Re: SQL/JSON features for v15

Hi Nikita,

On Wed, Aug 24, 2022 at 6:29 AM Nikita Glukhov <n(dot)gluhov(at)postgrespro(dot)ru> wrote:
> Here is my plan:
>
> 0. Take my last v7-0001 patch as a base. It already contains refactoring
> of JsonCoercion code. (Fix 0002 is not needed anymore, because it is for
> json[b] domains, which simply will not be supported.)
>
> 1. Replace JSON_COERCION_VIA_EXPR in JsonCoercion with new
> JsonCoercionType(s) for hardcoded coercions.
>
> 2. Disable all non-JSON-compatible output types in coerceJsonFuncExpr().
>
> 3. Add missing safe type input functions for integers, numerics, and
> maybe others.
>
> 4. Implement hardcoded coercions using these functions in
> ExecEvalJsonExprCoercion().
>
> 5. Try to allow only constants (and also maybe column/parameter
> references) in JSON_VALUE's DEFAULT expressions. This should be enough
> for the most of practical cases. JSON_QUERY even does not have DEFAULT
> expressions -- it has only EMPTY ARRAY and EMPTY OBJECT, which can be
> treated as simple JSON constants.
>
> But it is possible to allow all other expressions in ERROR ON ERROR
> case, and I don't know if it will be consistent enough to allow some
> expressions in one case and deny in other.
>
> And there is another problem: expressions can be only checked for
> Const-ness only after expression simplification. AFAIU, at the
> parsing stage they look like 'string'::type. So, it's unclear if it
> is correct to check expressions in ExecInitExpr().

IIUC, the idea is to remove the support for `DEFAULT expression` in
the following, no?

json_value ( context_item, path_expression
...
[ { ERROR | NULL | DEFAULT expression } ON EMPTY ]
[ { ERROR | NULL | DEFAULT expression } ON ERROR ])

json_query ( context_item, path_expression
...
[ { ERROR | NULL | EMPTY { [ ARRAY ] | OBJECT } | DEFAULT expression }
ON EMPTY ]
[ { ERROR | NULL | EMPTY { [ ARRAY ] | OBJECT } | DEFAULT expression }
ON ERROR ])

If that's the case, I'd imagine that `default_expr` in the following
will be NULL for now:

/*
* JsonBehavior -
* representation of JSON ON ... BEHAVIOR clause
*/
typedef struct JsonBehavior
{
NodeTag type;
JsonBehaviorType btype; /* behavior type */
Node *default_expr; /* default expression, if any */
} JsonBehavior;

And if so, no expression left to check the Const-ness of?

--
Thanks, Amit Langote
EDB: http://www.enterprisedb.com