[doc] improve tableoid description

[doc] improve tableoid description

Hi

Attached patch aims to improve the description of the tableoid system column [1]
by:

- mentioning it's useful for determining table names for partitioned tables as
  well as for those in inheritance hierarchies
- mentioning the possibility of casting tableoid to regclass (which is simpler
  than the currently suggested join on pg_class, which is only needed if
  the schema name is absolutely required)

[1] https://www.postgresql.org/docs/current/ddl-system-columns.html


Regards

Ian Barwick


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