Re: [gnome-db] Changing a field definition

[Rodrigo Moya <rodrigo gnome-db org>]

On Sun, 2004-04-25 at 15:02 +0200, Murray Cumming wrote:
> On Sun, 2004-04-25 at 12:53, Rodrigo Moya wrote:
> > On Sat, 2004-04-24 at 18:02 +0200, Murray Cumming wrote:
> > > gda_data_model_update_column() seems to be a way to change the column
> > > definition. But I think the data_model needs to be one with data in it,
> > > such as the result of a "SELECT" command.
> > > 
> > it doesn't need to be filled in with data. What is needed is the
> > structure of the data model, which only can be retrieved by knowing
> > which command is going to be executed.
> 
> Thans, but I don't understand the "knowing which command is going to be
> executed" part.
>
to get the structure of the data returned by the DB, you first need to
execute the command. This only executes it, which means no data is
retrieved by libgda. That is made later on, when the app requests the
data models' rows.

>  How could I create a suitable datamodel? Are you saying
> that the result of a "SELECT * FROM thetable" query is indeed a good
> thing to use?
> 
I guess so, once update_column is fully working, getting a GdaDataModel
from "SELECT * FROM table" might be much quicker for some providers than
calling the get_schema call, which in some cases needs to do nested
queries or more than one SQL query.

A better solution for this would be, I guess, to have create_table,
modify_table, etc methods on the provider.

What do you think?

> > Then, as providers usually only fetch the data when needed, it means you
> > get the model's structure without the need to retrieve all the data.
> > 
> > It will be as quick as doing the get_schema stuff, since in both cases
> > you'll have to do a SELECT, and in the model's case, you wont retrieve
> > any data at all (if not needed).
> 
> So gda_connection_execute_single_command() does not actually execute any
> command until I do a gda_data_model_get_value_at()?
> 
it does execute it, but data is not retrieved, at least for the RDBMS
cases (PostgreSQL, Oracle, etc), until you ask it to do so
(gda_data_model_get_value_at/get_row).

cheers