Re: proposal: auxiliary functions for record type - Mailing list pgsql-hackers

2010/12/11 Florian Pflug <fgp@phlo.org>:
> On Dec11, 2010, at 16:03 , Pavel Stehule wrote:
>> 2010/12/11 Florian Pflug <fgp@phlo.org>:
>>> On Dec11, 2010, at 06:20 , Pavel Stehule wrote:
>>>> I wrote a few functions for record type - record_expand,
>>>> record_get_fields, record_get_field, record_set_fields.
>>>
>>> Just FYI, I've created something similar a while ago. The code can be found at
>>> https://github.com/fgp/pg_record_inspect
>>>
>>> The main difference seems to be that my code allows you to retrieve fields from a record without casting it to
text.As a consequence, however, it lacks the set-returning functions that your's provides. It also doesn't provide a
wayto modify fields. 
>>
>> Casting to text is necessary for PL/pgSQL. I am not happy from this,
>> but there are not other way than using a common type - text - because
>> you don't know a target type.
>
>
> I use the anyarray/anyelement machinery to cheat there, at least a bit. My function fieldvalue() returns anyelement
andtakes a parameter <defval> of type anyelement, which serves two purposes. 
>
> First, by virtue of the anyelement machinery, the return type of fieldvalue() is that of <defval>. If the actual type
ofthe requested field matches that type, the value is returned. If they don't match, the parameter <coerce> decided
whetherfieldvalue() tries to cast the value to the requested type, or simply raises an error. 

It same trick, that I use in record_set_fields. But I don't want to
use it for reading of value. I don't like it. You don't need to know a
value, you have to know a type - NULL::type. it is just not nice :). I
though about it too, and maybe is a time for new polymorphic type
"anytype" - and then you don't need to write a litte bit strange
NULL::type

it can be "fieldvalue(myrec, type1, false)"

Regards

Pavel Stehule



>
> Second, to also give the *value*, not only the *type* of <defval> a meaning, it serves as the default return value.
Ifrequested field contains NULL, <defvalue> is returned instead. You are, of course, free to pass NULL for <defvalue>
itselfto turn that mapping into a NOP. 
>
> Note that the returned value's type is always the same as <defval>'s type, so the whole thing is perfectly type-safe
fromthe point of view of the rest of the system. 
>
> As long as you know all possible types than can appear in your record's fields, you can do in Pl/PgSQL something
alongthe line of 
> declare
>        v_value_type1 type1;
>        v_value_type2 type2;
>        ...
>        v_value_typeN typeN;
> begin
>        for v_field in select * from fieldinfos(myrec) loop
>                case
>                        when v_field.fieldtype = 'type1'::regtype then
>                                v_value_type1 := fieldvalue(myrec, NULL::type1, false);
>                                <Do something with v_value_type1>
>                        ...
>                        when v_field.fieldtype = 'typeN'::regtype then
>                                v_value_typeN := fieldvalue(myrec, NULL::typeN, false);
>                                <Do something with v_value_typeN>
>                        else raise exception 'Unexpected type % in record %', v_field.fieldtype, myrec;
>                end case;
>        end loop;
> end;
>
> It works pretty well for me...
>
> best regards,
> Florian Pflug
>
>