SELECT 
		SQL_NO_CACHE
			FPS0.IBLOCK_ELEMENT_ID as ID
		FROM
			b_iblock_element_prop_s2 FPS0

		WHERE
			(FPS0.PROPERTY_8 = '173032' OR FPS0.PROPERTY_9 = '173032')
		ORDER BY FPS0.IBLOCK_ELEMENT_ID desc
		LIMIT 0, 10

SELECT 
		SQL_NO_CACHE
			FPS0.IBLOCK_ELEMENT_ID as ID
		FROM
			b_iblock_element_prop_s2 FPS0
		USE INDEX(PRIMARY)
		WHERE
			(FPS0.PROPERTY_8 = '173032' OR FPS0.PROPERTY_9 = '173032')
		ORDER BY FPS0.IBLOCK_ELEMENT_ID desc
		LIMIT 0, 10

Array
(
    [id] => 1
    [select_type] => SIMPLE
    [table] => FPS0
    [type] => index_merge
    [possible_keys] => PROP_8_INDEX,PROP_9_INDEX,PROP_8_9_INDEX
    [key] => PROP_8_INDEX,PROP_9_INDEX
    [key_len] => 5,5
    [ref] => 
    [rows] => 1712980
    [Extra] => Using union(PROP_8_INDEX,PROP_9_INDEX); Using where; Using filesort
)

Array
(
    [id] => 1
    [select_type] => SIMPLE
    [table] => FPS0
    [type] => index
    [possible_keys] => 
    [key] => PRIMARY
    [key_len] => 4
    [ref] => 
    [rows] => 10
    [Extra] => Using where
)

CREATE TABLE IF NOT EXISTS `b_iblock_element_prop_s2` (
  `IBLOCK_ELEMENT_ID` int(11) NOT NULL,
...
  `PROPERTY_8` int(11) DEFAULT NULL,
  `PROPERTY_9` int(11) DEFAULT NULL,
...
  PRIMARY KEY (`IBLOCK_ELEMENT_ID`),
  KEY `PROP_8_INDEX` (`PROPERTY_8`),
  KEY `PROP_9_INDEX` (`PROPERTY_9`),
  KEY `PROP_8_9_INDEX` (`PROPERTY_8`,`PROPERTY_9`),
...
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;