Обсуждение: missing something about json syntax

> On 20/04/2023 18:35 CEST Marc Millas <marc.millas@mokadb.com> wrote:
>
> Hi,
>
> postgres 15
>
> looks Iike I am missing something, maybe obvious :-(
> In a table with a json column (_data) if I ask psql to select _data from
> mytable with a where clause to get only one line,, I get something beginning
> by
>
{"time":"2023-04-19T16:28:01.19780551+02:00","stream":"stderr","_p":"F","log":"{\"level\":\"info\",\"ts\":\"2023-04-19T14:28:01Z\",\"logger\":\"_audit\",\"msg\":\"record\",\"logging_pod\":\"cluster-pgsql\",\"record\":{\"log_time\":\"2023-04-19
14:28:01.197UTC\",\ 
> etc...

The value of property "log" is a string, not an object.  Notice the escaped
double quotes (\").

> if I create table anothertable as select _data as _data from mytable, it
> creates and feed that new table with all the appropriate data, and when I ask
> psql \d anothertable it says that its a table with a json column.named _data.
> fine !
>
> now if I select json_object_keys(_data) from mytable, I get a list of tags.
> time, stream, _p, log, fine.
> now, if i select json_object_keys(_data) from anothettable, I get an error:
> cannot call json_objet_keys on a scalar..
>
> ???
> both columns are fed and of type json. and postgres didn't throw any error
> feeding them.
> if I create a table with a jsonb column and feed it with the anothertable json
> column, same, fine... but still unusable.
>
> and unusable with all the other ways I did try, like simply
> select _data->'log'->>'level' from mytable, or
> select _data->'level' from anothertable
>
> sure if I look at the json field one is showed { "tag": "value", ...
> and the other is showed "{\"tag\":\"value\", ...

You executed

    create table anothertable as select _data->'log' as _data from mytable;

and not

    create table anothertable as select _data as _data from mytable;

So you end up with the scalar value of property "log" in anothertable._data.

> not the very same
>
> so 2 questions:
> 1) how postgres can feed a json or jsonb column and CANNOT use the values in
>    it ??
> 2) how to "transform" the inappropriate json into a usable one ?
>
> of course, if what I am missing is very obvious, I apologize...

Get the log value with operator ->> and cast the returned text to json:

    select (_data->>'log')::json->'level' from mytable;

--
Erik

Marc Millas <marc.millas@mokadb.com> writes:
> but it doesnt explain how postgres is able to put a scalar in a json or
> jsonb column without pb:
> I don't understand how this ('"{\"t\"}"') can be considered  a valid enough
> json to be inserted in a json column
> and at the same time invalid for all other json uses.

That's a bare string (it's not an object).  That's valid JSON per
RFC 7159:

   JSON can represent four primitive types (strings, numbers, booleans,
   and null) and two structured types (objects and arrays).
   ...
   A JSON text is a serialized value.  Note that certain previous
   specifications of JSON constrained a JSON text to be an object or an
   array.

However, there certainly are some operations that require the top-level
value to be an object or array.

            regards, tom lane