Re: remaining sql/json patches

This part is already committed.
ereport(ERROR,
(errcode(ERRCODE_UNDEFINED_OBJECT),
errmsg("could not find jsonpath variable \"%s\"",
pnstrdup(varName, varNameLength))));

but, you can simply use:
ereport(ERROR,
(errcode(ERRCODE_UNDEFINED_OBJECT),
errmsg("could not find jsonpath variable \"%s\"",varName)));

maybe not worth the trouble.
I kind of want to know, using `pnstrdup`, when the malloc related
memory will be freed?

json_query and json_query doc explanation is kind of crammed together.
Do you think it's a good idea to use </listitem> and </itemizedlist>?
it will look like bullet points. but the distance between the bullet
point and the first text in the same line is a little bit long, so it
may not look elegant.
I've attached the picture, json_query is using `</listitem> and
</itemizedlist>`, json_value is as of the v39.

other than this and previous points, v39, 0001 looks good to go.