/**
  * Returns attribute type for indexation.
  *
  * @param Mage_Catalog_Model_Resource_Eav_Attribute $attribute Attribute
  *
  * @return string
  */
 protected function _getAttributeType($attribute)
 {
     $type = 'string';
     if ($attribute->getBackendType() == 'int' || $attribute->getFrontendClass() == 'validate-digits') {
         $type = 'integer';
     } elseif ($attribute->getBackendType() == 'decimal') {
         $type = 'double';
     } elseif ($attribute->getSourceModel() == 'eav/entity_attribute_source_boolean') {
         $type = 'boolean';
     } elseif ($attribute->getBackendType() == 'datetime') {
         $type = 'date';
     } elseif ($attribute->usesSource() && $attribute->getSourceModel() === null) {
         $type = 'integer';
     } else {
         if ($attribute->usesSource()) {
             $type = 'string';
         }
     }
     return $type;
 }