SELECT 
  (
    CASE WHEN(value > 0) THEN 'vote_up' ELSE 'vote_down' END
  ) AS vote_type, 
  COUNT(1) count 
FROM 
  cscart_product_review_votes 
WHERE 
  product_review_id = 22 
GROUP BY 
  vote_type

Query time 0.00007

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "case when cscart_product_review_votes.`value` > 0 then 'vote_up' else 'vote_down' end",
      "temporary_table": {
        "table": {
          "table_name": "cscart_product_review_votes",
          "access_type": "ALL",
          "rows": 95,
          "filtered": 100,
          "attached_condition": "cscart_product_review_votes.product_review_id = 22"
        }
      }
    }
  }
}