Database upgrade, add fields to a table, prevent duplicate upgrade when SQL PIN is wrong

Create or Replace function Func_add_column () returns int as
$body $
Begin
Perform * from pg_tables where tablename = ' table name ';
If found then
Perform * from pg_attribute where attrelid = ' table name ':: Regclass and attname = ' field name ';
If not found then
ALTER TABLE name add column field name;
End If;

End If;


return 0;
End
$body $ language Plpgsql;
Select Func_add_column ();

Database upgrade, add fields to a table, prevent duplicate upgrade when SQL PIN is wrong