[Qt-interest] Using true database cursors with Qt

[John Moran]

Hi Robert,

> I believe that QSqlQuery provides exactly the same functionality as a
> cursor. There might be some things you could do to improve performance
> on the database side of things, though:

> (1) proper indexing;

Naturally, I already index things; My real concern is so-called
"impedance mismatch" though.

> (2) fetch pre-built thumbnails instead of entire full-sized images to
> populate your list;

They're already thumbnails.

> (3) call QApplication:;processEvents() within your loop which should
> help keep the GUI responsive.

That's what I intend to do, between cursor fetches; The point of the
cursor is that it encapsulates a result set. I don't have to transfer
all the results between the database and my app at once. I can fetch a
bunch of them at a time, and call QApplication:;processEvents()
between fetches. I think it's pretty doubtful that QSqlQuery is
transparently using a *native* cursor (the only useful kind - I think
some python DB drivers use the term cursor without actually using
native cursors). Surely the docs would say so, and surely I'd be able
to specify how many records I want to fetch at a time, between calls
to  QApplication:;processEvents(). This would typically work my
instantiating a cursor class, which we iterate through. Each iteration
would return a subset of the cursor's result set, perhaps in the form
of a QSqlQuery. We'd be using a nested loop. This is how the regular
C++ postgreSQL API, libpqxx, does it.

Thanks,
John Moran