Been doing more tracing...
The problem with UPDATE on inheritance hierarchies is that when it gets
down into ExecSeqScan, the value of...
node->scanstate->css_currentScanDesc->rs_rd->rd_id
is not equal to the value of...
node->plan.state->es_result_relation_info->ri_RelationDesc->rd_id
On the first scan, the former is equal to the relation for the base
class
and the latter is equal to the relation for the subclass.
Any thoughts anyone?