SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '6830', '6839', '6831', '6836', '6842', 
    '6840', '6832', '6833', '6834', '6828', 
    '6829', '92', '6835', '6843', '116', 
    '6837'
  ) 
  AND lang_code = 'ru'

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "11.21"
    },
    "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": 16,
      "rows_produced_per_join": 16,
      "filtered": "100.00",
      "index_condition": "((`zdrowy_db`.`cscart_product_feature_variant_descriptions`.`variant_id` in ('6830','6839','6831','6836','6842','6840','6832','6833','6834','6828','6829','92','6835','6843','116','6837')) and (`zdrowy_db`.`cscart_product_feature_variant_descriptions`.`lang_code` = 'ru'))",
      "cost_info": {
        "read_cost": "9.61",
        "eval_cost": "1.60",
        "prefix_cost": "11.21",
        "data_read_per_join": "61K"
      },
      "used_columns": [
        "variant_id",
        "lang_code",
        "yml2_unit"
      ]
    }
  }
}