Informix select trigger to update column

Question

Is it possible to increase the value of a number in a column with a trigger every time it gets selected? We have special tables where we store the new id and when we update it in the app, it tends to get conflicts before the update happens, even when it all takes less than a second. So I was wondering if it is not possible to set database to increase value after every select on that column? Do not ask me why we do not use autoincrement for ids because I do not know.

Answer 1

Informix provides the SERIAL and BIGSERIAL types (and also SERIAL8, but don't use that) which provide autoincrement support. It also provides SEQUENCES with more sophisticated autoincrements. You should aim to use one of those.

Trying to use a SELECT trigger to update the table being selected from is, at best, fraught with problems about transactions and the like (problems which both the types and sequences carefully avoid).

If your design team needs help making effective use of these, ask a new question outlining what you want to achieve.

Normally, the correct way to proceed is to make the ID column in each table that defines 'something' (the Orders table, the Customer table, …) into a SERIAL column and either not insert a value into the ID column or insert 0 into it. The generated value can be retrieved and used when creating auxilliary information — order items, etc.

Note that you could think about using:

CREATE TABLE xyz_sequence
(
    xyz     SERIAL NOT NULL PRIMARY KEY
);

and using:

INSERT INTO xyz_sequence VALUES(0);

and then retrieving the inserted value — in Informix ESQL/C, you'd use sqlca.sqlerrd[1], in other languages, other techniques. You can also delete the newly inserted record, or even all the records in the table. You can afford to ignore errors from the DELETE statement; sooner or later, the rows will be deleted. The next value inserted will continue where the prior ones left off.

In a stored procedure, you'd use DBINFO('sqlca.sqlerrd1') to get the inserted value. You'd use DBINFO('bigserial') to get the value if you use a BIGSERIAL type.

Answer 2

I found out possible answer in this question update with return value instead of doing it with select it seems better to return value directly from update as update use locks it should be more safer even when you use multithreading application. But these are just my assumptions. Hopefully it will help someone.