How to upgrade table inside a trigger function in POSTGRESQL? - postgresql

Question

I would like to create a trigger function inside my database which checks, if the newly "inserted" value (max_bid) is at least +1 greater than the largest max_bid value currently in the table.
If this is the case, the max_bid value inside the table should be updated, although not with the newly "inserted" value, but instead it should be increased by 1.
For instance, if max_bid is 10 and the newly "inserted" max_bid is 20, the max_bid value inside the table should be increased by +1 (in this case 11).
I tried to do it with a trigger, but unfortunatelly it doesn't work. Please help me to solve this problem.
Here is my code:
CREATE TABLE bidtable (
mail_buyer VARCHAR(80) NOT NULL,
auction_id INTEGER NOT NULL,
max_bid INTEGER,
PRIMARY KEY (mail_buyer),
);
CREATE OR REPLACE FUNCTION max_bid()
RETURNS TRIGGER LANGUAGE PLPGSQL AS $$
DECLARE
current_maxbid INTEGER;
BEGIN
SELECT MAX(max_bid) INTO current_maxbid
FROM bidtable WHERE NEW.auction_id = OLD.auction_id;
IF (NEW.max_bid < (current_maxbid + 1)) THEN
RAISE EXCEPTION 'error';
RETURN NULL;
END IF;
UPDATE bidtable SET max_bid = (current_maxbid + 1)
WHERE NEW.auction_id = OLD.auction_id
AND NEW.mail_buyer = OLD.mail_buyer;
RETURN NEW;
END;
$$;
CREATE OR REPLACE TRIGGER max_bid_trigger
BEFORE INSERT
ON bidtable
FOR EACH ROW
EXECUTE PROCEDURE max_bid();
Thank you very much for your help.

Answer 1

In a trigger function that is called for an INSERT operation the OLD implicit record variable is null, which is probably the cause of "unfortunately it doesn't work".
Trigger function
In a case like this there is a much easier solution. First of all, disregard the value for max_bid upon input because you require a specific value in all cases. Instead, you are going to set it to that specific value in the function. The trigger function can then be simplified to:
CREATE OR REPLACE FUNCTION set_max_bid() -- Function name different from column name
RETURNS TRIGGER LANGUAGE PLPGSQL AS $$
BEGIN
SELECT MAX(max_bid) + 1 INTO NEW.max_bid
FROM bidtable
WHERE auction_id = NEW.auction_id;
RETURN NEW;
END; $$;
That's all there is to it for the trigger function. Update the trigger to the new function name and it should work.
Concurrency
As several comments to your question pointed out, you run the risk of getting duplicates. This will currently not generate an error because you do not have an appropriate constraint on your table. Avoiding duplicates requires a table constraint like:
UNIQUE (auction_id, max_bid)
You cannot deal with any concurrency issue in the trigger function because the INSERT operation will take place after the trigger function completes with a RETURN NEW statement. What would be the most appropriate way to deal with this depends on your application. Your options are table locking to block any concurrent inserts, or looping in a function until the insert succeeds.
Avoid the concurrency issue altogether
If you can change the structure of the bidtable table, you can get rid of the whole concurrency issue by changing your business logic to not require the max_bid column. The max_bid column appears to indicate the order in which bids were placed for each auction_id. If that is the case then you could add a serial column to your table and use that to indicate order of bids being placed (for all auctions). That serial column could then also be the PRIMARY KEY to make your table more agile (no indexing on a large text column). The table would look something like this:
CREATE TABLE bidtable (
id SERIAL PRIMARY KEY,
mail_buyer VARCHAR(80) NOT NULL,
auction_id INTEGER NOT NULL
);
You can drop your trigger and trigger function and just depend on the proper id value being supplied by the system.
The bids for a specific action can then be extracted using a straightforward SELECT:
SELECT id, mail_buyer
FROM bidtable
WHERE auction_id = xxx
ORDER BY id;
If you require a max_bid-like value (the id values increment over the full set of auctions), you can use a simple window function:
SELECT mail_buyer, row_number() AS max_bid OVER (PARTITION BY auction_id ORDER BY id)
FROM bidtable
WHERE auction_id = xxx;