Marco Bubke authored Sep 19, 2017 and Tim Jenssen committed Sep 20, 2017

If you use the database in a multi-threaded environment, you must always
your statements in transactions.

For read-only statements, you use DeferredTransaction and write statements
you use ImmediateTransaction. If you mix read and write statements you have
to use ImmediateTransaction. Don't use DeferredTransaction because it leads
to undefined behavior.

Change-Id: Ida298a20f33423c8da09e768a7b658dd8e918f46
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>