[odb-users] operation can only be performed in transaction

[G G]

Hi.
On this code"    std:: string pragmaKey("PRAGMA key=\''0123456789\'");     std::cout << "Execute " << pragmaKey << " result = " << db->execute( pragmaKey ) << std::endl;" 
I got result: 
Execute PRAGMA key='0123456789'  result = operation can only be performed in transaction

Why???
odb-manual.pdf ("C++ Object Persistence with ODB", Revision 2.4, February 2015), page 45"The execute() functions allow us to execute native database statements on the connection. Their semantics are equivalent to the database::execute() functions (Section 3.12, "Executing NativeSQL Statements") except that they can be legally called outside a transaction."
Thanks.