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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