> Is it the same if there isn't any trigger? I mean, does the engine
> undo all the changes of a single statement that encountered an error?
>
> The situation could be a single statement that tries to insert lots of
> registers in a non-empty table and violates a unique key constraint in
> the middle of the procces.

Yup. For example, "UPDATE a SET pk = pk + 1000" will fail if there's a
unique constraint violation and any change made but this update is
discarded. All or nothing.

Best regards,
Roman Rokytskyy