Detecting if current transaction is modifying the database

Hi,

I'm trying to find a way to have Postgres tell me if the current
transaction would modify database if I committed it now. I can live with a
conservative approximation (sometimes – ideally, rarely – get a "yes" even
though nothing would be modified, but never get a "no" even though there
are pending modifications). It's acceptable (probably even desirable) if a
no-op write operation like "UPDATE foo SET bar = 1 WHERE bar = 1" is
considered a modification.

(The use case is an audit log mechanism vaguely similar to pgMemento.)

This sentence from
https://www.postgresql.org/docs/9.5/static/view-pg-locks.html :

> If a permanent ID is assigned to the transaction (which normally happens
> only if the transaction changes the state of the database), it also holds
> an exclusive lock on its permanent transaction ID until it ends.

makes me think that I can perhaps do it as follows:

SELECT count(*) FROM pg_locks WHERE pid=pg_backend_pid() AND
locktype='transactionid' AND mode='ExclusiveLock' AND granted;

Is that right? "Permanent transaction ID" refers to the XID, correct? Are
there other, better ways? Are there ways to avoid false positives due to
temp tables?

Thanks in advance,
Christian.