Why subqueried function does not insert new rows? - postgresql

Question

I need a function to insert rows because one column's (seriano) default value should be the same as PK id.
I have defined table:
CREATE SEQUENCE some_table_id_seq
INCREMENT 1
START 1
MINVALUE 1
MAXVALUE 9223372036854775807
CACHE 1;
CREATE TABLE some_table
(
id bigint NOT NULL DEFAULT nextval('some_table_id_seq'::regclass),
itemid integer NOT NULL,
serialno bigint,
CONSTRAINT stockitem_pkey PRIMARY KEY (id),
CONSTRAINT stockitem_serialno_key UNIQUE (serialno)
);
and function to insert count of rows:
CREATE OR REPLACE FUNCTION insert_item(itemid int, count int DEFAULT 1) RETURNS SETOF bigint AS
$func$
DECLARE
ids bigint[] DEFAULT '{}';
id bigint;
BEGIN
FOR counter IN 1..count LOOP
id := NEXTVAL( 'some_table_id_seq' );
INSERT INTO some_table (id, itemid, serialno) VALUES (id, itemid, id);
ids := array_append(ids, id);
END LOOP;
RETURN QUERY SELECT unnest(ids);
END
$func$
LANGUAGE plpgsql;
And inserting with it works fine:
$ select insert_item(123, 10);
insert_item
-------------
1
2
3
4
5
6
7
8
9
10
(10 rows)
$ select * from some_table;
id | itemid | serialno
----+--------+----------
1 | 123 | 1
2 | 123 | 2
3 | 123 | 3
4 | 123 | 4
5 | 123 | 5
6 | 123 | 6
7 | 123 | 7
8 | 123 | 8
9 | 123 | 9
10 | 123 | 10
(10 rows)
But if I want to use function insert_item as subquery, it seems not to work anymore:
$ select id, itemid from some_table where id in (select insert_item(123, 10));
id | itemid
----+--------
(0 rows)
I created dumb function insert_dumb to test in a subquery:
CREATE OR REPLACE FUNCTION insert_dumb(itemid int, count int DEFAULT 1) RETURNS SETOF bigint AS
$func$
DECLARE
ids bigint[] DEFAULT '{}';
BEGIN
FOR counter IN 1..count LOOP
ids := array_append(ids, counter::bigint);
END LOOP;
RETURN QUERY SELECT unnest(ids);
END
$func$
LANGUAGE plpgsql;
and this works in a subquery as expected:
$ select id, itemid from some_table where id in (select insert_dumb(123, 10));
id | itemid
----+--------
1 | 123
2 | 123
3 | 123
4 | 123
5 | 123
6 | 123
7 | 123
8 | 123
9 | 123
10 | 123
(10 rows)
Why does insert_item function not insert new rows when called as subquery? I tried to add raise notice to the loop and it runs as expected shouting new id every time (and increasing the sequence), but no new rows are appended to the table.
I made all the setup available as fiddle
I am using Postgres 11 on Ubuntu.
EDIT
Of course, I let out my real reason, and it pays off...
I need the insert_item function returning ids, so I could use it in update-statement, like:
update some_table set some_text = 'x' where id in (select insert_item(123, 10);)
And addition to the why-question: it is understandable I can get no ids in return (because they share the same snapshot), but the function runs all the needed INSERTs without affecting the table. Shouldn't those rows be available in the next query?

Answer 1

The problem is that the subquery and the surrounding query share the same snapshot, that is, they see the same state of the database. Hence the outer query cannot see the rows inserted by the inner query.
See the documentation (which explains that in the context of WITH, although it also applies here):
The sub-statements in WITH are executed concurrently with each other and with the main query. Therefore, when using data-modifying statements in WITH, the order in which the specified updates actually happen is unpredictable. All the statements are executed with the same snapshot (see Chapter 13), so they cannot “see” one another's effects on the target tables.
In addition, there is a second problem with your approach: if you run EXPLAIN (ANALYZE) on your statement, you will find that the subquery is not executed at all! Since the table is empty, there is no id, and running the subquery is not necessary to calculate the (empty) result.
You will have to run that in two different statements. Or, better, do it in a different fashion: updating a row that you just inserted is unnecessarily wasteful.

Answer 2

Laurenz explained the visibility problem, but you don't need the sub-query at all if you re-write your function to return the actual table, rather than just he IDs
CREATE OR REPLACE FUNCTION insert_item(itemid int, count int DEFAULT 1)
RETURNS setof some_table
AS
$func$
INSERT INTO some_table (id, itemid, serialno)
select NEXTVAL( 'some_table_id_seq' ), itemid, currval('some_table_id_seq')
from generate_series(1,count)
returning *;
$func$
LANGUAGE sql;
Then you can use it like this:
select id, itemid
from insert_item(123, 10);
And you get the complete inserted rows.
Online example