On Mon, Aug 18, 2014 at 4:42 PM, 土卜皿 <pengcz(dot)nwpu(at)gmail(dot)com> wrote:
> Recently, for understanding postgresql's optimizer, I read and debug the
> source code. these two days, I read these macros in sysattr.h:
>
> ...
> #define ObjectIdAttributeNumber (-2)
> ...
>
> and the relative codes, but I still did not know why authors designed them
> like that, especially the value '-2' ?
>
> what should I do if I want to see the thoughts behind them? thanks in
> advance!
Those values define the system attribute IDs associated to a relation.
You can have a look at the top of heap.c to see where they come from,
particularly the fields FormData_pg_attribute a1..a7.
Hope this helps.
Regards,
--
Michael