How to execute VACUUM command on SQLITE db

[Marmelade]

My program is really simple, a kind of address book.
The VACUUM call is done in SLOT and I am certain at this point that there are no other running queries.

For this to work I found this:

QSqlDatabase db= QSqlDatabase::database();
db.close();
db.open();
QSqlQuery query;
query.exec("VACUUM");

QSqlDatabase db= QSqlDatabase::database();
db.close();
db.open();
QSqlQuery query;
query.exec("VACUUM");

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Thank you for your advice