Modelling constraints on subset aggregates? I am using PostgreSQL but I figure most of the top-end db's must have some similar capabilities, and moreover, that solutions for them may inspire solutions for me, so don't consider this PostgreSQL-specific.

I know I am not the first to try to solve this problem so I figure it is worth asking here but I am trying to evaluate the costs of modelling accounting data such that every transaction is fundamentally balanced. The accounting data is append-only. The overall constraint (written in pseudo-code) here might look roughly like:

CREATE TABLE journal_entry (
    id bigserial not null unique, --artificial candidate key
    journal_type_id int references  journal_type(id),
    reference text, -- source document identifier, unique per journal
    date_posted date not null,
    PRIMARY KEY (journal_type_id, reference)
);

CREATE TABLE journal_line (
    entry_id bigint references journal_entry(id),
    account_id int not null references account(id),
    amount numeric not null,
    line_id bigserial not null unique,
    CHECK ((sum(amount) over (partition by entry_id) = 0) -- this won't work
);


Obviously such a check constraint will never work. It operates per row and might check over the entire db. So it will always fail and be slow doing it.

So my question is what is the best way to model this constraint? I have basically looked at two ideas so far. Wondering if these are the only ones, or if someone has a better way (other than leave it to the app level or a stored proc).


I could borrow a page from the accounting world's concept of the difference between a book of original entry and a book of final entry (general journal vs general ledger).  In this regard I could model this as an array of journal lines attached to the journal entry, enforce the constraint on the array (in PostgreSQL terms, select sum(amount) = 0 from unnest(je.line_items).  A trigger could expand and save these to a line items table, where individual column constraints could more easily be enforced, and where indexes etc could be more useful.  This is the direction I am leaning.
I could try to code a constraint trigger that would enforce this per transaction with the idea that the sum of a series of 0's will always be 0.


I am weighing these against the current approach of enforcing the logic in a stored procedure.  The complexity cost is being weighed against the idea that mathematical proof of constraints are superior to unit tests.  The major drawback of #1 above is that types as tuples is one of those areas in PostgreSQL where one runs into inconsistent behavior and changes in assumptions regularly and so I would even hope that behavior in this area might change over time.  Designing a future safe version is not so easy.

Are there other ways to solve this problem that will scale up to millions of records in each table? Am I missing something?  Is there a tradeoff I have missed?

In response to Craig's point below about versions, at a minimum, this will have to run on PostgreSQL 9.2 and higher (maybe 9.1 and higher, but probably we can go with straight 9.2).
As we have to span multiple rows it cannot be implemented with a simple CHECK constraint.

We can also rule out exclusion constraints. Those would span multiple rows, but only check for inequality. Complex operations like a sum over multiple rows are not possible.

The tool that seems to best fit your case is a CONSTRAINT TRIGGER (Or even just a plain TRIGGER - the only difference in the current implementation is that you can adjust the timing of the trigger with SET CONSTRAINTS.

So that's your option 2. 

Once we can rely on the constraint being enforced at all times, we need not check the whole table any more. Checking only rows inserted in the current transaction - at the end of the transaction - is sufficient. Performance should be ok.

Also, as 


  The accounting data is append-only.


... we only need to care about newly inserted rows. (Assuming UPDATE or DELETE are not possible.)

I use the system column xid and compare it to the function txid_current() - which returns the xid of the current transaction.
To compare the types, casting is needed ...
This should be reasonably safe. Consider this related, later answer with a safer method:


How to view tuples changed in a PostgreSQL transaction?


Demo

CREATE TABLE journal_line(amount int); -- simplistic table for demo

CREATE OR REPLACE FUNCTION trg_insaft_check_balance()
    RETURNS trigger AS
$func$
BEGIN
   IF sum(amount) &lt;&gt; 0
      FROM journal_line 
      WHERE xmin::text::bigint = txid_current()  -- consider link above
         THEN
      RAISE EXCEPTION 'Entries not balanced!';
   END IF;

   RETURN NULL;  -- RETURN value of AFTER trigger is ignored anyway
END;
$func$ LANGUAGE plpgsql;

CREATE CONSTRAINT TRIGGER insaft_check_balance
    AFTER INSERT ON journal_line
    DEFERRABLE INITIALLY DEFERRED
    FOR EACH ROW
    EXECUTE PROCEDURE trg_insaft_check_balance();


Deferred, so it is only checked at the end of the transaction.

Tests

INSERT INTO journal_line(amount) VALUES (1), (-1);


Works.

INSERT INTO journal_line(amount) VALUES (1);


Fails:


  ERROR:  Entries not balanced!


BEGIN;
INSERT INTO journal_line(amount) VALUES (7), (-5);
-- do other stuff
SELECT * FROM journal_line;
INSERT INTO journal_line(amount) VALUES (-2);
-- INSERT INTO journal_line(amount) VALUES (-1); -- make it fail
COMMIT;


Works. :)

If you need to enforce your constraint before the end of the transaction, you can do so at any point in the transaction, even at the start:

SET CONSTRAINTS insaft_check_balance IMMEDIATE;


Faster with plain trigger

If you operate with multi-row INSERT it is more effective to trigger per statement - which is not possible with constraint triggers:


  Constraint triggers can only be specified FOR EACH ROW.


Use a plain trigger instead and fire FOR EACH STATEMENT to ...


lose the option of SET CONSTRAINTS.
gain performance.


DELETE possible

In reply to your comment: If DELETE is possible you might add similar trigger doing a whole-table balance check after a DELETE has happened. This would be much more expensive, but won't matter much as it rarely happens.