mySQL和LAST_INSERT_ID问题

Question

I have a DB-Application and now we have to start with replication (master-master-replication). We build a stored-function which returns an BIGINT. This value is unique on all involved servers.

The situation: I have a table definition:

create table test (
 id BIGINT not null primary key auto_increment,
 col1 TEXT);

the table has a before insert trigger:

CREATE TRIGGER test_insert BEFORE INSERT ON test
 FOR EACH ROW BEGIN
  IF NEW.id = 0 THEN
   SET @my_uuid = MYUUID();
   SET NEW.id = @my_uuid;
  END IF;
 END;

after an insert into test (col1) values ("foo") I need the value of the LAST_INSERT_ID() - but I only get the value "0".

I tried this in the trigger:

SET NEW.id = LAST_INSERT_ID(@my_uuid);

but it don´t work.

I read the mysql manpage which says, that all changes on last_insert_id within triggers and functions will be canceled at the end of the trigger.

So I try to avoid changing the application (which use php.last_insert_id())...

any ideas how to solve this without changing php-code?

greatings.

Answer 1

I assume that you're trying to avoid an insert on the two masters ending up with the same ID.

One way to do this (assuming 2 masters) is to set auto_increment_increment to 2, and auto_increment_offset to 0 on one master, and 1 on the other.

This will result in ids on each master that cannot collide with the other.

Aside: with a bigint and random UUIDs, you current approach is likely to have a collision somewhere around 3 billion rows due to the birthday paradox.

Answer 2

The behavior of LAST_INSERT_ID in mysql in relation to triggers is actually quite "un-standard" when compared to most other database server technologies. But I find it safer and much easier to work with.

Anyway, there's no one definite answer to you question, as replication is always complex. But the answer of user83591 is sufficient to give you a solution for most cases. Notion of the birthday paradox is also very much in place. You should accept that as your answer.