default boolean isColumnFilterable(FacesContext context, UIColumn column) {
        Map<String, FilterMeta> filterBy = getFilterByAsMap();
        if (filterBy.containsKey(column.getColumnKey())) {
            return true;
        }

        // lazy init - happens in cases where the column is initially not rendered
        FilterMeta f = FilterMeta.of(context, getVar(), column, isFilterNormalize());
        if (f != null) {
            filterBy.put(f.getColumnKey(), f);
        }
        setFilterByAsMap(filterBy);

        return f != null;
    }
ID of the global filter component
primefaces/src/main/java/org/primefaces/component/api/UITable.java