Subject About enforcing uniqueness with triggers
Author Raul Chirea
Hello everybody,

I keep seeing a common missconception apearing, from time to time, in this
kind of forums. It is that one could enforce uniqueness of one or more
fields in a table with triggers instead of a unique key.

It isn't just a bad practice is is a really a dangerous thing ! Let me show

Consider a case when, for simplicity, we forbid any update and we check the
uniqueness with a before insert trigger.

Let's say that the trigger tries to find a row in the table with same values
in the "key fields" as the one that triggered the insert and if it does it
raises an exception.

At a first look it apears that everything is OK, we enforced uniqueness. BUT
THIS IS ONLY AN IMPRESSION if the trigger is running inside a transaction
other than "dirty read" one !
Consider the following scenario:
- the table is "TABLE" with the field we are trying to make unique named
- the following transactions are other than "dirty read" ones

1. start transaction "A"
2. insert into TABLE (ID) values (1);
3. start transaction "B"
4. insert into TABLE (ID) values (1);
/* at this time transaction "A" is not commited yet, so the row created
by the first insert
isn't visible in transaction "B", so our trigger will NOT raise the
exception !!! */
5. commit transaction "A"
6. commit transaction "B"

After that our "TABLE" contains two records with the same value for the ID

It might be usefull to put a chapter in docs with this type of
(printed in uppercase :-).
But this is up to FB gurus !

Regards !

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