Why does ExecComputeStoredGenerated() form a heap tuple

Hi,

while rebasing the remaining tableam patches (luckily a pretty small set
now!), I had a few conflicts with ExecComputeStoredGenerated(). While
resolving I noticed:

    oldtuple = ExecFetchSlotHeapTuple(slot, true, &should_free);
    newtuple = heap_modify_tuple(oldtuple, tupdesc, values, nulls, replaces);
    ExecForceStoreHeapTuple(newtuple, slot);
    if (should_free)
        heap_freetuple(oldtuple);

    MemoryContextSwitchTo(oldContext);

First off, I'm not convinced this is correct:

ISTM you'd need at least an ExecMaterializeSlot() before the
MemoryContextSwitchTo() in ExecComputeStoredGenerated().

But what actually brought me to reply was that it seems like it'll cause
unnecessary slowdowns for !heap AMs. First, it'll form a heaptuple if
the slot isn't in that form, and then it'll cause a conversion by
storing a heap tuple even if the target doesn't use heap representation.

ISTM the above would be much more efficiently - even more efficient if
only heap is used - implemented as something roughly akin to:

    slot_getallattrs(slot);
    memcpy(values, slot->tts_values, ...);
    memcpy(nulls, slot->tts_isnull, ...);

    for (int i = 0; i < natts; i++)
    {
        if (TupleDescAttr(tupdesc, i)->attgenerated == ATTRIBUTE_GENERATED_STORED)
        {
            values[i] = ...
        }
        else
            values[i] = datumCopy(...);
    }

    ExecClearTuple(slot);
    memcpy(slot->tts_values, values, ...);
    memcpy(slot->tts_isnull, nulls, ...);
    ExecStoreVirtualTuple(slot);
    ExecMaterializeSlot(slot);

that's not perfect, but more efficient than your version...

Greetings,

Andres Freund