Postgresql create a log schema - postgresql

Question

So my problem is simple. I have a schema prod with many tables, and another one log with the exact same tables and structure (primary keys change that's it).
When I do UPDATE or DELETE in the schema prod, I want to record old data in the log schema.
I have the following function called after a update or delete:
CREATE FUNCTION prod.log_data() RETURNS trigger
LANGUAGE plpgsql AS $$
DECLARE
v RECORD;
column_names text;
value_names text;
BEGIN
-- get column names of current table and store the list in a text var
column_names = '';
value_names = '';
FOR v IN SELECT * FROM information_schema.columns WHERE table_name = quote_ident(TG_TABLE_NAME) AND table_schema = quote_ident(TG_TABLE_SCHEMA) LOOP
column_names = column_names || ',' || v.column_name;
value_names = value_names || ',$1.' || v.column_name;
END LOOP;
-- remove first char ','
column_names = substring( column_names FROM 2);
value_names = substring( value_names FROM 2);
-- execute the insert into log schema
EXECUTE 'INSERT INTO log.' || TG_TABLE_NAME || ' ( ' || column_names || ' ) VALUES ( ' || value_names || ' )' USING OLD;
RETURN NULL; -- no need to return, it is executed after update
END;$$;
The annoying part is that I have to get column names from information_schema for each row.
I would rather use this:
EXECUTE 'INSERT INTO log.' || TG_TABLE_NAME || ' SELECT ' || OLD;
But some values can be NULL so this will execute:
INSERT INTO log.user SELECT 2,,,"2015-10-28 13:52:44.785947"
instead of
INSERT INTO log.user SELECT 2,NULL,NULL,"2015-10-28 13:52:44.785947"
Any idea to convert ",," to ",NULL,"?
Thanks
-Quentin

Answer 1

First of all I must say that in my opinion using PostgreSQL system tables (like information_schema) is the proper way for such a usecase. Especially that you must write it once: you create the function prod.log_data() and your done. Moreover it may be dangerous to use OLD in that context (just like *) as always because of not specified elements order.
But,
to answer your exact question the only way I know is to do some operations on OLD. Just observe that you cast OLD to text by doing concatenation ... ' SELECT ' || OLD. The default casting create that ugly double-commas. So, next you can play with that text. In the end I propose:
DECLARE
tmp TEXT
...
BEGIN
...
/*to make OLD -> text like (2,,3,4,,)*/
SELECT '' || OLD INTO tmp; /*step 1*/
/*take care of commas at the begining and end: '(,' ',)'*/
tmp := replace(replace(tmp, '(,', '(NULL,'), ',)', ',NULL)'); /*step 2*/
/* replace rest of commas to commas with NULL between them */
SELECT array_to_string(string_to_array(tmp, ',', ''), ',', 'NULL') INTO tmp; /*step 3*/
/* Now we can do EXECUTE*/
EXECUTE 'INSERT INTO log.' || TG_TABLE_NAME || ' SELECT ' || tmp;
Of course you can do steps 1-3 in one big step
SELECT array_to_string(string_to_array(replace(replace('' || NEW, '(,', '(NULL,'), ',)', ',NULL)'), ',', ''), ',', 'NULL') INTO tmp;
In my opinion this approach isn't any better from using information_schema, but it's your call.