The select code below is possible without using Bootstrap-Select plugin, but not when using it:
<div class="container">
    <select>
        <optgroup label="Group 1">
            <optgroup label="Sub Group 1">
                <option>Item 1</option>
                <option>Item 2</option>
                <option>Item 3</option>
            </optgroup>
            <option>Item 2</option>
            <option>Item 3</option>
        </optgroup>
        <optgroup label="Group 2">
            <option>Item 1</option>
            <option>Item 2</option>
            <option>Item 3</option>
        </optgroup>
    </select>
</diV>
How can I use subgroups (nested optiongroups) using this plugin?
The first option group is not shown if I use nested. Only the second (children) optgroup and its options.
Here is the plugin website: http://silviomoreto.github.io/bootstrap-select/