How to use a newly inserted value from another table in PostgreSQL? - sql

Question

Assuming I have two tables final table and table_1, I want to use the the newest values from table_1 and insert them with a trigger in the final_table with every INSERT ON table_1. When I create the triggerfunction inserttrigger() as shown in the example and create the trigger, I get the newest value times the number of rows in table_1. How to write the trigger proper that I get only the single newest record in table1?
Doing:
-- Create tables and inserting example values
CREATE TABLE final_table(id INTEGER, value_fin INTEGER);
CREATE TABLE table_1(id INTEGER, value INTEGER);
INSERT INTO table_1 VALUES(1, 200), (2,203), (3, 209);
-- Create Triggerfunction
CREATE OR REPLACE FUNCTION inserttrigger()
RETURNS TRIGGER AS
$func$
BEGIN
INSERT INTO final_table
SELECT latest.id, latest.value
FROM (SELECT NEW.id, NEW.value FROM table_1) AS latest;
RETURN NEW;
END;
$func$ language plpgsql;
-- Create Trigger
CREATE TRIGGER final_table_update BEFORE INSERT ON table_1
FOR EACH ROW EXECUTE PROCEDURE inserttrigger() ;
--Insert example values
INSERT INTO table_1 VALUES(4, 215);
Results in:
SELECT * FROM final_table
id | value_fin
4 215
4 215
4 215
But should look like:
id | value_fin
4 215
While:
CREATE TRIGGER final_table_update BEFORE INSERT ON table_1
EXECUTE PROCEDURE inserttrigger() ;
Results in:
ERROR: record "new" is not assigned yet
DETAIL: The tuple structure of a not-yet-assigned record is indeterminate.

Answer 1

I would recommend the VALUES() syntax:
CREATE OR REPLACE FUNCTION inserttrigger()
RETURNS TRIGGER AS
$func$
BEGIN
INSERT INTO final_table VALUES(NEW.id, NEW.value);
RETURN NEW;
END;
$func$ language plpgsql;
CREATE TRIGGER final_table_update BEFORE INSERT ON table_1
FOR EACH ROW EXECUTE PROCEDURE inserttrigger();
Note that you could also get the same behavior with a common-table-expression and the returning syntax, which avoids the need for a trigger:
with t1 as (
insert into table_1(id, value_fin) values(4, 215)
returning id, value_fin
)
insert into final_table(id, value) select id, value_fin from t1