Use Flat Catalog Category disable

Question

Use Flat Catalog Category in Catalog is disable or Hide, Why?

Answer 1

The flat category config field has a frontend model adminhtml/system_config_form_field_select_flatcatalog. Look for it in app/code/core/Mage/Catalog/etc/system.xml.

The frontend model is actually a block that renders the field.
This block is Mage_Adminhtml_Block_System_Config_Form_Field_Select_Flatcatalog. If you take a look at it, you will see this method:

protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element)
{
    if (!Mage::helper('catalog/category_flat')->isBuilt()) {
        $element->setDisabled(true)
            ->setValue(0);
    }
    return parent::_getElementHtml($element);
}

$this means that the element is disabled if Mage_Catalog_Helper_Category_Flat::isBuilt() return false.
Digging deeper you end up in the method Mage_Catalog_Model_Resource_Category_Flat::isBuilt() that returns false if there is no flat table for any store view or if the flat tables don't have entities.

Reindexing should fix everything.

Answer 2

It's probably disabled because catalog_category_flat_store_x table is empty. Go to System > Index Management and reindex Category Products option and see if that enables the option.

Answer 3

This option will always be disabled if no products are in the store (so by default, it will be disabled until you add a product & reindex)