Re: Ada DB bindings and APQ

From: Warren W. Gay VE3WWG (ve3wwg_at_NoSpam.cogeco.ca)
Date: 12/17/04 

Date: Fri, 17 Dec 2004 06:13:47 -0500

Brian May wrote:
>>>>>>"Warren" == Warren W Gay VE3WWG <ve3wwg@NoSpam.cogeco.ca> writes:
>
>
> Warren> I like the neatness of this approach, but the danger is
> Warren> that you might do the "..." part on a row that might
> Warren> not exist. Though I suppose the Query_Type object could
> Warren> maintain enough state such that it can raise an exception
> Warren> if any "Values" were fetched from the row that is not there.
> Warren> Just a quibble: I'd prefer something like "No_Row(Q)" perhaps.
>
> Warren> An even neater loop is this, IMHO ;-)
>
> >> loop
> >> Fetch(Q);
> >> exit when No_More_Data(Q);
> >>
> >> ...
> >> end loop;
>
> How about
>
> Valid_Row : Boolean;
>
> loop
> Fetch(Q, Valid_Row);
> exit when not Valid_Row;
> ...
> end loop;
>
> Where Valid_Row is an out variable from Fetch.
>
> This would avoid breaking any existing software, and make it obvious
> that Valid_Row has to be checked.

I like that idea.

> Other questions that might be significant, especially to some of my
> proposed changes.
>
> 1. What happens if an unexpected exception occurs within the above
> loop, and it prevents continuing the loop. In fact, the code that
> handles the exception may not realize it occurred in middle of the
> fetch operation. Is there any way of recovering the database
> connection?

An unexpected error would raise SQL_Error (or somesuch). So
if you expect to recover from it, you must obviously be willing
to catch the exception at some level.

> 2. With Mysql what happens if I do this:
>
> declare
> Q1 : Root_Query_Type'Class := New_Query(C);
> Q2 : Root_Query_Type'Class := New_Query(C);
> begin
> Prepare(Q1, ...);
> Prepare(Q2, ...)
>
> Set_Fetch_Mode(Q1, Sequential_Fetch);
> Set_Fetch_Mode(Q2, Sequential_Fetch);
>
> Execute(Q1, C);
> Execute(Q2, C);
>
> loop
> Fetch(Q1);
> Fetch(Q2);
> ...
> end loop;
> end;
>
> Will this work, trigger an error, or will it kill the Mysql connection
> as requests are intermixed?

I haven't tried this, but I know that PostgreSQL is supposed
to support this, and I believe it will. Whether MySQL and Sybase
can, I am not sure.

In MySQL's case, what I believe happens is that any Q2 rows that
are skipped over to fetch rows for Q1, will just be loaded
into the C client memory for later use. But I have not taken
the time to test this. 

--
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg

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