Does the NDB membership query (“IN” operation) performance degrade with lots of possible values?

问题

The documentation for the IN query operation states that those queries are implemented as a big OR'ed equality query:

qry = Article.query(Article.tags.IN(['python', 'ruby', 'php']))

is equivalent to:

qry = Article.query(ndb.OR(Article.tags == 'python',
                           Article.tags == 'ruby',
                           Article.tags == 'php'))

I am currently modelling some entities for a GAE project and plan on using these membership queries with a lot of possible values:

qry = Player.query(Player.facebook_id.IN(list_of_facebook_ids))

where list_of_facebook_ids could have thousands of items.

Will this type of query perform well with thousands of possible values in the list? If not, what would be the recommended approach for modelling this?

回答1:

This won't work with thousands of values (in fact I bet it starts degrading with more than 10 values). The only alternative I can think of are some form of precomputation. You'll have to change your schema.

回答2:

One way you can you do it is to create a new model called FacebookPlayer which is an index. This would be keyed by facebook_id. You would update it whenever you add a new player. It looks something like this:

class FacebookUser(ndb.Model):
    player = ndb.KeyProperty(kind='Player', required=True)

Now you can avoid queries altogether. You can do this:

# Build keys from facebook ids.
facebook_id_keys = []
for facebook_id in list_of_facebook_ids:
    facebook_id_keys.append(ndb.Key('FacebookPlayer', facebook_id))

keysOfUsersMatchedByFacebookId = []
for facebook_player in ndb.get_multi(facebook_id_keys):
    if facebook_player:
        keysOfUsersMatchedByFacebookId.append(facebook_player.player)
usersMatchedByFacebookId = ndb.get_multi(keysOfUsersMatchedByFacebookId)

If list_of_facebook_ids is thousands of items, you should do this in batches.

来源：https://stackoverflow.com/questions/11936035/does-the-ndb-membership-query-in-operation-performance-degrade-with-lots-of