ResetSystemCaches(was Re: relation ### modified while in use)

Hi

RelationCacheInvalidate() is called from ResetSystemCaches()
and calles RelationFlushRelation() for all relation descriptors
except some nailed system relations.
I'm wondering why nailed relations could be exceptions.
Conversely why must RelationCacheInvalidate() call
RelationFlushRelation() for other system relations ?
Isn't it sufficient to call smgrclose() and replace rd_rel
member of system relations by the latest ones instead
of calling RelationFlushRelation() ?
There's -O option of postmaster(postgres) which allows
system table structure modification.  I'm suspicious
if it has been used properly before.

Comments ?

Regards.
Hiroshi Inoue