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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