Re: remaining sql/json patches

hi.

+/*
+ * Recursively transform child JSON_TABLE plan.
+ *
+ * Default plan is transformed into a cross/union join of its nested columns.
+ * Simple and outer/inner plans are transformed into a JsonTablePlan by
+ * finding and transforming corresponding nested column.
+ * Sibling plans are recursively transformed into a JsonTableSibling.
+ */
+static Node *
+transformJsonTableChildPlan(JsonTableParseContext *cxt,
+ List *columns)
this comment is not the same as the function intention for now.
maybe we need to refactor it.

/*
* Each call to fetch a new set of rows - of which there may be very many
* if XMLTABLE is being used in a lateral join - will allocate a possibly
* substantial amount of memory, so we cannot use the per-query context
* here. perTableCxt now serves the same function as "argcontext" does in
* FunctionScan - a place to store per-one-call (i.e. one result table)
* lifetime data (as opposed to per-query or per-result-tuple).
*/
MemoryContextSwitchTo(tstate->perTableCxt);

maybe we can replace "XMLTABLE" to "XMLTABLE or JSON_TABLE"?

/* Transform and coerce the PASSING arguments to to jsonb. */
there should be only one "to"?

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json_table_column clause doesn't have a passing clause.
we can only have one passing clause in json_table.
but during JsonTableInitPathScan, for each output columns associated
JsonTablePlanState
we already initialized the PASSING arguments via `planstate->args = args;`
also transformJsonTableColumn already has a passingArgs argument.
technically we can use the jsonpath variable for every output column
regardless of whether it's nested or not.

JsonTable already has the "passing" clause,
we just need to pass it to function transformJsonTableColumns and it's callees.
based on that, I implemented it. seems quite straightforward.
I also wrote several contrived, slightly complicated tests.
It seems to work just fine.

simple explanation:
previously the following sql will fail, error message is that "could
not find jsonpath variable %s".
now it will work.

SELECT sub.* FROM
JSON_TABLE(jsonb '{"a":{"za":[{"z1": [11,2222]},{"z21": [22,
234,2345]}]},"c": 3}',
'$' PASSING 22 AS x, 234 AS y
COLUMNS(
xx int path '$.c',
NESTED PATH '$.a.za[1]' as n1 columns
(NESTED PATH '$.z21[*]' as n2
COLUMNS (z21 int path '$?(@ == $"x" || @ == $"y" )' default 0 on empty)),
NESTED PATH '$.a.za[0]' as n4 columns
(NESTED PATH '$.z1[*]' as n3
COLUMNS (z1 int path '$?(@ > $"y" + 1988)' default 0 on empty)))
)sub;