SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '9058', '8807', '8809', '8819', '8820', 
    '8823', '9071', '9042', '9030', '9036', 
    '8877', '8881', '9080', '9040', '9091', 
    '9082', '9090', '9086', '9084', '6967', 
    '6970', '6968', '6965', '6966', '7959', 
    '9148', '9153', '9151', '9157', '9150', 
    '9147', '9167', '9162', '9170', '9168'
  ) 
  AND lang_code = 'ru'

Query time 0.00076

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variant_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "rows": 35,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('9058','8807','8809','8819','8820','8823','9071','9042','9030','9036','8877','8881','9080','9040','9091','9082','9090','9086','9084','6967','6970','6968','6965','6966','7959','9148','9153','9151','9157','9150','9147','9167','9162','9170','9168') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}