> "The data types of all the result expressions must be convertible to a
> single output type."
The type of the field pg_class.relkind appears to be "char" which is
described in the notes as:
The type "char" (note the quotes) is different from char(1) in that it
only uses one byte of storage. It is internally used in the system catalogs
as a poor-man's enumeration type.
http://www.postgresql.org/docs/8.3/interactive/datatype-character.html
But one would expect "char" to be "convertible" to text for the purposes
of CASE. Both implicit and explicit cast to text seems to work fine.