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From "Bill Bell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (SOLR-2155) Geospatial search using geohash prefixes
Date Mon, 14 Feb 2011 10:09:57 GMT

    [ https://issues.apache.org/jira/browse/SOLR-2155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12994256#comment-12994256
] 

Bill Bell commented on SOLR-2155:
---------------------------------

I did more research. You cannot get from doc to multiple values in the cache for a field.
It does not exist for what I can see. The "docToTermOrd" property (type Direct8) is an array
that is indexed by the document ID, and has one value (the term ord). It does not appear to
be easy to get a list since there is one value. This was created to easily count the number
of documents for facets (does it have 1 or more). I could do something like the following
(but it would be really slow).

Document doc = searcher.doc(id, fields);

It would be better if you copied each lat long into the index with a prefix added to the sfield.
Like "store_1", "store_2", "store_3", when you index the values. Then I can grab them easily.
Of course you could also just sore them in one field like that I did but name it store_1 :
"lat,lon|lat,lon". If we did this during indexing it would make it easier for people to use
(not having to copy it) with bars. Asking for 2,3,4 term lists by document ID is probably
slower than just doing the "|" separation. 

I keep going back to my patch, and I think it is still pretty good. I hope others have not
went down this same path, since it was not fun.

Improvements potential:

1. Auto populate sfieldmulti when indexing geohash field into "|"
2. Multi-thread the brute force looking for lat longs
3. Use DistanceUtils for hsin
4. Remove split() to improve performance

Bill



> 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.p2.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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