How to evaluate if a query is correct?

Hi my friends...
I wrote in the last post a question similiar to this. But in this post I
clarify better the previous question.
I need know how to evaluated if a query is correct without execute it.
When I say if a query is correct, is that if I run the query, it did not
throw an exception.
For example...

create or replace function is_correct(query text) returns boolean as
$body$
  Declare
  Begin
   -- Here I check if the query is correct, but I can't execute this
query, because it may make changes in the data base
  End;
$body$
language 'plpgsql';

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