SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1', '158', '289', '125', '126', '504', 
    '508', '1829', '1826', '509', '160', 
    '122', '240', '121', '127', '1349', 
    '149', '120', '123', '270', '234', 
    '146', '50', '57', '58', '147', '20', 
    '21', '15', '18', '19', '1813', '507', 
    '287', '262', '162', '265', '422', 
    '282', '111', '1300', '237', '238', 
    '98', '165', '320'
  ) 
  AND lang_code = 'ru'

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "32.21"
    },
    "table": {
      "table_name": "cscart_product_feature_variant_descriptions",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "variant_id",
        "lang_code"
      ],
      "key_length": "9",
      "rows_examined_per_scan": 46,
      "rows_produced_per_join": 46,
      "filtered": "100.00",
      "index_condition": "((`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`variant_id` in ('1','158','289','125','126','504','508','1829','1826','509','160','122','240','121','127','1349','149','120','123','270','234','146','50','57','58','147','20','21','15','18','19','1813','507','287','262','162','265','422','282','111','1300','237','238','98','165','320')) and (`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`lang_code` = 'ru'))",
      "cost_info": {
        "read_cost": "27.61",
        "eval_cost": "4.60",
        "prefix_cost": "32.21",
        "data_read_per_join": "143K"
      },
      "used_columns": [
        "variant_id",
        "lang_code",
        "yml2_unit"
      ]
    }
  }
}