I should post that in the general section but I am confused by the sentence "A parent partition is always going to have a lower relid than its children"
It's a little confusing since RelOptInfo has a relid field and so does RangeTblEntry. They both have completely different meanings. RelOptInfo's relid is a number starting at 1 and continues in a gapless sequence increasing by 1 with each RelOptInfo. These relids are completely internal to the server and don't appear in the system catalog tables. RangeTblEntry's relid is what's in pg_class.oid.
I was talking about RelOptInfo's relid.
Using relids starting at 1 is quite convenient for allowing direct array lookups in various data structures in the planner. However it's also required to uniquely identify a relation as a single table may appear many times in a query, so trying to identify them by their oid could be ambiguous. Also, some RTEKinds don't have storage, e.g a VALUES() clause.
