How to use variable settings in trigger functions? - postgresql

Question

I would like to record the id of a user in the session/transaction, using SET, so I could be able to access it later in a trigger function, using current_setting. Basically, I'm trying option n2 from a very similar ticket posted previously, with the difference that I'm using PG 10.1 .
I've been trying 3 approaches to setting the variable:
SET local myvars.user_id = 4, thereby setting it locally in the transaction;
SET myvars.user_id = 4, thereby setting it in the session;
SELECT set_config('myvars.user_id', '4', false), which depending of the last argument, will be a shortcut for the previous 2 options.
None of them is usable in the trigger, which receives NULL when getting the variable through current_setting. Here is a script I've devised to troubleshoot it (can be easily used with the postgres docker image):
database=$POSTGRES_DB
user=$POSTGRES_USER
[ -z "$user" ] && user="postgres"
psql -v ON_ERROR_STOP=1 --username "$user" $database <<-EOSQL
DROP TRIGGER IF EXISTS add_transition1 ON houses;
CREATE TABLE IF NOT EXISTS houses (
id SERIAL NOT NULL,
name VARCHAR(80),
created_at TIMESTAMP WITHOUT TIME ZONE DEFAULT now(),
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS transitions1 (
id SERIAL NOT NULL,
house_id INTEGER,
user_id INTEGER,
created_at TIMESTAMP WITHOUT TIME ZONE DEFAULT now(),
PRIMARY KEY(id),
FOREIGN KEY(house_id) REFERENCES houses (id) ON DELETE CASCADE
);
CREATE OR REPLACE FUNCTION add_transition1() RETURNS TRIGGER AS \$\$
DECLARE
user_id integer;
BEGIN
user_id := current_setting('myvars.user_id')::integer || NULL;
INSERT INTO transitions1 (user_id, house_id) VALUES (user_id, NEW.id);
RETURN NULL;
END;
\$\$ LANGUAGE plpgsql;
CREATE TRIGGER add_transition1 AFTER INSERT OR UPDATE ON houses FOR EACH ROW EXECUTE PROCEDURE add_transition1();
BEGIN;
%1% SELECT current_setting('myvars.user_id');
%2% SELECT set_config('myvars.user_id', '55', false);
%3% SELECT current_setting('myvars.user_id');
INSERT INTO houses (name) VALUES ('HOUSE PARTY') RETURNING houses.id;
SELECT * from houses;
SELECT * from transitions1;
COMMIT;
DROP TRIGGER IF EXISTS add_transition1 ON houses;
DROP FUNCTION IF EXISTS add_transition1;
DROP TABLE transitions1;
DROP TABLE houses;
EOSQL
The conclusion I came to was that the function is triggered in a different transaction and a different (?) session. Is this something that one can configure, so that all happens within the same context?

Answer 1

Handle all possible cases for the customized option properly:
option not set yet
All references to it raise an exception, including current_setting() unless called with the second parameter missing_ok. The manual:
If there is no setting named setting_name, current_setting throws an error unless missing_ok is supplied and is true.
option set to a valid integer literal
option set to an invalid integer literal
option reset (which burns down to a special case of 3.)
For instance, if you set a customized option with SET LOCAL or set_config('myvars.user_id3', '55', true), the option value is reset at the end of the transaction. It still exists, can be referenced, but it returns an empty string now ('') - which cannot be cast to integer.
Obvious mistakes in your demo aside, you need to prepare for all 4 cases. So:
CREATE OR REPLACE FUNCTION add_transition1()
RETURNS trigger AS
$func$
DECLARE
_user_id text := current_setting('myvars.user_id', true); -- see 1.
BEGIN
IF _user_id ~ '^\d+$' THEN -- one or more digits?
INSERT INTO transitions1 (user_id, house_id)
VALUES (_user_id::int, NEW.id); -- valid int, cast is safe
ELSE
INSERT INTO transitions1 (user_id, house_id)
VALUES (NULL, NEW.id); -- use NULL instead
RAISE WARNING 'Invalid user_id % for house_id % was reset to NULL!'
, quote_literal(_user_id), NEW.id; -- optional
END IF;
RETURN NULL; -- OK for AFTER trigger
END
$func$ LANGUAGE plpgsql;
db<>fiddle here
Notes:
Avoid variable names that match column names. Very error prone. One popular naming convention is to prepend variable names with an underscore: _user_id.
Assign at declaration time to save one assignment. Note the data type text. We'll cast later, after sorting out invalid input.
Avoid raising / trapping an exception if possible. The manual:
A block containing an EXCEPTION clause is significantly more expensive
to enter and exit than a block without one. Therefore, don't use
EXCEPTION without need.
Test for valid integer strings. This simple regular expression allows only digits (no leading sign, no white space): _user_id ~ '^\d+$'. I reset to NULL for any invalid input. Adapt to your needs.
I added an optional WARNING for your debugging convenience.
Cases 3. and 4. only arise because customized options are string literals (type text), valid data types cannot be enforced automatically.
Related:
User defined variables in PostgreSQL
Is there a way to define a named constant in a PostgreSQL query?
All that aside, there may be more elegant solutions for what you are trying to do without customized options, depending on your exact requirements. Maybe this:
Fastest way to get current user's OID in Postgres?

Answer 2

It is not clear why you are trying to concat NULL to user_id but it is obviously the cause of the problem. Get rid of it:
CREATE OR REPLACE FUNCTION add_transition1() RETURNS TRIGGER AS $$
DECLARE
user_id integer;
BEGIN
user_id := current_setting('myvars.user_id')::integer;
INSERT INTO transitions1 (user_id, house_id) VALUES (user_id, NEW.id);
RETURN NULL;
END;
$$ LANGUAGE plpgsql;
Note that
SELECT 55 || NULL
always gives NULL.

Answer 3

You can catch the exception when the value doesn't exist - here's the changes I made to get this to work:
CREATE OR REPLACE FUNCTION add_transition1() RETURNS TRIGGER AS $$
DECLARE
user_id integer;
BEGIN
BEGIN
user_id := current_setting('myvars.user_id')::integer;
EXCEPTION WHEN OTHERS THEN
user_id := 0;
END;
INSERT INTO transitions1 (user_id, house_id) VALUES (user_id, NEW.id);
RETURN NULL;
END;
$$ LANGUAGE plpgsql;
CREATE OR REPLACE FUNCTION insert_house() RETURNS void as $$
DECLARE
user_id integer;
BEGIN
PERFORM set_config('myvars.user_id', '55', false);
INSERT INTO houses (name) VALUES ('HOUSE PARTY');
END; $$ LANGUAGE plpgsql;