accumulo-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Josh Elser <>
Subject Re: Geospatial + Partitioned Index
Date Fri, 16 Jan 2015 22:40:01 GMT
Thanks for the clarification, Russ, I assumed something of the sort was 
the case.

It's important to remember that there is still benefit to "partition 
elimination". Doing the entire table, while it won't read all the data 
on the backend, you'll likely incur extra RPC to servers, file opens, 
iterator creation, etc. If your query is only going to match a few 
records, this can turn out to be a significant portion of your execution 
time. Something to keep in mind :)

Russ Weeks wrote:
> Hi, Josh,
> Thanks for your response. I think I should clarify something. When I
> said, "the client would just scan (-inf, +inf)", I didn't mean that the
> net effect would be to read all data. I just meant that my custom
> Iterator would seek() to ranges which are a function of its
> configuration and its knowledge of the partitioning scheme, just like
> the IntersectingIterator. Except that instead of its configuration
> defining a set of keyword terms, it would define a set of disjoint
> intervals on a space-filling curve.
> My understanding is that setting the scan range to (-inf,+inf) in this
> case is just a way to tell Accumulo, "run this scan across all tablets".
> -Russ
> On Fri, Jan 16, 2015 at 12:17 PM, Josh Elser <
> <>> wrote:
>     Russ Weeks wrote:
>         Hey, all,
>         I'm looking at switching my geospatial index to a partitioned
>         index to
>         smooth out some hotspots. So for any query, I'll have a bunch of
>         ranges
>         representing intervals on a Hilbert curve, plus a bunch of
>         partitions,
>         each of which needs to be scanned for every range.
>         The way that the (excellent!) Accumulo Recipes geospatial store
>         addresses this is to take the product of the partitions and the
>         curve
>         intervals[1]. It seems like an alternative would be to encode
>         the curve
>         intervals as a property of a custom iterator (I need one anyways to
>         filter out extraneous points from the search area) and then the
>         client
>         would just scan (-inf, +inf), which I think is more typical when
>         querying a partitioned index?
>     I'm no expert on storing geo-spatial data, but having to scan
>     (-inf,+inf) on a table for a query is typically the reason people
>     deal with the pain of hot-spotting, although it is the easiest to
>     implement.
>     If you can be "tricky" in how you're encoding your data in the row
>     such that you can reduce the search space over your partitioned
>     index, you can try to get the best of both worlds (avoid reading all
>     data and still get a good distribution).
>     Since that was extremely vague, here's an example: say you had a
>     text index and wanted to look up the word "the" and your index had
>     100 partitions, [0,99]. If you knew that it was only possible for
>     "the" to show up on partitions 5, 27 and 83 (typically by use of
>     some hashing function), you could drastically reduce your search
>     space while still avoiding hot spotting on a single server.
>         Can anybody comment on which approach is preferred? Is it common to
>         expose the number of partitions in the index and the encoding of
>         those
>         partitions to client code? Am I needlessly worried that taking the
>         product of the curve intervals and the partitions will produce
>         too many
>         ranges?
>     In the trivial sense, the client doesn't need to know the partitions
>     and would just scan the entire index like you said earlier. You
>     could also track the partitions that you have created in a separate
>     table and the client could read that table to know ahead of time (if
>     you have a reason to do so in your implementation).
>     Depending on the amount of data you have, lots of ranges to check
>     could take some time. YMMV
>         Thanks,
>         -Russ
>         1:
>         <>

View raw message