SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1', '3', '1351', '289', '125', '126', 
    '414', '1348', '419', '1347', '424', 
    '428', '122', '121', '127', '1349', 
    '149', '120', '124', '123', '234', 
    '146', '59', '50', '57', '147', '24', 
    '25', '26', '426', '423', '421', '422', 
    '111', '1296', '237', '238', '165', 
    '320'
  ) 
  AND lang_code = 'ru'

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "27.31"
    },
    "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": 39,
      "rows_produced_per_join": 39,
      "filtered": "100.00",
      "index_condition": "((`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`variant_id` in ('1','3','1351','289','125','126','414','1348','419','1347','424','428','122','121','127','1349','149','120','124','123','234','146','59','50','57','147','24','25','26','426','423','421','422','111','1296','237','238','165','320')) and (`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`lang_code` = 'ru'))",
      "cost_info": {
        "read_cost": "23.41",
        "eval_cost": "3.90",
        "prefix_cost": "27.31",
        "data_read_per_join": "121K"
      },
      "used_columns": [
        "variant_id",
        "lang_code",
        "yml2_unit"
      ]
    }
  }
}