MYSQL READ-COMMITTED For this to work correctly, all tables must have a primary key.

The only relevant from that is poll_vote.

However the code suggests it already has a primary key? Look at poll.install:

'primary key' => array('pid', 'uid', 'hostname'),

advpoll removes the primary key.

Can a primary key be added back to support the function of advpoll?

/**
* implements hook_install
*/
function advpoll_install() {
// remove primary key lock on poll vote table so that other vote types can be
// recorded.
$schema = Database::getConnection()->schema();
$schema->dropPrimaryKey('poll_vote');
}