MySQL 8 - Trigger on INSERT - duplicate AUTO_INCREMENT id for VCS - mysql

Question

Trying to
create trigger that is called on INSERT & sets originId = id (AUTO_INCREMENT),
I've used SQL suggested here in 1st block:
CREATE TRIGGER insert_example
BEFORE INSERT ON notes
FOR EACH ROW
SET NEW.originId = (
SELECT AUTO_INCREMENT
FROM information_schema.TABLES
WHERE TABLE_SCHEMA = DATABASE()
AND TABLE_NAME = 'notes'
);
Due to information_schema caching I have also set
information_schema_stats_expiry = 0
in my.cnf file. Now information gets updated almost instantly on every INSERT, as I've noticed..
But, performing "direct" INSERTs via console with ~2min intervals, I keep getting not updated AUTO_INCREMENT values in originId.
(They shoud be equal to id fields)
While explicit queries, fetching AUTO_) result in updated correct values.
Thus I suspect that the result of SELECT AUTO_INCREMENT... subquery gets somehow.. what? cached?
How can one get around this?
Thank you.
Edit 1
I intended to implement sort of VCS this way:
User creates new Note, app marks it as 'new' and performs an INSERT in MySQL table. It is the "origin" note.
Then user might edit this Note (completely) in UI, app will mark is as 'update' and INSERT it in MySQL table as a new row, again. But this time originId should be filled with an id of "origin" Note (by app logics). And so on.
This allows PARTITIONing by originId on SELECT, fetching only latest versions to UI.
initial Problem:
If originId of "origin" Note is NULL, MySQL 8 window function(s) in default (and only?) RESPECT_NULL mode perform(s) framing not as expected ("well, duh, it's all about your NULLs in grouping-by column").
supposed Solution:
Set originId of "origin" Notes to id on their initial and only INSERT, expecting 2 benefits:
Easily fetch "origin" Notes via originId = id,
perform correct PARTITION by originId.
resulting Problem:
id is AUTO_INCREMENT, so there's no way (known to me) of getting its new value (for the new row) on INSERT via backend (namely, PHP).
supposed Solution:
So, I was hoping to find some MySQL mechanism to solve this (avoiding manipulations with id field) and TRIGGERs seemed a right way...
Edit 2
I believed automated duplicating id AUTO_INCREMENT field (or any field) within MySQL to be extra fast & super easy, but it totally doesn't appear so now..
So, possibly, better way is to have vcsGroupId UNSIGNED INT field, responsible for "relating" Note's versions:
On create and "origin" INSERT - fill it with MAX(vcsGroupId) + 1,
On edit and "version" INSERT - fill it with "sibling"/"origin" vcsGroupId value (fetched with CTE),
On view and "normal" SELECT - perform framing with Window Function by PARTITION BY vcsGroupId, ORDER BY id or timestamp DESC, then just using 1st (or ascending order by & using last) row,
On view and "origin" SELECT - almost the same, but reversed..
It seems easier, doesn't it?

Answer 1

What you are doing is playing with fire. I don't know exactly what can go wrong with your trigger (beside that it doesn't work for you already), but I have a strong feeling that many things can and will go wrong. For example: What if you insert multiple rows in a single statement? I don't think, that the engine will update the information_schema for each row. And it's going to be even worse if you run an INSERT ... SELECT statement. So using the information_schema for this task is a very bad idea.
However - The first question is: Why do you need it at all? If you need to save the "origin ID", then you probably plan to update the id column. That is already a bad idea. And assuming you will find a way to solve your problem - What guarantees you, that the originId will not be changed outside the trigger?
However - the alternative is to keep the originId column blank on insert, and update it in an UPDATE trigger instead.
Assuming this is your table:
create table vcs_test(
id int auto_increment,
origin_id int null default null,
primary key (id)
);
Use the UPDATE trigger to save the origin ID, when it is changed for the first time:
delimiter //
create trigger vcs_test_before_update before update on vcs_test for each row begin
if new.id <> old.id then
set new.origin_id = coalesce(old.origin_id, old.id);
end if;
end;
delimiter //
Your SELECT query would then be something like this:
select *, coalesce(origin_id, id) as origin_id from vcs_test;
See demo on db-fiddle
You can even save the full id history with the following schema:
create table vcs_test(
id int auto_increment,
id_history text null default null,
primary key (id)
);
delimiter //
create trigger vcs_test_before_update before update on vcs_test for each row begin
if new.id <> old.id then
set new.id_history = concat_ws(',', old.id_history, old.id);
end if;
end;
delimiter //
The following test
insert into vcs_test (id) values (null), (null), (null);
update vcs_test set id = 5 where id = 2;
update vcs_test set id = 4 where id = 5;
select *, concat_ws(',', id_history, id) as full_id_history
from vcs_test;
will return
| id | id_history | full_id_history |
| --- | ---------- | --------------- |
| 1 | | 1 |
| 3 | | 3 |
| 4 | 2,5 | 2,5,4 |
View on DB Fiddle