SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '8823', '8855', '8856', '9042', '9090', 
    '9086', '9084', '6967', '6970', '9138', 
    '9132', '9134', '9151', '9147', '9170', 
    '9168'
  ) 
  AND lang_code = 'ru'

Query time 0.00212

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variant_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('8823','8855','8856','9042','9090','9086','9084','6967','6970','9138','9132','9134','9151','9147','9170','9168') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}