how to understand these macro defines such as ObjectIdAttributeNumber in sysattr.h?

From: 土卜皿 <pengcz(dot)nwpu(at)gmail(dot)com>
To: PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: how to understand these macro defines such as ObjectIdAttributeNumber in sysattr.h?
Date: 2014-08-18 07:42:05
Message-ID: CADT5_18DwDyX=mQ34AHDhK9TLm_BW6=_cezGhGok2=8TJnivSg@mail.gmail.com
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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!

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