Re: minimal update

Tom Lane wrote:
> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>
>> Tom Lane wrote:
>>
>>> Well, you could write the trigger in C and it'd work for any table.
>>> I think it could be as simple as a memcmp of the tuples' data areas,
>>> since we now require padding bytes to be 0 ...
>>>
>
>
>> Something like this fragment?
>>
>
>
>> newtuple = trigdata->tg_newtuple;
>> oldtuple = trigdata->tg_trigtuple;
>> rettuple = newtuple;
>>
>
>
>> if (newtuple->t_len == oldtuple->t_len &&
>> newtuple->t_data->t_hoff == oldtuple->t_data->t_hoff &&
>> memcmp(GETSTRUCT(newtuple),GETSTRUCT(oldtuple),
>> newtuple->t_len - newtuple->t_data->t_hoff) == 0)
>> rettuple = NULL;
>>
>
>
>> return PointerGetDatum(rettuple);
>>
>
> Close, but I think you also need to take care to compare natts and
> the null bitmaps (if any). Might be worth comparing OIDs too, though
> AFAIR there is no mechanism for substituting a different OID during
> UPDATE. Probably the easiest coding is to memcmp all the way from
> offsetof(t_bits) to t_len, after comparing natts and the HASNULL and
> HASOID flags.
>

How does this look?

if (newtuple->t_len == oldtuple->t_len &&
newtuple->t_data->t_hoff == oldtuple->t_data->t_hoff &&
HeapTupleHeaderGetNatts(newtuple) == HeapTupleHeaderGetNatts(oldtuple) &&
(newtuple->t_data->t_infomask & (HEAP_HASOID|HEAP_HASNULL)) == (oldtuple->t_data->t_infomask & (HEAP_HASOID|HEAP_HASNULL)) &&
memcmp(newtuple->t_data + offsetof(HeapTupleHeaderData, t_bits),
oldtuple->t_data + offsetof(HeapTupleHeaderData, t_bits)
newtuple->t_len - offsetof(HeapTupleHeaderData, t_bits)) == 0)

rettuple = NULL;

return PointerGetDatum(rettuple);

cheers

andrew
>
>> Also, when did we first require padding bytes to be 0?
>>
>
> The 8.3 varvarlena patch is what requires it, but in practice
> heap_formtuple has always started with a palloc0, so I think it would
> work a long ways back.
>
> regards, tom lane
>
>