How to get Agg attribute

Hi,
I am implementing a new feature of postgres. The problem is
how to get the attribute of Aggregation. In
/src/backend/commands/explain.c,
I want to add a function after "case AGG_HASHED" to get the name
of this aggregation attribute in the table. How can I get it.

I tried to see (Agg *) plan)->grpColIdx, which just is a integer number.

Best Regards,
Zhe HE
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