Re: SQL design pattern for a delta trigger?

Thanks all.  I tried the appended code in a trigger
function, but postgresql won't take it.

It complains that assets.quantity is not a scalar.
However, the WHERE clause in that select statement
guarantees that at most only one record will be
returned.  An open position on a given kind of asset
is represented by null in the end_valid_time field,
and the combination of asset_type_id, portfolio_id and
end_valid_time is certain to be unique, if there is a
record for that asset type in that porfolio at all.

I thought I'd try checking for an open position first
because the manual indicated that exception handling
is quite expensive.  But I must have missed something,
because it doesn't like how I tried to define my
trigger function.

I have four sequences, one each for four tables.  Two
of the tables are just look up tables, for asset types
and portfolios; trivial for test case with only an
autoincrementing integer primary key and a "name".
The other two are the ones of interest.  Assets is
treated as read only as far as the user is concerned.
The user's data in the assets table is mediated
through transactions inserted (and NEVER deleted or
updated), into the transactions table.  Assets has the
minimal suite of columns (autoincrementing integer
primary key, asset_typeID, portfolio_id, all integers,
quantity with is a floating point number and two
dates: start_valid_time and end_valid_time).
Transactions has only a transaction_id, portfolio_id,
asset_type_id, quantity and transaction_date.  There
are of course foreign keys connectin the assets and
transactions tables to the lookup tables, and a
composite index on assets to make looking up records
based on portfolio_id, asset_id and end_valid_time as
quick as possible.  It couldn't be simpler,
conceptually!  yet I must have missed something, cause
postgresql won't accept the function body I show
below.

If I can't get this working quickly, I may just resort
to creating a stored procedure that takes the
transaction details as arguments and processes both
tables appropriately without relying on a trigger.
:-(

Thanks for everyone's help.

Ted



===========================================
DECLARE
  id BIGINT;
  q DOUBLE PRECISION;
BEGIN
  SELECT assets.id INTO id, assets.quantity INTO q
    FROM assets
    WHERE assets.asset_type_id = NEW.asset_type_id
      AND assets.portfolio_id = NEW.portfolio_id
      AND assets.end_valid_time IS NULL;
  IF (id IS NULL) THEN
    INSERT INTO assets (asset_type_id,
portfolio_id,quantity,start_valid_stime,end_valid_time)
      VALUES (NEW.asset_type_id,NEW.portfolio_id,
NEW.quantity, NEW.transaction_date,NULL);
  ELSE
    UPDATE assets SET end_valid_time =
NEW.transaction_date WHERE id = id;
    INSERT INTO assets (asset_type_id,
portfolio_id,quantity,start_valid_stime,end_valid_time)
      VALUES (NEW.asset_type_id,NEW.portfolio_id, q +
NEW.quantity, NEW.transaction_date,NULL);
  END
END