minimal update trigger

As discussed a little while back, I would like to add a generic trigger
function which will force an update to skip if the new and old tuples
are identical.

The guts of this is the following snippet of code:

| rettuple = newtuple = trigdata->tg_newtuple;
oldtuple = trigdata->tg_trigtuple;

if (newtuple->t_len == oldtuple->t_len &&
newtuple->t_data->t_hoff == oldtuple->t_data->t_hoff &&
HeapTupleHeaderGetNatts(newtuple->t_data) == HeapTupleHeaderGetNatts(oldtuple->t_data) &&
(newtuple->t_data->t_infomask & ~HEAP_XACT_MASK) ==
(oldtuple->t_data->t_infomask & ~HEAP_XACT_MASK) &&
memcmp(((char *)newtuple->t_data) + offsetof(HeapTupleHeaderData, t_bits),
((char *)oldtuple->t_data) + offsetof(HeapTupleHeaderData, t_bits),
newtuple->t_len - offsetof(HeapTupleHeaderData, t_bits)) == 0)
{
rettuple = NULL;
}

return rettuple;

I propose to call the function pg_minimal_update.

Unless there is an objection I will put together a patch + docs for this shortly. Not quite sure what section of the docs to put it in - maybe a new subsection of the Functions chapter?

cheers

andrew
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