test_table ADD COLUMN testedit_date date

4. Click OK to save these settings

In the Model Designer, click the Run model button or F5 to run the model

In pgAdmin, check that the new field has been added to the data, and right-click > Delete/Drop to remove it so we can run the process again

In the Graphical Modeler, add another PostgreSQL Execute SQL process to the model in the same way as before

This time, rename the process Add values, and add some data to the table you have just updated, using the following SQL in the SQL query field
INSERT INTO test_table(col1,col2, test_from_qgis) VALUES('3','4','test_value')

Under Dependencies, add a dependency on the previous process - this will make this process run after the previous one

Click OK - your model should look something like this

Run the model, then check the results in pgAdmin

When you are finished, save the model for future use