Thread: how to understand these macro defines such as ObjectIdAttributeNumber in sysattr.h?
hi, all
I am so sorry for asking some questions like this!
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!
Re: how to understand these macro defines such as ObjectIdAttributeNumber in sysattr.h?
From
Michael Paquier
Date:
On Mon, Aug 18, 2014 at 4:42 PM, 土卜皿 <pengcz.nwpu@gmail.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
Re: how to understand these macro defines such as ObjectIdAttributeNumber in sysattr.h?
From
土卜皿
Date:
hi, Michael
thanks a lot! your reminding lead me go a step forward!
the 7 attributes are system attributes of some table such as pg_class, pg_type. but now I have a new question:
where and when using these system attributes?
2014-08-18 15:57 GMT+08:00 Michael Paquier <michael.paquier@gmail.com>:
On Mon, Aug 18, 2014 at 4:42 PM, 土卜皿 <pengcz.nwpu@gmail.com> wrote:Those values define the system attribute IDs associated to a relation.
> 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!
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
Re: how to understand these macro defines such as ObjectIdAttributeNumber in sysattr.h?
From
Tom Lane
Date:
土卜皿 <pengcz.nwpu@gmail.com> writes: > where and when using these system attributes? http://www.postgresql.org/docs/devel/static/ddl-system-columns.html regards, tom lane