PostgreSQL 11 trigger can’t select a value in a table to use it in a second table

Basically we would like to create a TRIGGER for each table we want to be notified for an UPDATE/INSERT/DELETE operation. Once this trigger fires it will execute a function that will simply append a new row (encoding the event) to a log table that we will then poll from an external service.

That's a pretty standard use for a trigger.

Before going all in with Postgres TRIGGER(s) we would like to know how they scale: how many triggers can we create on a single Postgres installation?

If you keep creating them, eventually you'll run out of disk space.

There's no specific limit for triggers.

PostgreSQL limits are documented on the about page.

Does they affect query performance?

It depends on the trigger type, trigger language, and what the trigger does.

A simple PL/PgSQL BEFORE ... FOR EACH STATEMENT trigger that doesn't do anything has near-zero overhead.

FOR EACH ROW triggers have higher overhead than FOR EACH STATEMENT triggers. Scaling, obviously, with the affected row counts.

AFTER triggers are more expensive than BEFORE triggers because they must be queued up until the statement finishes doing its work, then executed. They aren't spilled to disk if the queue gets big (at least in 9.4 and below, may change in future) so huge AFTER trigger queues can cause available memory to overrun, resulting in the statement aborting.

A trigger that modifies the NEW row before insert/update is cheaper than a trigger that does DML.

The specific use case you want would perform better with an in-progress enhancement that might make it into PostgreSQL 9.5 (if we're lucky), where FOR EACH STATEMENT triggers can see virtual OLD and NEW tables. This isn't possible in current PostgreSQL versions, so you must use FOR EACH ROW triggers instead.

Did anyone before tried this ?

Of course. It's a pretty standard use for triggers, along with auditing, sanity checking, etc.

You'll want to look into LISTEN and NOTIFY for a good way to wake up your worker when changes to the task table happen.

You're already doing the most important thing by avoiding talking to external systems directly from triggers. That tends to be problematic for performance and reliability. People often try to do things like send mail directly from a trigger, and that's bad news.

First, you need to specified when do you want to run the trigger inside the table. I've edited your trigger and use AFTER UPDATE and AFTER INSERT:

AFTER INSERT:

USE `TEST`;
DELIMITER $$
DROP TRIGGER IF EXISTS MEDICAMENTO.monthUpdateTriggerAI$$
USE `TEST`$$
    CREATE DEFINER = CURRENT_USER TRIGGER `TEST`.`monthUpdateTriggerAI` AFTER INSERT ON `TestTable` FOR EACH ROW
    BEGIN
    SET @COUNT=(SELECT COUNT(*) FROM TestTable2 WHERE (ItemId=NEW.ItemId AND Year=YEAR(NEW.Date) AND Month=MONTH(NEW.Date)));
    IF @COUNT=0 THEN
        INSERT INTO TestTable2 (ItemId,Year,Month,Open,Close,Increase ) VALUES 
        (NEW.ItemId , YEAR(NEW.Date), MONTH(NEW.Date),NEW.Value,NEW.Close,0.0); 
    ELSE
        UPDATE TestTable2 SET TestTable2.Close=NEW.Close AND Increase=(NEW.Close/(SELECT 
        Open FROM TestTable WHERE (Year=YEAR(NEW.Date) and Month=MONTH(NEW.Date)) LIMIT 1))
        WHERE (Year=YEAR(NEW.Date) and Month=MONTH(NEW.Date));
    END IF;
    END;
$$
DELIMITER ;

AFTER UPDATE:

USE `TEST`;
DELIMITER $$
DROP TRIGGER IF EXISTS MEDICAMENTO.monthUpdateTriggerAU$$
USE `TEST`$$
    CREATE DEFINER = CURRENT_USER TRIGGER `TEST`.`monthUpdateTriggerAU` AFTER UPDATE ON `TestTable` FOR EACH ROW
BEGIN

    UPDATE TestTable2 SET TestTable2.Close=NEW.Close AND Increase=(NEW.Close/(SELECT 
    Open FROM TestTable WHERE (Year=YEAR(NEW.Date) and Month=MONTH(NEW.Date)) LIMIT 1))
    WHERE (Year=YEAR(NEW.Date) and Month=MONTH(NEW.Date));

END;
$$
DELIMITER ;

Note: Be careful about the SELECT Open FROM TestTable WHERE (Year=YEAR(NEW.Date) and Month=MONTH(NEW.Date)), I've added a LIMIT 1 to avoid duplicate rows.

Change the database TEST.

You can also use ON DUPLICATE KEY on the two triggers execution.

You can get more information in the MySQL's documentation manual.