Getting Results of Dynamic Query As A Table? - postgresql

Question

My company is going to start generating documents using data from our database and I am designing the function that will spit out the document text. These documents will need to contain data taken from multiple tables, with hundreds of columns and invariably some records will be missing data.
I am trying to make a function that will take null fields and replace them with a little error message that makes it clear to the end user that a piece of data is missing. Since the end user is totally unfamiliar with the backend, I want these messages to reference something intelligible to them.
My solution is pretty simple yet I for the life of me can't get it to work. The record identifier, table name are set as parameters in the function. The function then loops through names for each of the columns in the specified table, building a query that contains a bunch of case statements. Once the loop is complete, the identifier is appended and then the query is executed, returning the results to the calling function.
Despite reading around quite a bit, the best I can is a single column/row containing all the results - not useful to me at all, because I need to be able to easily reference specific pieces of data in the parent query. I am a beginner with Postgres and the documentation is too complex for me to understand, any help would be appreciated.
-- Function: data_handler(text, text)
-- DROP FUNCTION data_handler(text, text);
CREATE OR REPLACE FUNCTION data_handler(target_uri text, target_table TEXT)
RETURNS SETOF record AS
$BODY$
DECLARE
c text;
strSQL text;
site_only text;
result record;
BEGIN
--We need the schema for strSQL but the loop needs just the table name.
site_only = split_part(target_table, '.', 2);
FOR c IN
SELECT column_name
FROM information_schema.columns
WHERE table_name = site_only
LOOP
strSQL = concat(strSQL, chr(10), '(SELECT CASE WHEN ', c::text, '::text IS NULL THEN concat(', chr(39), '<Error:', chr(39), ', (SELECT lkp_value FROM alb_cr.lkp_field_values WHERE column_name = ', chr(39), c::text, chr(39), ')::text, ', chr(39), ' value not found>', chr(39), ')::text ELSE ',
c::text, '::text END AS ', c::text, '_convert) AS ', c::text, ',');
END LOOP;
strSQL = LEFT(strSQL, character_length(strSQL) - 1);
strSQL = concat('SELECT ', strSQL, ' FROM ', target_table, ' WHERE nm_site_id = ', chr(39), target_uri, chr(39));
RETURN QUERY EXECUTE strSQL;
RAISE NOTICE 'strSQL: %', strSQL;
--RETURN strSQL;
--RETURN QUERY EXECUTE format('SELECT ' || strSQL || 'FROM %s WHERE nm_site_id = $1', pg_typeof(target_table)) USING target_uri;
END
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION data_handler(text, text)
OWNER TO inti;

Answer 1

You could create views for that as well, in the following example on a schema nullsbegone:
-- create the schema to hold the views
create schema if not exists nullsbegone;
-- create a function to create the views (any and all that you might need)
create or replace function nullsbegone.f_make_view_of(p_tablename text) returns void as $f$
begin
execute ($$
create or replace view nullsbegone.$$||(select relname from pg_class where oid = $1::regclass)||$$
returns void as
select $$||array_to_string(array(
select case when not attnotnull then 'COALESCE('||quote_ident(attname)||$$::text, (SELECT '<Error:'''||lkp_value||''' value not found>' FROM alb_cr.lkp_field_values
WHERE column_name = $$||quote_literal(attname)||$$)) AS $$
else '' end || quote_ident(attname)
from pg_attribute
where attrelid = $1::regclass and attnum > 0 order by attnum
), E', \n')||$$
from $$||$1);
end;$f$ language plpgsql;
-- create the view based on a given table
select nullsbegone.f_make_view_of('yourschema.yourtable');
-- select from your view as if you were selecting from the actual table
select * from nullsbegone.yourtable
where nm_site_id = 'yoursite';