MariaDB: multiple table update does not update a single row multiple times? Why? - sql-update

Question

Today I was just bitten in the rear end by something I didn't expect. Here's a little script to reproduce the issue:
create temporary table aaa_state(id int, amount int);
create temporary table aaa_changes(id int, delta int);
insert into aaa_state(id, amount) values (1, 0);
insert into aaa_changes(id, delta) values (1, 5), (1, 7);
update aaa_changes c join aaa_state s on (c.id=s.id) set s.amount=s.amount+c.delta;
select * from aaa_state;
The final result in the aaa_state table is:
ID
Amount
1
5
Whereas I would expect it to be:
ID
Amount
1
12
What gives? I checked the docs but cannot find anything that would hint at this behavior. Is this a bug that I should report, or is this by design?

Answer 1

The behavior you are seeing is consistent with two updates happening on the aaa_state table. One update is assigning the amount to 7, and then this amount is being clobbered by the second update, which sets to 5. This could be explained by MySQL using a snapshot of the aaa_state table to fetch the amount for each step of the update. If true, the actual steps would look something like this:
1. join the two tables
2. update the amount using the "first" row from the changes table.
now the cached result for the amount is 7, but this value will not actually
be written out to the underlying table until AFTER the entire update
3. update the amount using the "second" row from the changes table.
now the cached amount is 5
5. the update is over, write 5 out for the actual amount
Your syntax is not really correct for what you want to do. You should be using something like the following:
UPDATE aaa_state as
INNER JOIN
(
SELECT id, SUM(delta) AS delta_sum
FROM aaa_changes
GROUP BY id
) ac
ON ac.id = as.id
SET
as.amount = as.amount + ac.delta_sum;
Here we are doing a proper aggregation of the delta values for each id in a separate bona-fide subquery. This means that the delta sums will be properly computed and materialized in the subquery before MySQL does the join, to update the first table.