google-app-engine - single-property GQL causes unexpected composite index

Question

If I execute this GQL query:

GQLQuery("SELECT user FROM MyUser WHERE foo = :1", fooz)

It will cause my index.yaml to make a composite index with both the user and foo properties.

- kind: MyUser
  properties:
  - name: user
  - name: foo

And if i delete that index, when it's uploaded to App Engine and my app runs, an error appears no matching index found.

I just don't understand why GAE must create a composite index for that query??

Answer 1

此行为在此处完整记录：

投影查询要求投影中指定的所有属性都包含在数据存储索引中。