Nested coalesce #6074

Is your feature request related to a problem or challenge?

coalsece currently does not support structs. If it would however, I think it should only consider top-level NULLs, so coalsece(NULL, {a: 1, b: NULL}, {a: 2, b: 3}) = {a: 1, b: NULL}. However sometimes it would be handy to fill in fields recursively. So coalsece_nested(NULL, {a: 1, b: NULL}, {a: 2, b: 3}) = {a: 1, b: 3}

Describe the solution you'd like

Have a coalesce variant that supports structs by traversing them recursively. So:

coalesce_nested(
  NULL,
  {
    a: 1,
    b: NULL,
    c: NULL,
    d: NULL,
  },
  {
    a: 2,
    b: NULL,
    c: {a: NULL},
    d: {a: 2, b: NULL},
  },
  {
    a: 3,
    b: NULL,
    c: NULL,
    d: {a: 3, b: 3},
  },
)

=

{
  a: 1,
  b: NULL,
  c: {a: NULL},
  d: {a: 2, b: 3},
}

Describe alternatives you've considered

-

Additional context

A user that fully knows the type could manually construct this using coalesce and named_struct (proposed in #5861).

Prior art:

• PostgresQL pg_rowalesce extension