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From "David Smiley (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-2155) Geospatial search using geohash prefixes
Date Fri, 01 Mar 2013 16:53:14 GMT

    [ https://issues.apache.org/jira/browse/SOLR-2155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13590706#comment-13590706
] 

David Smiley commented on SOLR-2155:
------------------------------------

bq. I was just thinking that in order to do the ranking, the filter has to go through all
the values of the field and it shouldn't be hard to persist this information and return to
the client

It doesn't; this is a common misconception!  It does *not* calculate the distance between
every indexed matched point and the center point of a circle query shape.  If it did, it wouldn't
be so fast :-)  If you want an implementation like that then look at LatLonType which is a
brute force algorithm and hence is not as scalable, and doesn't support multi-value either.
 To try and help explain how this can possibly be, understand that there are large grid cells
that fit within the query shape and for these large grid cells, the index knows which documents
are all in that grid cell and so it simply matches those documents without knowing or calculating
more precisely where those underlying points actually are.  So it's not like the filter code
has all this information you want and simply isn't exposing it to you.  I need to drive this
point home at my next conference presentation at Lucene/Solr Revolution 2013 in May (San Diego
CA).
                
> Geospatial search using geohash prefixes
> ----------------------------------------
>
>                 Key: SOLR-2155
>                 URL: https://issues.apache.org/jira/browse/SOLR-2155
>             Project: Solr
>          Issue Type: Improvement
>            Reporter: David Smiley
>            Assignee: David Smiley
>         Attachments: GeoHashPrefixFilter.patch, GeoHashPrefixFilter.patch, GeoHashPrefixFilter.patch,
Solr2155-1.0.2-project.zip, Solr2155-1.0.3-project.zip, Solr2155-1.0.4-project.zip, Solr2155-for-1.0.2-3.x-port.patch,
SOLR-2155_GeoHashPrefixFilter_with_sorting_no_poly.patch, SOLR.2155.p3.patch, SOLR.2155.p3tests.patch
>
>
> {panel:title=NOTICE} The status of this issue is a plugin for Solr 3.x located here:
https://github.com/dsmiley/SOLR-2155.  Look at the introductory readme and download the plugin
.jar file.  Lucene 4's new spatial module is largely based on this code.  The Solr 4 glue
for it should come very soon but as of this writing it's hosted temporarily at https://github.com/spatial4j.
 For more information on using SOLR-2155 with Solr 3, see http://wiki.apache.org/solr/SpatialSearch#SOLR-2155
 This JIRA issue is closed because it won't be committed in its current form.
> {panel}
> There currently isn't a solution in Solr for doing geospatial filtering on documents
that have a variable number of points.  This scenario occurs when there is location extraction
(i.e. via a "gazateer") occurring on free text.  None, one, or many geospatial locations might
be extracted from any given document and users want to limit their search results to those
occurring in a user-specified area.
> I've implemented this by furthering the GeoHash based work in Lucene/Solr with a geohash
prefix based filter.  A geohash refers to a lat-lon box on the earth.  Each successive character
added further subdivides the box into a 4x8 (or 8x4 depending on the even/odd length of the
geohash) grid.  The first step in this scheme is figuring out which geohash grid squares cover
the user's search query.  I've added various extra methods to GeoHashUtils (and added tests)
to assist in this purpose.  The next step is an actual Lucene Filter, GeoHashPrefixFilter,
that uses these geohash prefixes in TermsEnum.seek() to skip to relevant grid squares in the
index.  Once a matching geohash grid is found, the points therein are compared against the
user's query to see if it matches.  I created an abstraction GeoShape extended by subclasses
named PointDistance... and CartesianBox.... to support different queried shapes so that the
filter need not care about these details.
> This work was presented at LuceneRevolution in Boston on October 8th.

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