detect initiator of update/delete action

Dear list,

I want to create a function on a table that does something before deletion
on a table, e.g. like

create or replace function table1_del() returns trigger as $$   begin       update table1 set column1=
substr(column2,1,10);      insert into table1_hist values (           old.column1;       return OLD;   end;
 
$$
language plpgsql volatile;
create trigger table1_del before delete on table1 for each row execute procedure table1_del();

However, I need to separate this function with an if-statement depending on
how the deletion was initiated. 
There are two cases: 
1. if the deletion was executed through a user action, both the update and
insert statement of the 'table1_del' function should be executed
2. if the deletion was initiated through a function fired from another
table, only the insert statement should be executed

Is there a way in postgres to detect if an action (here:deletion) was
initiated by user interaction or a function?

Any help greatly appreciated!

Stefan




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