Re: preserving forensic information when we freeze

On Fri, Dec 20, 2013 at 9:56 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Dec 20, 2013 at 7:51 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>>> I wondered that, too, but it's not well-defined for all tuples. What
>>> happens if you pass in constructed tuple rather than an on-disk tuple?
>>
>> Those should be discernible I think, t_self/t_tableOid won't be set for
>> generated tuples.
>
> I went looking for existing precedent for code that does things like
> this and found record_out, which does this:
>
> HeapTupleHeader rec = PG_GETARG_HEAPTUPLEHEADER(0);
> ...
> /* Extract type info from the tuple itself */
> tupType = HeapTupleHeaderGetTypeId(rec);
> tupTypmod = HeapTupleHeaderGetTypMod(rec);
> tupdesc = lookup_rowtype_tupdesc(tupType, tupTypmod);
> ncolumns = tupdesc->natts;
>
> /* Build a temporary HeapTuple control structure */
> tuple.t_len = HeapTupleHeaderGetDatumLength(rec);
> ItemPointerSetInvalid(&(tuple.t_self));
> tuple.t_tableOid = InvalidOid;
> tuple.t_data = rec;
>
> This appears to be a typical pattern, although interestingly I noticed
> that row_to_json() doesn't bother setting t_tableOid or t_self, which
> I think it's supposed to do. The problem I see here is that this code
> seems to imply that a function passed a record doesn't actually have
> enough information to know what sort of a thing it's getting. The use
> of HeapTupleHeaderGetTypeId and HeapTupleHeaderGetTypMod implies that
> it's safe to assume that t_choice will contain DatumTupleFields rather
> than HeapTupleFields, which doesn't seem to bode well for your
> approach.
>
> Am I missing a trick?

If not, here's a patch done the way I originally proposed.

--
Robert Haas
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