How to inspect tuples during execution of a plan?

Hello all,

I'd like to inspect the content of tuples as they are sent during the
execution of a query in order to react to their values.

I guess I could do it with a FDW, but that's a bit clumsy so I took a
look at the hooks but have two questions:

1.) Would ExecutorRun_hook be the correct place to implement such an
'tuple inspector'?
2.) If yes, how? As far as I understand the source code, I would have
to provide my own implementation based on standard_ExecutorRun and
replace the ExecutePlan function with my own one that takes a look at
each 'slot' like so

if (estate->es_junkFilter != NULL)
1589 slot = ExecFilterJunk(estate->es_junkFilter, slot);
1590

Tuple inspection here

1591 /*
1592 * If we are supposed to send the tuple somewhere, do so. (In
1593 * practice, this is probably always the case at this point.)
1594 */
1595 if (sendTuples)
1596 {

If there is a better way, please advise. I'm really a newbie to this.

Best regards,

Ernst-Georg Schmid