Log_statement behaviour a little misleading?

This caught me today :

I switched on "log_statement=true" whilst examining a possible foreign
key concurrency problem. I noticed that the generated foreign key check

"SELECT 1 FROM ONLY ... WHERE id = ...FOR UPDATE..."

on the parent table seemed to be only appearing every now and again.
This caused some scratching of the head :-)

Finally light dawned (ok - after reading ri_triggers.c and querying
pg_locks) - that the backend saves the execution plan for the generated
statement, so it is only planned once...and I guess log_statement is
triggered in the plan stage somewhere...

So setting "log_statement=true" does all *statements* - but not all
*executions* of each statement. Is this the intention?

(BTW - I am using 7.4.1)

best wishes

Mark