On 9/28/22 05:00, Ulf Hermann wrote:
I think Andre's point is that you should not use Qt containers for such
large amounts of data, but rather some other data structure better
suited for your case (most trivially, std::vector instead of QList).
My point was you don't get a choice given the single-thread-i-ness of Qt
and trying to stuff everything in the main event loop.
https://doc.qt.io/qt-5/qtablewidget.html
int *currentRow <https://doc.qt.io/qt-5/qtablewidget.html#currentRow>*()
const
https://doc.qt.io/qt-5/qsqltablemodel.html
virtual int *rowCount
<https://doc.qt.io/qt-5/qsqltablemodel.html#rowCount>*(const QModelIndex
&/parent/ = QModelIndex()) const override
When your database has segments and each segment can have a ROWID 0 -
ULLONG_MAX, honestly, even ULONG_MAX gets you in trouble when you have
multiple segments in your return query.
Somewhere in all of that code is a "container." Not every database
engine will allow one to move bidirectionally through a cursor. If you
want to support a user being able to drag the scrollbar all the way to
the bottom (or just clicking there) then going back to the first row,
the data is either loaded into a container or the cursor must have
bidirectional capabilities.
I could be wrong, but I believe part of Quentin's point of stuffing
everything into RAM is the fact sliding cursor windows aren't
implemented and many database engines don't support bidirectional
movement. Leaving data in a cursor allows the OS and database engine to
determine what should be "in RAM" without having to do all of the I/O.
If one is stuck with low end databases and tools, then one has little
choice, they have to do the I/O and stuff all that data into RAM. This
is especially true if you are making a decision spread across multiple
databases from multiple vendors. The capabilities of Qt don't quite
measure up to the 1980s product Cognos PowerHouse
https://www.teamblue.unicomsi.com/products/powerhouse-4gl/
which would let you pull a cursor from say, Oracle, and drive it against
an indexed file or one of the other supported database engines. I think
you could have up to seven different database engines in a single
transaction, but that may be an outdated number. It never loaded that
stuff into RAM more than a few records at a time. I know this because
the databases I was using were many times the size of physical + virtual
memory. It all "just worked."
People don't want data dictionaries anymore, but they all want the data
capacity and throughput.
Yes, developers can customize their own QSQLTableModel to periscope over
the cursor, and force bidirectional in, but they shouldn't have to in
2022. The underlying code needs to also realize that ROWID is no longer
a table unique number. It is __segment__ unique. I forget the official
name for the groups of segments, but there is one.
--
Roland Hughes, President
Logikal Solutions
(630)-205-1593
http://www.theminimumyouneedtoknow.com
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