SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '3', '502', '252', '366', '125', '172', 
    '296', '148', '414', '367', '155', 
    '173', '122', '121', '127', '149', 
    '120', '123', '152', '234', '146', 
    '59', '50', '57', '58', '147', '176', 
    '42', '32', '33', '31', '44', '43', 
    '26', '298', '299', '503', '257', '154', 
    '489', '165', '320'
  ) 
  AND lang_code = 'ru'

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "29.41"
    },
    "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": 42,
      "rows_produced_per_join": 42,
      "filtered": "100.00",
      "index_condition": "((`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`variant_id` in ('3','502','252','366','125','172','296','148','414','367','155','173','122','121','127','149','120','123','152','234','146','59','50','57','58','147','176','42','32','33','31','44','43','26','298','299','503','257','154','489','165','320')) and (`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`lang_code` = 'ru'))",
      "cost_info": {
        "read_cost": "25.21",
        "eval_cost": "4.20",
        "prefix_cost": "29.41",
        "data_read_per_join": "130K"
      },
      "used_columns": [
        "variant_id",
        "lang_code",
        "yml2_unit"
      ]
    }
  }
}