I am trying to detect, after a user registers, which friends from Facebook have already registered for my service. My current implementation is very CPU intensive:
for eachFriend in facebookFriends:
friendUser = User.get_by_key_name(eachFriend['id'])
if friendUser:
friendUsers.append(friendUser)
I tried optimizing the query by using the IN operator:
users = User.all().filter('id IN', idList).fetch(10) # the idList is the list of IDs for a users facebook friends
This method fails as the maximum subqueries of the IN operator is 30.
Any tips?
Using an IN operator actually makes your query less efficient: Instead of doing a fast get operation for each friend, you're doing a slow query operation (IN and != filters are broken out into multiple queries on the backend).
Instead, do a single batch fetch for all the matching suers:
friendUsers = User.get_by_key_name([x['id'] for x in facebookFriends])
This returns a list of all friends, with None values for any friends that don't yet exist.
Yeah, you can have each registered user store his friends in a ListProperty so that when I register, you can do an = query on that property to see who has me as a friend. = queries on ListProperties return all entities having the filtered-on value anywhere in the list, and they don't generate subqueries the way IN queries do.
Just be aware of per-entity index limits if some of your users have tons of friends.
Related
I have a following model in the GAE app.
class User
school_name = db.StringProperty(Indexed=True)
country = db.StringProperty(Indexed=True)
city = db.StringProperty(Indexed=True)
sex = db.StringProperty(Indexed=True)
profession = db.StringProperty(Indexed=True)
joined_date = db.DateTimeProperty(Indexed=True)
And I want to filter the users by combinations of these fields. Result of the filter should show a user at first who is joined recently. So which means any query end by order operation, I suppose. like that:
User.all().filter('country =','US').filter('profession =','SE').order('-joined_date')
User.all().filter('school_name =','AAA').filter('profession =','SE').order('-joined_date')
....
User.all().filter('sex =','Female').filter('profession =','HR').order('-joined_date')
All these fields combination would be C(5,1)+C(5,2)+...+C(5,5) = 31.
My question is to implement it, do I need to create indexes for all these cases(31) in the Google AppEngine. Or can you suggest other way to implement it?
Note: C(n,k) is combination formula, see more on http://en.wikipedia.org/wiki/Combination
Thanks in advance!
You have several options:
Create all 31 indexes, as you suggest.
Do the sorting in memory. Without a sort order, all your queries can be executed with the built-in merge-join strategy, and so you won't need any indexes at all.
Restrict queries to those that are more likely, or those that eliminate most of the non-matching results, and perform additional filtering in memory.
Put all your data in a ListProperty for indexing as "key:value" strings, and filter only on that. You will need to create multiple indexes with different occurrence counts on that field (eg, indexing it once, twice, etc), and it will result in the same number of index entries, but fewer custom indexes used.
For example, if I have: List A; and a String B;
What is the difference, in JDO (AppEngine), between the following two conditions in a query: B == A; and A.contains(B);?
Also, does the query in Slides 23-25 of http://dl.google.com/io/2009/pres/W_0415_Building_Scalable_Complex_App_Engines.pdf work efficiently in AppEngine (JDO) for more than 30 receivers? How so, especially since I read in AppEngine documentation that each contains() query can have a maximum of 30 items in the list. Do I not use a contains() query to imitate the above slides (written in Python)? If not, then how can I achieve the same results in JDO?
Any suggestions/comments are highly welcome. I'm trying to build a messaging system in AppEngine but having trouble trying to get used to the platform.
Thanks.
There's no difference - in App Engine, equality checks on lists are the same as checking for containment, due to the way things are indexed in the datastore.
By the query on slides 23-25, I presume you mean this one?
indexes = db.GqlQuery(
"SELECT __key__ FROM MessageIndex "
"WHERE receivers = :1", me)
keys = [k.parent() for k in indexes]
messages = db.get(keys)
This works just fine, as it's a list containment check as described above, and results in a single datastore query. The limitation you're thinking about is on the reverse: if you have a list, and you want to find a record that contains any item in that list, a subquery will be created for each element in the list.
Select all records, ID which is not in the list
How to make like :
query = Story.all()
query.filter('ID **NOT IN** =', [100,200,..,..])
There's no way to do this efficiently in App Engine. You should simply select everything without that filter, and filter out any matching entities in your code.
This is now supported via GQL query
The 'IN' and '!=' operators in the Python runtime are actually
implemented in the SDK and translate to multiple queries 'under the
hood'.
For example, the query "SELECT * FROM People WHERE name IN ('Bob',
'Jane')" gets translated into two queries, equivalent to running
"SELECT * FROM People WHERE name = 'Bob'" and "SELECT * FROM People
WHERE name = 'Jane'" and merging the results. Combining multiple
disjunctions multiplies the number of queries needed, so the query
"SELECT * FROM People WHERE name IN ('Bob', 'Jane') AND age != 25"
generates a total of four queries, for each of the possible conditions
(age less than or greater than 25, and name is 'Bob' or 'Jane'), then
merges them together into a single result set.
source: appengine blog
This is an old question, so I'm not sure if the ID is a non-key property. But in order to answer this:
query = Story.all()
query.filter('ID **NOT IN** =', [100,200,..,..])
...With ndb models, you can definitely query for items that are in a list. For example, see the docs here for IN and !=. Here's how to filter as the OP requested:
query = Story.filter(Story.id.IN([100,200,..,..])
We can even query for items that in a list of repeated keys:
def all(user_id):
# See if my user_id is associated with any Group.
groups_belonged_to = Group.query().filter(user_id == Group.members)
print [group.to_dict() for group in belong_to]
Some caveats:
There's docs out there that mention that in order to perform these types of queries, Datastore performs multiple queries behind the scenes, which (1) might take a while to execute, (2) take longer if you searching in repeated properties, and (3) will up your costs with more operations.
Simple one really. In SQL, if I want to search a text field for a couple of characters, I can do:
SELECT blah FROM blah WHERE blah LIKE '%text%'
The documentation for App Engine makes no mention of how to achieve this, but surely it's a common enough problem?
BigTable, which is the database back end for App Engine, will scale to millions of records. Due to this, App Engine will not allow you to do any query that will result in a table scan, as performance would be dreadful for a well populated table.
In other words, every query must use an index. This is why you can only do =, > and < queries. (In fact you can also do != but the API does this using a a combination of > and < queries.) This is also why the development environment monitors all the queries you do and automatically adds any missing indexes to your index.yaml file.
There is no way to index for a LIKE query so it's simply not available.
Have a watch of this Google IO session for a much better and more detailed explanation of this.
i'm facing the same problem, but i found something on google app engine pages:
Tip: Query filters do not have an explicit way to match just part of a string value, but you can fake a prefix match using inequality filters:
db.GqlQuery("SELECT * FROM MyModel WHERE prop >= :1 AND prop < :2",
"abc",
u"abc" + u"\ufffd")
This matches every MyModel entity with a string property prop that begins with the characters abc. The unicode string u"\ufffd" represents the largest possible Unicode character. When the property values are sorted in an index, the values that fall in this range are all of the values that begin with the given prefix.
http://code.google.com/appengine/docs/python/datastore/queriesandindexes.html
maybe this could do the trick ;)
Altough App Engine does not support LIKE queries, have a look at the properties ListProperty and StringListProperty. When an equality test is done on these properties, the test will actually be applied on all list members, e.g., list_property = value tests if the value appears anywhere in the list.
Sometimes this feature might be used as a workaround to the lack of LIKE queries. For instance, it makes it possible to do simple text search, as described on this post.
You need to use search service to perform full text search queries similar to SQL LIKE.
Gaelyk provides domain specific language to perform more user friendly search queries. For example following snippet will find first ten books sorted from the latest ones with title containing fern
and the genre exactly matching thriller:
def documents = search.search {
select all from books
sort desc by published, SearchApiLimits.MINIMUM_DATE_VALUE
where title =~ 'fern'
and genre = 'thriller'
limit 10
}
Like is written as Groovy's match operator =~.
It supports functions such as distance(geopoint(lat, lon), location) as well.
App engine launched a general-purpose full text search service in version 1.7.0 that supports the datastore.
Details in the announcement.
More information on how to use this: https://cloud.google.com/appengine/training/fts_intro/lesson2
Have a look at Objectify here , it is like a Datastore access API. There is a FAQ with this question specifically, here is the answer
How do I do a like query (LIKE "foo%")
You can do something like a startWith, or endWith if you reverse the order when stored and searched. You do a range query with the starting value you want, and a value just above the one you want.
String start = "foo";
... = ofy.query(MyEntity.class).filter("field >=", start).filter("field <", start + "\uFFFD");
Just follow here:
init.py#354">http://code.google.com/p/googleappengine/source/browse/trunk/python/google/appengine/ext/search/init.py#354
It works!
class Article(search.SearchableModel):
text = db.TextProperty()
...
article = Article(text=...)
article.save()
To search the full text index, use the SearchableModel.all() method to get an
instance of SearchableModel.Query, which subclasses db.Query. Use its search()
method to provide a search query, in addition to any other filters or sort
orders, e.g.:
query = article.all().search('a search query').filter(...).order(...)
I tested this with GAE Datastore low-level Java API. Me and works perfectly
Query q = new Query(Directorio.class.getSimpleName());
Filter filterNombreGreater = new FilterPredicate("nombre", FilterOperator.GREATER_THAN_OR_EQUAL, query);
Filter filterNombreLess = new FilterPredicate("nombre", FilterOperator.LESS_THAN, query+"\uFFFD");
Filter filterNombre = CompositeFilterOperator.and(filterNombreGreater, filterNombreLess);
q.setFilter(filter);
In general, even though this is an old post, a way to produce a 'LIKE' or 'ILIKE' is to gather all results from a '>=' query, then loop results in python (or Java) for elements containing what you're looking for.
Let's say you want to filter users given a q='luigi'
users = []
qry = self.user_model.query(ndb.OR(self.user_model.name >= q.lower(),self.user_model.email >= q.lower(),self.user_model.username >= q.lower()))
for _qry in qry:
if q.lower() in _qry.name.lower() or q.lower() in _qry.email.lower() or q.lower() in _qry.username.lower():
users.append(_qry)
It is not possible to do a LIKE search on datastore app engine, how ever creating an Arraylist would do the trick if you need to search a word in a string.
#Index
public ArrayList<String> searchName;
and then to search in the index using objectify.
List<Profiles> list1 = ofy().load().type(Profiles.class).filter("searchName =",search).list();
and this will give you a list with all the items that contain the world you did on the search
If the LIKE '%text%' always compares to a word or a few (think permutations) and your data changes slowly (slowly means that it's not prohibitively expensive - both price-wise and performance-wise - to create and updates indexes) then Relation Index Entity (RIE) may be the answer.
Yes, you will have to build additional datastore entity and populate it appropriately. Yes, there are some constraints that you will have to play around (one is 5000 limit on the length of list property in GAE datastore). But the resulting searches are lightning fast.
For details see my RIE with Java and Ojbectify and RIE with Python posts.
"Like" is often uses as a poor-man's substitute for text search. For text search, it is possible to use Whoosh-AppEngine.
Simple one really. In SQL, if I want to search a text field for a couple of characters, I can do:
SELECT blah FROM blah WHERE blah LIKE '%text%'
The documentation for App Engine makes no mention of how to achieve this, but surely it's a common enough problem?
BigTable, which is the database back end for App Engine, will scale to millions of records. Due to this, App Engine will not allow you to do any query that will result in a table scan, as performance would be dreadful for a well populated table.
In other words, every query must use an index. This is why you can only do =, > and < queries. (In fact you can also do != but the API does this using a a combination of > and < queries.) This is also why the development environment monitors all the queries you do and automatically adds any missing indexes to your index.yaml file.
There is no way to index for a LIKE query so it's simply not available.
Have a watch of this Google IO session for a much better and more detailed explanation of this.
i'm facing the same problem, but i found something on google app engine pages:
Tip: Query filters do not have an explicit way to match just part of a string value, but you can fake a prefix match using inequality filters:
db.GqlQuery("SELECT * FROM MyModel WHERE prop >= :1 AND prop < :2",
"abc",
u"abc" + u"\ufffd")
This matches every MyModel entity with a string property prop that begins with the characters abc. The unicode string u"\ufffd" represents the largest possible Unicode character. When the property values are sorted in an index, the values that fall in this range are all of the values that begin with the given prefix.
http://code.google.com/appengine/docs/python/datastore/queriesandindexes.html
maybe this could do the trick ;)
Altough App Engine does not support LIKE queries, have a look at the properties ListProperty and StringListProperty. When an equality test is done on these properties, the test will actually be applied on all list members, e.g., list_property = value tests if the value appears anywhere in the list.
Sometimes this feature might be used as a workaround to the lack of LIKE queries. For instance, it makes it possible to do simple text search, as described on this post.
You need to use search service to perform full text search queries similar to SQL LIKE.
Gaelyk provides domain specific language to perform more user friendly search queries. For example following snippet will find first ten books sorted from the latest ones with title containing fern
and the genre exactly matching thriller:
def documents = search.search {
select all from books
sort desc by published, SearchApiLimits.MINIMUM_DATE_VALUE
where title =~ 'fern'
and genre = 'thriller'
limit 10
}
Like is written as Groovy's match operator =~.
It supports functions such as distance(geopoint(lat, lon), location) as well.
App engine launched a general-purpose full text search service in version 1.7.0 that supports the datastore.
Details in the announcement.
More information on how to use this: https://cloud.google.com/appengine/training/fts_intro/lesson2
Have a look at Objectify here , it is like a Datastore access API. There is a FAQ with this question specifically, here is the answer
How do I do a like query (LIKE "foo%")
You can do something like a startWith, or endWith if you reverse the order when stored and searched. You do a range query with the starting value you want, and a value just above the one you want.
String start = "foo";
... = ofy.query(MyEntity.class).filter("field >=", start).filter("field <", start + "\uFFFD");
Just follow here:
init.py#354">http://code.google.com/p/googleappengine/source/browse/trunk/python/google/appengine/ext/search/init.py#354
It works!
class Article(search.SearchableModel):
text = db.TextProperty()
...
article = Article(text=...)
article.save()
To search the full text index, use the SearchableModel.all() method to get an
instance of SearchableModel.Query, which subclasses db.Query. Use its search()
method to provide a search query, in addition to any other filters or sort
orders, e.g.:
query = article.all().search('a search query').filter(...).order(...)
I tested this with GAE Datastore low-level Java API. Me and works perfectly
Query q = new Query(Directorio.class.getSimpleName());
Filter filterNombreGreater = new FilterPredicate("nombre", FilterOperator.GREATER_THAN_OR_EQUAL, query);
Filter filterNombreLess = new FilterPredicate("nombre", FilterOperator.LESS_THAN, query+"\uFFFD");
Filter filterNombre = CompositeFilterOperator.and(filterNombreGreater, filterNombreLess);
q.setFilter(filter);
In general, even though this is an old post, a way to produce a 'LIKE' or 'ILIKE' is to gather all results from a '>=' query, then loop results in python (or Java) for elements containing what you're looking for.
Let's say you want to filter users given a q='luigi'
users = []
qry = self.user_model.query(ndb.OR(self.user_model.name >= q.lower(),self.user_model.email >= q.lower(),self.user_model.username >= q.lower()))
for _qry in qry:
if q.lower() in _qry.name.lower() or q.lower() in _qry.email.lower() or q.lower() in _qry.username.lower():
users.append(_qry)
It is not possible to do a LIKE search on datastore app engine, how ever creating an Arraylist would do the trick if you need to search a word in a string.
#Index
public ArrayList<String> searchName;
and then to search in the index using objectify.
List<Profiles> list1 = ofy().load().type(Profiles.class).filter("searchName =",search).list();
and this will give you a list with all the items that contain the world you did on the search
If the LIKE '%text%' always compares to a word or a few (think permutations) and your data changes slowly (slowly means that it's not prohibitively expensive - both price-wise and performance-wise - to create and updates indexes) then Relation Index Entity (RIE) may be the answer.
Yes, you will have to build additional datastore entity and populate it appropriately. Yes, there are some constraints that you will have to play around (one is 5000 limit on the length of list property in GAE datastore). But the resulting searches are lightning fast.
For details see my RIE with Java and Ojbectify and RIE with Python posts.
"Like" is often uses as a poor-man's substitute for text search. For text search, it is possible to use Whoosh-AppEngine.