Describe the problem
Came across this while trying to union two selects that contained a json column, and narrowed it down to the json column. The following example fails:

To Reproduce

> CREATE TABLE t1 (j JSON);
> CREATE TABLE t2 (j JSON);
> INSERT INTO t1 VALUES ('{"a": "b"}');
> INSERT INTO t2 VALUES ('{"c": "d"}');
> (SELECT j FROM t1) UNION (SELECT j FROM t2);

Output is pq: unable to encode table key: *tree.DJSON

Expected behavior
If this is related to #24436 (not key-encoding JSON values) then a clearer error message. Otherwise I would expect:

       j         
+------------+
  {"a": "b"}
  {"c": "d"}  
(2 rows)

Environment:

• CockroachDB version: 2.1.4
• Server OS: MacOS High Sierra
• Client app:cockroach sql