Re: pvDataCPP implementations

[Benjamin F. <ben...@he...>]

Am Samstag, 3. September 2011, um 21:53:46 schrieb Benjamin Franksen:
> Am Samstag, 3. September 2011, um 14:32:26 schrieb Benjamin Franksen:
> > Am Freitag, 2. September 2011, um 17:09:05 schrieb White, Greg:
> > > I'm thinking
> > > about the common class we've found, of which returning data from *any*
> > > SQL database query is a good example. Until runtime, you don't know the
> > > number of rows, nor the number or type of columns that must be
> > > returned.
> >
> > This is the perfect example where you do *not* need heterogenous arrays.
>
> I have thought some more about this and I understand now what you want to
> achieve. This is something where you need dynamic typing, but not
> heterogeneous arrays.

...but it might indeed be a case for arrays of arrays, for instance if the
database is untyped (like sqlite, where all columns are strings). However, if
I have understood things right, it is easy to emulate arrays of arrays on top
of pvData by using a structureArray where the structure contains just one
array, i.e.

  structure resultSet
    string[] columnNames
      string "id"
      string "value"
    structure[] rows
      structure /* do I need a name here? which one? */
        string[]
          string "398576"
          string "first value"
      structure /* do I need a name here? which one? */
        string[]
          string "273465"
          string "second value"
      ...

(I hope I have got the data specification syntax right; in the examples I have
seen there is always a name attached to a structure but I would not know what
structure name to use here nor why it would be expected.)

Cheers
Ben

> See ChannelRPC...
>
> Cheers
> Ben
>
> > Each single SQL query returns a homogeneous array. A generic client that
> > wants to support arbitrary SQL queries needs to initiate a new request
> > for each new SQL query, of course. The server responds with a
> > description of how the result set is structured (i.e. what type the rows
> > have).
> >
> > > Server
> > > side code has to dynamically construct the single return structure,
> > > composed of either, an array of structures (little used we found), or
> > > an array of differently typed arrays. In a full implementation, the
> > > leaf node structures, may themselves be structures (that is, one of
> > > the db table columns is of structure type). My point is, what's
> > > returned was dynamically constructed, and heterogenous.
> >
> > I don't know what SQL dialect you are using but I have never seen a
> > relational database where the result set of a query was not a homogeneous
> > array of structures.
> >
> > Cheers
> > Ben
> >
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> ________________________________
>
> Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
>
> Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren
> e.V.
>
> Aufsichtsrat: Vorsitzender Prof. Dr. Dr. h.c. mult. Joachim Treusch, stv.
> Vorsitzende Dr. Beatrix Vierkorn-Rudolph Geschäftsführer: Prof. Dr. Anke
> Rita Kaysser-Pyzalla, Dr. Ulrich Breuer
>
> Sitz Berlin, AG Charlottenburg, 89 HRB 5583
>
> Postadresse:
> Hahn-Meitner-Platz 1
> D-14109 Berlin
>
> http://www.helmholtz-berlin.de
>
> ---------------------------------------------------------------------------
> --- Special Offer -- Download ArcSight Logger for FREE!
> Finally, a world-class log management solution at an even better
> price-free! And you'll get a free "Love Thy Logs" t-shirt when you
> download Logger. Secure your free ArcSight Logger TODAY!
> http://p.sf.net/sfu/arcsisghtdev2dev


________________________________

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V.

Aufsichtsrat: Vorsitzender Prof. Dr. Dr. h.c. mult. Joachim Treusch, stv. Vorsitzende Dr. Beatrix Vierkorn-Rudolph
Geschäftsführer: Prof. Dr. Anke Rita Kaysser-Pyzalla, Dr. Ulrich Breuer

Sitz Berlin, AG Charlottenburg, 89 HRB 5583

Postadresse:
Hahn-Meitner-Platz 1
D-14109 Berlin

http://www.helmholtz-berlin.de