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Revision 10 as of 2009-03-09 14:09:00
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Editor: adsl-242-246-180
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Revision 11 as of 2009-06-18 15:26:12
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Editor: YonikSeeley
Comment: first pass at cleaning up faceting overview
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Solr provides a [http://lucene.apache.org/solr/docs/api/org/apache/solr/request/SimpleFacets.html Simple Faceting toolkit] which can be reused by various Request Handlers to include "Facet counts" based on some simple criteria. Both the StandardRequestHandler and the DisMaxRequestHandler currently use these utilities. Detailed descriptions of the parameters used to control faceting can be found (along with several examples) at SimpleFacetParameters. Solr provides a [http://wiki.apache.org/solr/SimpleFacetParameters faceting component] which is part of the standard request handler and can be used by various other request handlers to include "Facet counts" based on some simple criteria.
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  * They are not tokenized into separate words
  * They are not mapped into lower case
  * Human-readable punctuation is not removed (other than double-quotes)
  * They are often not tokenized into separate words
  * They are often not mapped into lower case
  * Human-readable punctuation is often not removed (other than double-quotes)
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  * Depending on how the field is defined, the SimpleFacets mechanism may only allow for a single value per field per document (see below)
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Then when the user drills down on the "Schildt, Herbert" string I would reissue the query with an added fq=author:"Schild, Herbert" parameter.  If you wanted to drill-down or query by multiple authors you would add more 'fq' parameters as needed, e.g. fq=author:"Schield, Herbet"&fq=author:"Wolpert, Lewis". Then when the user drills down on the "Schildt, Herbert" string I would reissue the query with an added fq=author:"Schild, Herbert" parameter.
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Currently SimpleFacets has 3 modes of operation, selected by a combination of SimpleFacetParameters, Response Handler parameters and [:SchemaXml: schema.xml] Field definitions: Currently SimpleFacets has 3 modes of operation, selected by a combination of SimpleFacetParameters and [:SchemaXml: schema.xml] Field definitions:
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Any number of [:SimpleFacetParameters#facet.query:facet.query] parameters can be passed to the request handler. Each distinct facet.query will first be executed against the entire index, with the results cached as a hashed set (if fewer than hashDocSet) or a bit set (if greater) of document IDs (see [:SolrCaching#The hashDocSet Max Size:hashDocSet]). Then, every time that facet.query is used for faceting a query, the cached set will be intersected against the set of document IDs returned by the query to count the number of documents for which the facet.query condition is true. Any number of [:SimpleFacetParameters#facet.query:facet.query] parameters can be passed to the request handler. The filter for each distinct facet.query is retrieved from the filterCache (or generated if not cached yet) and intersected with the filter for the base query to obtain the count.
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    * '''Enum Based Field Queries''': If {{{facet.method=enum}}} or the field is defined in the schema as boolean, then every indexed value for the field will be iterated and a facet query will be executed and cached (as described above). This is excellent for fields where there is a small set of distinct values. For example, faceting on a field with U.S. States e.g. `Alabama, Alaska, ... Wyoming` would lead to fifty cached queries which would be used over and over again. However, it requires excessive amounts of memory and time when the number of field values is large, and especially when it exceeds the filter cache size defined in [:SolrCaching#filterCache:filterCache]
    
    * '''Field Cache''': If {{{facet.method=fc}}} then a field-cache approach will be used. This is currently implemented using either the the Lucene [http://lucene.apache.org/java/docs/api/org/apache/lucene/search/FieldCache.html FieldCache] or (starting in Solr 1.4) an !UnInvertedField if the field is multivalued or tokenized. Every time that {{{facet.field}}} is used for faceting a query, all the document IDs resulting from the query are looked up in the cache and any value found has its tally incremented. This is excellent for situations where the number of indexed values for the field is too large to be practical using the field queries mechanism, such as faceting against authors or titles. However it is currently much slower and more memory-intensive than the field query mechanism for fields with a small number of values.
    * '''Enum Based Field Queries''': If {{{facet.method=enum}}} or the field is defined in the schema as boolean, then Solr will iterate over all of the indexed terms for the field, and for each term it will get a filter from the filterCache and calculate the intersection with the filter for the base query. This is excellent for fields where there is a small set of distinct values. The average number of values per document does not matter. For example, faceting on a field with U.S. States e.g. `Alabama, Alaska, ... Wyoming` would lead to fifty cached filters which would be used over and over again. The [:SolrCaching#filterCache:filterCache] should be large enough to hold all of the cached filters.

    * '''Field Cache''': If {{{facet.method=fc}}} then a field-cache approach will be used. This is currently implemented using either the the Lucene [http://lucene.apache.org/java/docs/api/org/apache/lucene/search/FieldCache.html FieldCache] or (starting in Solr 1.4) an !UnInvertedField if the field is multivalued or tokenized. Each document is looked up in the cache to wee what terms/values it contains, and a tally is incremented for each value. This is excellent for situations where the number of indexed values for the field is high, but the number of values per document is low. For multi-valued fields, a hybrid approach is used that uses term filters from the filterCache for terms that match many documents.

Solr provides a [http://wiki.apache.org/solr/SimpleFacetParameters faceting component] which is part of the standard request handler and can be used by various other request handlers to include "Facet counts" based on some simple criteria.

This page briefly provides some general background information:

Facet Indexing

Faceting is done on indexed rather than stored values. This is because the primary use for faceting is drill-down into a subset of hits resulting from a query, and so the chosen facet value is used to construct a filter query which literally matches that value in the index. For the stock Solr request handlers this is done by adding an fq=<facet-field>:<quoted facet-value> parameter and resubmitting the query.

Because faceting fields are often specified to serve two purposes, human-readable text and drill-down query value, they are frequently indexed differently from fields used for searching and sorting:

  • They are often not tokenized into separate words
  • They are often not mapped into lower case
  • Human-readable punctuation is often not removed (other than double-quotes)
  • There is often no need to store them, since stored values would look much like indexed values and the faceting mechanism is used for value retrieval.

As an example, if I had an "author" field with a list of authors, such as:

  • Schildt, Herbert; Wolpert, Lewis; Davies, P.

I might want to index the same data differently in three different fields (perhaps using the Solr [:SchemaXml#Copy Fields:copyField] directive):

  • For searching: Tokenized, case-folded, punctuation-stripped:
    • schildt / herbert / wolpert / lewis / davies / p
  • For sorting: Untokenized, case-folded, punctuation-stripped:
    • schildt herbert wolpert lewis davies p
  • For faceting: Primary author only, using a solr.StringField:

    • Schildt, Herbert

Then when the user drills down on the "Schildt, Herbert" string I would reissue the query with an added fq=author:"Schild, Herbert" parameter.

Facet Operation

Currently SimpleFacets has 3 modes of operation, selected by a combination of SimpleFacetParameters and [:SchemaXml: schema.xml] Field definitions:

FacetQueries

Any number of [:SimpleFacetParameters#facet.query:facet.query] parameters can be passed to the request handler. The filter for each distinct facet.query is retrieved from the filterCache (or generated if not cached yet) and intersected with the filter for the base query to obtain the count.

FacetFields

Any number of [:SimpleFacetParameters#facet.field:facet.field] parameters can be passed to the request handler. For each facet.field, one of two approaches will be used based on the [:SimpleFacetParameters#facet.method:facet.method] or the field type:

  • Enum Based Field Queries: If facet.method=enum or the field is defined in the schema as boolean, then Solr will iterate over all of the indexed terms for the field, and for each term it will get a filter from the filterCache and calculate the intersection with the filter for the base query. This is excellent for fields where there is a small set of distinct values. The average number of values per document does not matter. For example, faceting on a field with U.S. States e.g. Alabama, Alaska, ... Wyoming would lead to fifty cached filters which would be used over and over again. The [:SolrCaching#filterCache:filterCache] should be large enough to hold all of the cached filters.

  • Field Cache: If facet.method=fc then a field-cache approach will be used. This is currently implemented using either the the Lucene [http://lucene.apache.org/java/docs/api/org/apache/lucene/search/FieldCache.html FieldCache] or (starting in Solr 1.4) an UnInvertedField if the field is multivalued or tokenized. Each document is looked up in the cache to wee what terms/values it contains, and a tally is incremented for each value. This is excellent for situations where the number of indexed values for the field is high, but the number of values per document is low. For multi-valued fields, a hybrid approach is used that uses term filters from the filterCache for terms that match many documents.

SolrFacetingOverview (last edited 2014-03-06 14:39:38 by 84)