blocks/myoverview/db/upgrade.php

    $instances = $DB->get_recordset_select('block_instances', $instanceselect, $instanceparams);

    $total = $DB->count_records_select('block_instances', $instanceselect, $instanceparams);
    if ($total > 0) {
        // Show a progress bar.
        $pbar = new progress_bar('deleteblockinstances', 500, true);
        $pbar->update($i, $total, "Deleting block instance - $i/$total.");
        foreach ($instances as $instance) {
            delete_block_instance($instance);
            // Update progress.
            $pbar->update($i, $total, "Deleting block instance - $i/$total.");
            $i++;
        }
        $instances->close();
    }