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From Benjamin Black...@b3k.us>
Subject Re: How to perform queries on Cassandra?
Date Sun, 11 Apr 2010 07:43:16 GMT
You would have a Column Family, not a column for that; let's call it
the Users CF.  You'd use username as the row key and have a column
called 'password'.  For your example query, you'd retrieve row key
'usr2', column 'password'.  The general pattern is that you create CFs
to act as indices for each query you want to perform.  There is no
equivalent to a relational store to perform arbitrary queries.  You
must structure things to permit the queries of interest.


b

On Sat, Apr 10, 2010 at 8:34 PM, dir dir <sikerasakti@gmail.com> wrote:
> I have already read the API spesification. Honestly I do not understand
> how to use it. Because there are not an examples.
>
> For example I have a column like this:
>
> UserName    Password
> usr1                abc
> usr2                xyz
> usr3                opm
>
> suppose I want query the user's password using SQL in RDBMS
>
>       Select Password From Users Where UserName = "usr2";
>
> Now I want to get the password using OODBMS DB4o Object Query  and Java
>
>      ObjectSet QueryResult = db.query(new Predicate()
>      {
>             public boolean match(Users Myusers)
>             {
>                  return Myuser.getUserName() == "usr2";
>             }
>      });
>
> After we get the Users instance in the QueryResult, hence we can get the
> usr2's password.
>
> How we perform this query using Cassandra API and Java??
> Would you tell me please??  Thank You.
>
> Dir.
>
>
> On Sat, Apr 10, 2010 at 11:06 AM, Paul Prescod <paul@prescod.net> wrote:
>>
>> No. Cassandra has an API.
>>
>> http://wiki.apache.org/cassandra/API
>>
>> On Fri, Apr 9, 2010 at 8:00 PM, dir dir <sikerasakti@gmail.com> wrote:
>> > Does Cassandra has a default query language such as SQL in RDBMS
>> > and Object Query in OODBMS?  Thank you.
>> >
>> > Dir.
>> >
>> > On Sat, Apr 10, 2010 at 7:01 AM, malsmith
>> > <malsmith@treehousesystems.com>
>> > wrote:
>> >>
>> >>
>> >> It's sort of an interesting problem - in RDBMS one relatively simple
>> >> approach would be calculate a rectangle that is X km by Y km with User
>> >> 1's
>> >> location at the center.  So the rectangle is UserX - 10KmX ,
>> >> UserY-10KmY to
>> >> UserX+10KmX , UserY+10KmY
>> >>
>> >> Then you could query the database for all other users where that each
>> >> user
>> >> considered is curUserX > UserX-10Km and curUserX < UserX+10KmX and
>> >> curUserY
>> >> > UserY-10KmY and curUserY < UserY+10KmY
>> >> * Not the 10KmX and 10KmY are really a translation from Kilometers to
>> >> degrees of  lat and longitude  (that you can find on a google search)
>> >>
>> >> With the right indexes this query actually runs pretty well.
>> >>
>> >> Translating that to Cassandra seems a bit complex at first - but you
>> >> could
>> >> try something like pre-calculating a grid with the right resolution
>> >> (like a
>> >> square of 5KM per side) and assign every user to a particular grid ID.
>> >> That
>> >> way you just calculate with grid ID User1 is in then do a direct key
>> >> lookup
>> >> to get a list of the users in that same grid id.
>> >>
>> >> A second approach would be to have to column families -- one that maps
>> >> a
>> >> Latitude to a list of users who are at that latitude and a second that
>> >> maps
>> >> users who are at a particular longitude.  You could do the same
>> >> rectange
>> >> calculation above then do a get_slice range lookup to get a list of
>> >> users
>> >> from range of latitude and a second list from the range of longitudes.
>> >> You would then need to do a in-memory nested loop to find the list of
>> >> users
>> >> that are in both lists.  This second approach could cause some trouble
>> >> depending on where you search and how many users you really have --
>> >> some
>> >> latitudes and longitudes have many many people in them
>> >>
>> >> So, it seems some version of a chunking / grid id thing would be the
>> >> better approach.   If you let people zoom in or zoom out - you could
>> >> just
>> >> have different column families for each level of zoom.
>> >>
>> >>
>> >> I'm stuck on a stopped train so -- here is even more code:
>> >>
>> >> static Decimal GetLatitudeMiles(Decimal lat)
>> >> {
>> >> Decimal f = 0.0M;
>> >> lat = Math.Abs(lat);
>> >> f = 68.99M;
>> >>          if (lat >= 0.0M && lat < 10.0M) { f = 68.71M;
}
>> >> else if (lat >= 10.0M && lat < 20.0M) { f = 68.73M; }
>> >> else if (lat >= 20.0M && lat < 30.0M) { f = 68.79M; }
>> >> else if (lat >= 30.0M && lat < 40.0M) { f = 68.88M; }
>> >> else if (lat >= 40.0M && lat < 50.0M) { f = 68.99M; }
>> >> else if (lat >= 50.0M && lat < 60.0M) { f = 69.12M; }
>> >> else if (lat >= 60.0M && lat < 70.0M) { f = 69.23M; }
>> >> else if (lat >= 70.0M && lat < 80.0M) { f = 69.32M; }
>> >> else if (lat >= 80.0M) { f = 69.38M; }
>> >>
>> >> return f;
>> >> }
>> >>
>> >>
>> >> Decimal MilesPerDegreeLatitude = GetLatitudeMiles(zList[0].Latitude);
>> >> Decimal MilesPerDegreeLongitude = ((Decimal) Math.Abs(Math.Cos((Double)
>> >> zList[0].Latitude))) * 24900.0M / 360.0M;
>> >>                         dRadius = 10.0M  // ten
miles
>> >> Decimal deltaLat = dRadius / MilesPerDegreeLatitude;
>> >> Decimal deltaLong = dRadius / MilesPerDegreeLongitude;
>> >>
>> >> ps.TopLatitude = zList[0].Latitude - deltaLat;
>> >> ps.TopLongitude = zList[0].Longitude - deltaLong;
>> >> ps.BottomLatitude = zList[0].Latitude + deltaLat;
>> >> ps.BottomLongitude = zList[0].Longitude + deltaLong;
>> >>
>> >>
>> >>
>> >> On Fri, 2010-04-09 at 16:30 -0700, Paul Prescod wrote:
>> >>
>> >> 2010/4/9 Onur AKTAS <onur.aktas@live.com>:
>> >> > ...
>> >> > I'm trying to find out how do you perform queries with calculations
>> >> > on
>> >> > the
>> >> > fly without inserting the data as calculated from the beginning.
>> >> > Lets say we have latitude and longitude coordinates of all users and
>> >> > we
>> >> > have
>> >> >  Distance(from_lat, from_long, to_lat, to_long) function which
>> >> > gives distance between lat/longs pairs in kilometers.
>> >>
>> >> I'm not an expert, but I think that it boils down to "MapReduce" and
>> >> "Hadoop".
>> >>
>> >> I don't think that there's any top-down tutorial on those two words,
>> >> you'll have to research yourself starting here:
>> >>
>> >>  * http://en.wikipedia.org/wiki/MapReduce
>> >>
>> >>  * http://hadoop.apache.org/
>> >>
>> >>  * http://wiki.apache.org/cassandra/HadoopSupport
>> >>
>> >> I don't think it is all documented in any one place yet...
>> >>
>> >>  Paul Prescod
>> >>
>> >
>> >
>
>

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