This is used in a lot of places, and the overhead of checking it in all of those places is not necessarily nil. Syscache lookups aren't free. What if we didn't create a new catalog for this and instead just added a relpartitionbound attribute to pg_class? It seems a bit silly to have a whole extra catalog to store one extra column...
It looks like in most of the places where this function is called it's using relid_is_partition(RelationGetRelid(rel)). Instead probably we should check existence of rd_partdesc or rd_partkey within Relation() and make sure that those members are always set for a partitioned table. That will avoid cache lookup and may give better performance.
That brings up another question. Can we have rd_partdesc non null and rd_partkey null or vice-versa. If not, should we club those into a single structure like Partition (similar to Relation)?
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Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company