Re: [geomesa-users] Is it possible to query for objects that were in pos

[Emilio Lahr-Vivaz <elahrvivaz@xxxxxxxx>]

Hello,

You may be able to do better than issuing two queries, but GeoMesa's support for that type of query is not entirely robust right now. For the future, it may be fully answerable using the idea of a trajectory API, which is something that the GeoMesa team has been interested in for a while, but hasn't yet had the time/funding to pursue.

For now, you may be able to get closer by storing each track in a single feature. GeoMesa supports List-type attributes, so you could store your positions as a single LineString, and then have a corresponding List[Date] which stores the time for each point. Then you could query for features that were in both positions and both times - you would still have to do some manual comparisons to ensure that the times and positions align, but the first query should substantially narrow down the results. You could probably only take advantage of either a spatial or temporal index, but not both as GeoMesa doesn't currently have an index on time durations (although our existing XZ3 index could support that, it hasn't been implemented). In addition, you may hit some issues when you query on multiple dates, as there are some places that we check for 'disjoint' queries, which may not account for having multiple dates (e.g. generally dtg == today AND dtg == yesterday would never return a result, however in the case of a List[Date] attribute it could) - I am not sure if that is handled correctly, you would likely have to try it out to see. If not, then you could potentially query for both positions and one date, then do the second date and alignments checks post-query.

One note, if you are writing frequent updates to a single track, you may hit throughput issues writing to the database. The GeoMesa Lambda data store is designed to solve this problem[1].

Thanks,

Emilio

[1] https://www.geomesa.org/documentation/user/lambda/index.html

On 4/28/19 7:54 AM, Yoav Zibin wrote:

Hi,
I'm looking for a SpatialTemporal DB that can track moving objects and answer queries of the form: find all objects that were in position X at time X' and also were in position Y at time Y'.
Is it possible to write such a query?

The closest query I could find is:

BBOX(geom,0,0,10,10) AND dtg DURING 2017-01-01T00:00:00.000Z/2017-01-02T00:00:00.000Z 

I could issue two such queries, and do the intersection of the two results, but I was hoping for a better way (because each query could return millions of results, but the intersection is much much smaller).

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