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From "David Smiley (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-2155) Geospatial search using geohash prefixes
Date Thu, 12 Jan 2012 20:46:40 GMT

    [ https://issues.apache.org/jira/browse/SOLR-2155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13185224#comment-13185224
] 

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

Srikanth,
# According to the table on wikipedia, a geohash length of 8 translates to +- 19 meters error.
 This means that if you do a filter query, it may match points that are <=19 meters outside
of the query shape erroneously, or it may exclude points <= 19 meters within the query
shape erroneously.  The accuracy is associated to the indexing of the data, not the query
shape.
# At query time, if you only do geospatial filters, then there is *no* RAM requirement for
this field.  *If* you sort by distance or influence relevancy by distance, then the indexed
points are all brought into memory "inverted" so that the center of the query shape can be
compared against all indexed points for documents matching the query.  LatLonType does the
same thing for the same reason, but this code is multiValue aware, returning the closest indexed
point a document has to the query shape.  When a commit happens on Solr, this in-memory cache
is unused and garbage collected, and the data is brought into memory again.
                
> 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
>         Attachments: GeoHashPrefixFilter.patch, GeoHashPrefixFilter.patch, GeoHashPrefixFilter.patch,
SOLR-2155_GeoHashPrefixFilter_with_sorting_no_poly.patch, SOLR.2155.p3.patch, SOLR.2155.p3tests.patch,
Solr2155-1.0.2-project.zip, Solr2155-1.0.3-project.zip, Solr2155-for-1.0.2-3.x-port.patch
>
>
> 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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