SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1', '3', '366', '491', '125', '492', 
    '494', '496', '497', '164', '122', 
    '240', '121', '127', '149', '120', 
    '123', '152', '234', '146', '228', 
    '50', '57', '147', '39', '32', '6', 
    '237', '238', '98', '170', '489', '165'
  ) 
  AND lang_code = 'ru'

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "23.11"
    },
    "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": 33,
      "rows_produced_per_join": 33,
      "filtered": "100.00",
      "index_condition": "((`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`variant_id` in ('1','3','366','491','125','492','494','496','497','164','122','240','121','127','149','120','123','152','234','146','228','50','57','147','39','32','6','237','238','98','170','489','165')) and (`test_uchur_k`.`cscart_product_feature_variant_descriptions`.`lang_code` = 'ru'))",
      "cost_info": {
        "read_cost": "19.81",
        "eval_cost": "3.30",
        "prefix_cost": "23.11",
        "data_read_per_join": "102K"
      },
      "used_columns": [
        "variant_id",
        "lang_code",
        "yml2_unit"
      ]
    }
  }
}