Re: pgsql: Add more SQL/JSON constructor functions

While reviewing the patch, I found some inconsistency on json_table EXISTS.

--tested based on your patch and master.
src4=# SELECT * FROM JSON_TABLE(jsonb '"a"', '$' COLUMNS (a jsonb
EXISTS PATH '$'));
ERROR: cannot cast behavior expression of type boolean to jsonb
src4=# SELECT * FROM JSON_TABLE(jsonb '"a"', '$' COLUMNS (a jsonb
EXISTS PATH '$' error on error));
a
------
true
(1 row)

Why explicitly "error on error" not report error while not explicitly
mentioning it yields an error?

"(a jsonb EXISTS PATH '$' error on error)" returns jsonb 'true'
imply that no errors happened.
so "(a jsonb EXISTS PATH '$')" should not have any errors.

but boolean cannot cast to jsonb so for JSON_TABLE,
we should reject
COLUMNS (a jsonb EXISTS PATH '$' error on error ));
COLUMNS (a jsonb EXISTS PATH '$' unknown on error ));
at an earlier stage.

because json_populate_type will use literal 'true'/'false' cast to
jsonb, which will not fail.
but JsonPathExists returns are *not* quoted true/false.
so rejecting it earlier is better than handling it at ExecEvalJsonExprPath.

attached patch trying to solve the problem, changes applied based on
your 0001, 0002.
after apply attached patch:

create domain djsonb as jsonb check(value = 'true');
SELECT * FROM JSON_TABLE(jsonb '"a"', '$' COLUMNS (a djsonb EXISTS
PATH '$' error on error));
ERROR: cannot cast type boolean to djsonb
SELECT * FROM JSON_TABLE(jsonb '"a"', '$' COLUMNS (a djsonb EXISTS
PATH '$' unknown on error));
ERROR: cannot cast type boolean to djsonb
SELECT * FROM JSON_TABLE(jsonb '"a"', '$' COLUMNS (a jsonb EXISTS PATH '$'));
ERROR: cannot cast type boolean to jsonb

i found out a typo in
src/test/regress/expected/sqljson_queryfuncs.out,
src/test/regress/sql/sqljson_queryfuncs.sql
"fixed-legth" should be "fixed-length"