fast defaults in heap_getattr vs heap_deform_tuple

Hi,

While working on the patch to slotify trigger.c I got somewhat confused
by the need to expand tuples in trigger.c:

static HeapTuple
GetTupleForTrigger(EState *estate,
                   EPQState *epqstate,
                   ResultRelInfo *relinfo,
                   ItemPointer tid,
                   LockTupleMode lockmode,
                   TupleTableSlot **newSlot)
{
...
    if (HeapTupleHeaderGetNatts(tuple.t_data) < relation->rd_att->natts)
        result = heap_expand_tuple(&tuple, relation->rd_att);
    else
        result = heap_copytuple(&tuple);
    ReleaseBuffer(buffer);

There's no explanation why GetTupleForTrigger() needs expanding tuples,
but most other parts of postgres don't. Normally deforming ought to take
care of expanding missing attrs.

As far as I can tell, the reason that it's needed is that heap_gettar()
wasn't updated along the rest of the functions. heap_deform_tuple(),
heap_attisnull() etc look for missing attrs, but heap_getattr() doesn't.

That, to me, makes no sense. The reason that we see a difference in
regression test output is that composite_to_json() uses heap_getattr(),
if it used heap_deform_tuple (which'd be considerably faster), we'd get
the default value.

Am I missing something?

Greetings,

Andres Freund