Friday, November 8, 2013

Interlude: A cool PL/PGSQL trick

I will be getting back to the report writing in LedgerSMB next or the week after.   I have an open source, PostgreSQL-based point of sale project I am hoping to make to a minimally useful version in the mean time.

In the mean time, here's a useful trick I discovered while trying to solve a problem.  The basic problem is one of Object-Relational design, in the sense that we may want to use types as classes and therefore return a tuple which is garanteed to be of the type defined by one table or view.

However, we may want to override some of values in this tuple with values from another table.

In older versions of PostgreSQL, I seem to recall it was possible to add additional columns onto the end of a result which would be silently ignored on return.  This doesn't work in current versions and isn't very safe anyway.  A better way is to work with composite tuples, where each member is a tuple you need for processing.

So for example, suppose we have a table which contains pricing information, and a second table which contains temporary pricing information for a given customer, and these are different table structures.  Suppose, for sake of argument we have tables like this:

   id int serial not null unique,
   sku text primary key,
   sell numeric not null,  -- sell price
   buy numeric not null  -- purchase price

CREATE TABLE pricematrix (
   customer_id int not null references customer(id),
   goods_id int not null references goods(id),
   valid_from date,
   valid_to date,
   sell numeric not null

CREATE VIEW pricematrix_today AS 
select * from pricematrix
WHERE (valid_from is null or valid_from <= 'today') and
              (valid_to is null or valid_to >= 'today');

Now, what I want to do is pull records from the goods table, but override the sell column with the one from the pricematrix if it exists, choosing the lowest available price.

A simple approach (this code is not tested unlike my actual production code based on the same principle):

CREATE OR REPLACE FUNCTION get_goods_by_sku(in_sku text, in_customer_id int)
DECLARE t_rec record;
       retval goods;
      pmatrix record;
           SELECT goods, pm INTO t_rec
             FROM goods
        LEFT JOIN (SELECT *, 
                          rank() OVER 
                           (ORDER BY sell
                            PARTITION BY goods_id) as ranking
                     FROM pricematrix_today
                    WHERE customer_id = in_customer_id
                  ) pm ON ( = pm.goods_id and ranking = 1)
            WHERE goods.sku = in_sku
       pmatrix :=;
       retval  := t_rec.goods;
       IF pmatrix.sell < retval.sell THEN
          retval.sell = pmatrix.sell;
       END IF;
       RETURN retval;

By keeping our types isolated we can safely manipulate them inside the function before returning them and still be guaranteed a named type tuple on output.

Now obviously this is a contrived example and it is less likely we will usually be selecting from tables than views, but the same goes for views since we can define "type methods" on those as well.