How/When do views generate columns - postgresql

Question

Consider the following setup:
CREATE TABLE person (
first_name varchar,
last_name varchar,
age INT
);
INSERT INTO person (first_name, last_name, age)
VALUES ('pete', 'peterson', 16),
('john', 'johnson', 20),
('dick', 'dickson', 42),
('rob', 'robson', 30);
Create OR REPLACE VIEW adult_view AS
SELECT
first_name || ' ' || last_name as full_name,
age
FROM person;
If I run:
SELECT *
FROM adult_view
WHERE age > 18;
Will the view generate the full_name column for pete even though he gets filtered out?
Similarly, if I run:
SELECT age
FROM adult_view;
Will the view generate any full_name columns at all?

Answer 1

Concerning your first query:
EXPLAIN (VERBOSE, COSTS OFF)
SELECT full_name
FROM adult_view
WHERE age > 18;
QUERY PLAN
══════════════════════════════════════════════════════════════════════════════════
Seq Scan on laurenz.person
Output: (((person.first_name)::text || ' '::text) || (person.last_name)::text)
Filter: (person.age > 18)
Query Identifier: 5675263059379476127
So the table is scanned, people under 19 are filtered out, and then the result is calculated. So full_name won't be computed for Pete.
Concerning your second query:
EXPLAIN (VERBOSE, COSTS OFF)
SELECT age
FROM adult_view
WHERE age > 18;
QUERY PLAN
════════════════════════════════════════
Seq Scan on laurenz.person
Output: person.age
Filter: (person.age > 18)
Query Identifier: -8981994317495194105
(4 rows)
full_name is not calculated at all.