Re: remaining sql/json patches

I noticed that JSON_TABLE uses an explicit FORMAT JSON in one of the
rules, instead of using json_format_clause_opt like everywhere else.  I
wondered why, and noticed that it's because it wants to set coltype
JTC_FORMATTED when the clause is present but JTC_REGULAR otherwise.
This seemed a little odd, but I thought to split json_format_clause_opt
in two productions, one without the empty rule (json_format_clause) and
another with it.  This is not a groundbreaking improvement, but it seems
more natural, and it helps contain the FORMAT stuff a little better.

I also noticed while at it that we can do away not only with the
json_encoding_clause_opt clause, but also with makeJsonEncoding().

The attach patch does it.  This is not derived from the patches you're
currently working on; it's more of a revise of the previous SQL/JSON
code I committed in 7081ac46ace8.

It goes before your 0003 and has a couple of easily resolved conflicts
with both 0003 and 0004; then in 0004 you have to edit the JSON_TABLE
rule that has FORMAT_LA and replace that with json_format_clause.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"El que vive para el futuro es un iluso, y el que vive para el pasado,
un imbécil" (Luis Adler, "Los tripulantes de la noche")