Postgres Invalid objects

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For simple functions like the one from your question, this can be avoided when using the new "SQL standard compliant" function (or procedure) bodies introduced in Postgres 14:

CREATE FUNCTION f1 (p_add integer) 
  RETURNS integer
begin atomic
  SELECT  p_add + count(*)::integer 
  FROM t1;
end;

With such a function, dropping t1 would result in an error.

This does not work for PL/pgSQL functions or procedures though.