python - 从 AppEngine 数据存储区删除之前获取部分祖先

Question

我想在事务中从 AppEngine 数据存储区删除一堆对象:

def F():
  items_to_delete = []
  for item in db.Query().ancestor(...):
    if item.aaa ... item.bbb:
      items_to_delete.append(item)
  db.delete(items_to_delete)
db.run_in_transaction(F)

是否可以仅获取项目的几个字段(aaa 和 bbb)？这样的获取会对性能产生积极影响吗？

Answer 1

不可能只获取几个字段；即，您将始终获取整个实体及其所有字段(或仅获取实体键)。来自 App Engine GQL reference docs :

The GQL syntax can be summarized as follows:   
    SELECT [* | __key__]
        [FROM <kind>]]
        [WHERE <condition> [AND <condition> ...]]
        [ORDER BY <property> [ASC | DESC] [, <property> [ASC | DESC] ...]]
        [LIMIT [<offset>,]<count>]
        [OFFSET <offset>]

和

A GQL query returns zero or more entities or Keys of the requested kind. Every GQL query always begins with either SELECT * or SELECT __key__. (A GQL query cannot perform a SQL-like "join" query.) Tip: SELECT __key__ queries are faster and cost less CPU than SELECT * queries.