Reference an arbitrary row and field in another table - sql

Question

Is there any form (data type, inherence..) of implement in postgresql something like this:
CREATE TABLE log (
datareferenced table_row_column_reference,
logged boolean
);
The referenced data may be any row field from the database. My objective is implement something like this without use Procedural Language or implement it in a higher layer, using only a relational approach and without modify the rest of the tables. Another feature may be referencial integrity, example:
-- Table foo (id, field1, field2, fieldn)
-- ('bar', '2014-01-01', 4.33, Null)
-- Table log (datareferenced, logged)
-- ({table foo -> id:'bar' -> field2 } <=> 4.33, True)
DELETE FROM foo where id='bar';
-- as result, on cascade, deleted both rows.
I have an application build onto a MVC pattern. The logic is written in Python. The application is a management tool, very data intensive. My goal is implement a module that could store additional information per every data present in the DDBB. Per example, a client have a serie of attributes (name, address, phone, email ...) across multiple tables, and I want that the app could store metadata-like for every registry from all the DDBB. A metadata could be last modification, or a user flag, etc.
I have implemented the metadata model (in postgres), its mapping to objects and a parcial API. But the part left is the most important, the glue. My plan B is create that glue in the data mapping layer as a module. Something like this:
address= person.addresses[0]
address.saveMetadata('foo', 'bar')
-- in the superclass of Address
def saveMetadata(self, code, value):
self.mapper.metadata_adapter.save(self, code, value)
-- in the metadata adapter class:
def save(self, entity, code, value):
sql = """update value=%s from metadata_values
where code=%s and idmetadata=
(select id from metadata_rels mr
where mr.schema=%s and mr.table=%s and
mr.field=%s and mr.recordpk=%s)"""%
(value, code,
self.class2data[entity.__class__]["schema"],
self.class2data[entity.__class__]["table"],
self.class2data[entity.__class__]["field"],
entity.id)
self.mapper.execute(sql)
def read(self, entity , code):
sql = """select mv.value
from metadata_values mv
join metadata_rels mr on mv.idmetadata=mr.id
where mv.code=%s and mr.schema=%s and mr.table=%s and
mr.field=%s and mr.recordpk=%s"""%
(code,
self.class2data[entity.__class__]["schema"],
self.class2data[entity.__class__]["table"],
self.class2data[entity.__class__]["field"],
entity.id )
return self.mapper.execute(sql)
But it would add overhead between python and postgresql, complicate Python logic, and using PL and triggers may be very laborious and bug-prone. That is why i'm looking at doing the same at the DDBB level.

Answer 1

No, there's nothing like that in PostgreSQL.
You could build triggers yourself to do it, probably using a composite type. But you've said (for some reason) you don't want to use PL/PgSQL, so you've ruled that out. Getting RI triggers right is quite hard, though, and you must apply a trigger to the referencing and referenced ends.
Frankly, this seems like a square peg, round hole kind of problem. Are you sure PostgreSQL is the right choice for this application?
Describe your needs and goal in context. Why do you want this? What problem are you trying to solve? Maybe there's a better way to approach the same problem one step back...