SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN ('1529', '1533', '1542', '1429') 
  AND lang_code = 'ru'

Query time 0.00028

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": 4,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('1529','1533','1542','1429') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}