How to set/get a field of a nested composite type variable?

Question

For example:

create type t1 as (a int2, b text, c boolean);
create type t2 as (a t1, b boolean);
create type t3 as (a t2, b int4);

Now, how can I do the following for a variable x of type t3:

x.a.a.c := true;
y := x.a.a.c;

The first gives me an error.

Answer 1

There's one (awful, in my opinion) workaround... you can access only one level down at a time. Use intermediate variables.

DO LANGUAGE PLPGSQL
$$
DECLARE 
    x t3 ;
    x2 t2 ;
    x1 t1 ;
BEGIN
    -- x.a.a.c := true;   -- Syntax error

    x2 := x.a ;
    x1 := x2.a ;
    x1.c := true ;
    x2.a := x1 ;
    x.a := x2 ;
END ;
$$

dbfiddle here

Answer 2

You can assign the row type as a whole, which does what you ask for. But it also overwrites every other (sub-)field. (I use NULL for the rest.) See x in the demo.

You can also assign a whole composite type value to a column at the outer level. This preserves all other column values in the outer level. See y in the demo.

But you cannot do the same for nested composite types. See z in the demo.

DO
$$
DECLARE 
   x t3;
   y t3;
   z t3;
BEGIN
   x      := '("(""(,,true)"",)",)'::t3;  -- works
   y.a    :=    '("(,,true)",)'::t2;      -- works
--(z.a).a :=      '(,,true)'::t1;         -- does not work
   RAISE NOTICE E'\nx: %\ny: %\nz: %', x, y, z;
END
$$

dbfiddle here
(Using an EXCEPTION to make the result visible in the fiddle.)