Re: SQL/JSON features for v15

On Wed, Aug 24, 2022 at 11:55 AM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> 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 ])

Or is the idea rather to restrict the set of data types we allow in `[
RETURNING data_type ]`?

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