json functions for dynamic field modification in plpgsql, including in triggers.

One of the occasionally annoying things about Plpgsql is that it's very static, and any sort of dynamic access to records, say by using a trigger argument, is difficult. I just conducted a little experiment that seems to make life a bit easier. The json_populate_record() function returns a record of the same type as its first argument. Mostly we just use NULL::record_type here, but it turns out that in a trigger function it works perfectly happily with NEW and OLD, and they can be modified using any sort of dynamic values you like, including values from TG_ARGV

Here's a simple example:

mydb=# create type rainbow as enum('r','o','y','g','b','i','v');
CREATE TYPE
mydb=# create table foo (a text, b int, c boolean, d rainbow);
CREATE TABLE
mydb=# create or replace function foo_tr()
returns trigger
language plpgsql as $$
begin
  if tg_op = 'INSERT' or tg_op = 'UPDATE'
  then
    return json_populate_record(NEW,json_object(TG_ARGV));
  else
    return old;
  end if;
end;
$$;
CREATE FUNCTION
mydb=# create trigger foo_tri before insert on foo for each row execute procedure foo_tr(d,i);
CREATE TRIGGER
mydb=# create trigger foo_tru before update on foo for each row execute procedure foo_tr(c,true);
CREATE TRIGGER
mydb=# insert into foo values ('x',1,null,'r') returning *;
 a | b | c | d 
---+---+---+---
 x | 1 |   | i
(1 row)

INSERT 0 1
mydb=# update foo set c = null, d = 'o' returning *;
 a | b | c | d 
---+---+---+---
 x | 1 | t | o
(1 row)

UPDATE 1
mydb=#