Re: SQL/JSON path issues/questions

I'm going to push attached 3 patches if no objections.

Regarding 0003-Separate-two-distinctive-json-errors.patch, I think it
requires more thoughts.

        RETURN_ERROR(ereport(ERROR,
                             (errcode(ERRCODE_SINGLETON_JSON_ITEM_REQUIRED),
                              errmsg("left operand of jsonpath
operator %s is not a single numeric value",
-                                    jspOperationName(jsp->type)))));
+                                    jspOperationName(jsp->type)),
+                             (llen != 1 ?
+                              errdetail("It was an array with %d
elements.", llen):
+                              errdetail("The only element was not a
numeric.")))));

When we have more than 1 value, it's no exactly array.  Jsonpath can
extract values from various parts of json document, which never
constitute and array.  Should we say something like "There are %d
values"?  Also, probably we should display the type of single element
if it's not numeric.  jsonb_path_match() also throws
ERRCODE_SINGLETON_JSON_ITEM_REQUIRED, should we add similar
errdetail() there?

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company