data modeling - App engine datastore denormalization: index properties in the main entity or the denormalized entity? -

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consider classic example of blog data modelling, have blog entity many properties, , want list latest blogs in page.

it makes sense denormalize blogpost entity blogpostsummary entity shown in list view, avoiding fetching , deserializing many unwanted properties.

class blogpost(db.model):   title = db.stringproperty()   content = db.textproperty()   created = db.dateproperty()   ...  class blogpostsummary(db.model):   title = db.stringproperty()   content_excerpt = db.textproperty()

the question is: entity should hold indexed properties? there 3 options:

1. index properties in both

  • pros:
    • easy query on both entities.
  • cons:
    • maintaining denormalized indexes expensive.

2. index properties in main entity only

  • pros:
    • indexing properties in main entity more safe, denormalized entity treated redundancy.
  • cons:
    • querying list view need double roundtrip datastore: 1 key-only query blogpost entities, followed batch blogpostsummary.

3. index in denormalized entity only

  • pros:
    • the list view can built single query.
  • cons:
    • the main entity cannot queried properties anymore.
    • the indexes occupy more space when denormalized entity child of main entity.

which option work better? there other options?

would double round trip datastore in option 2 problem?

this abstract question not have "correct" answer. choice of data model depends on specific requirements of project, including:

  • usage patterns (how need access different data)
  • update patterns (e.g. separating updated properties stable properties in order reduce write costs)
  • average performance , extreme-case performance requirements (e.g average blog may have 10 posts, popular blog may have 10,000 posts)
  • ability use memcache reduce datastore trips , improve performance
  • data complexity (i.e. how many different kids of entities depend on particular entity kind)
  • transaction requirements
  • security , access roles considerations (e.g. not exposing private data mistake)

by way, there way model data in datastore - using child entities. example, blog posts may child entities of blog entity. way can retrieve blog posts single query providing parent key - without storing post ids or keys in blog entity or blog id/key in post entities.


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