Coalesce sentence containing an insert into clause fails in PostgreSQL - postgresql

Question

This is my trivial test table,
create table test (
id int not null generated always as identity,
first_name. varchar,
primary key (id),
unique(first_name)
);
As an alternative to insert-into-on-conflict sentences, I was trying to use the coalesce laziness to execute a select whenever possible or an insert, only when select fails to find a row.
coalesce laziness is described in documentation. See https://www.postgresql.org/docs/current/functions-conditional.html
Like a CASE expression, COALESCE only evaluates the arguments that are needed to determine the result; that is, arguments to the right of the first non-null argument are not evaluated. This SQL-standard function provides capabilities similar to NVL and IFNULL, which are used in some other database systems.
I also want to get back the id value of the row, having being inserted or not.
I started with:
select coalesce (
(select id from test where first_name='carlos'),
(insert into test(first_name) values('carlos') returning id)
);
but an error syntax error at or near "into" was found.
See it on this other DBFiddle
https://www.db-fiddle.com/f/t7TVkoLTtWU17iaTAbEhDe/0
Then I tried:
select coalesce (
(select id from test where first_name='carlos'),
(with r as (
insert into test(first_name) values('carlos') returning id
) select id from r
)
);
Here I am getting a WITH clause containing a data-modifying statement must be at the top level error that I don't understand, as insert is the first and only sentence within the with.
I am testing this with DBFiddle and PostgreSQL 13. The source code can be found at
https://www.db-fiddle.com/f/hp8T1iQ8eS4wozDCBhBXDw/5

Answer 1

Different method: chained CTEs:
CREATE TABLE test
( id INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY PRIMARY KEY
, first_name VARCHAR UNIQUE
);
WITH sel AS (
SELECT id FROM test WHERE first_name = 'carlos'
)
, ins AS (
INSERT INTO test(first_name)
SELECT 'carlos'
WHERE NOT EXISTS (SELECT 1 FROM test WHERE first_name = 'carlos')
RETURNING id
)
, omg AS (
SELECT id FROM sel
UNION ALL
SELECT id FROM ins
)
SELECT id
FROM omg
;

Answer 2

It seems that the returning value from the insert into clause is not equivalent in nature to the scalar query of a select clause. So I try encapsulating the insert into into an SQL function and it worked.
create or replace function insert_first_name(
_first_name varchar
) returns int
language sql as $$
insert into test (first_name)
values (_first_name)
returning id;
$$;
select coalesce (
(select id from test where first_name='carlos'),
(select insert_first_name('carlos'))
);
See it on https://www.db-fiddle.com/f/73rVXgqGfrG4VmjrAk6Z3i/2

Answer 3

This is a refinement on #wildplasser accepted answer. it avoids comparing first_name twice and uses coalesce instead of union all. Kind of an selsert in just one sentence.
with sel as (
select id from test where first_name = 'carlos'
)
, ins as (
insert into test(first_name)
select 'carlos'
where (select id from sel) is null
returning id
)
select coalesce (
(select id from sel),
(select id from ins)
);
See it at https://www.db-fiddle.com/f/goRh4TyAebTkEZFHk6WbtK/6