Solution to UPDATE...INSERT problem

Hi Guys,

I just thought I'd share with you guys a very clever solution to the old
'update row. if no rows affected, then insert the row' race condition
problem. A guy at my work came up with it.

We were discussing this earlier on -hackers, but no-one could find a
solution that didn't involve locking the entire table around the
update...insert commands.

The problem is that sometimes the row will be inserted by another process
between your update and insert, causing your insert to fail with a unique
constraint violation.

So, say this is the insert:

INSERT INTO table VALUES (1, 'foo'); // 1 is in the primary key column

Rewrite it like this:

INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
pkcol=1;

See? So now that INSERT statement will insert the row if it doesn't exist,
or insert zero rows if it does. You are then guaranteed that your
transaction will not fail and rollback, so you can repeat your update, or do
the insert first and then the update, etc.

Hope that's handy for people,

Chris