Re: minimal update

Andrew Dunstan <andrew@dunslane.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.

> 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