[Solr Wiki] Update of "SolrCaching" by ChrisHarris

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The following page has been changed by ChrisHarris:
http://wiki.apache.org/solr/SolrCaching

The comment on the change is:
A few more notes about what the filter cache is for

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  == filterCache ==
  
- This cache stores '''unordered''' sets of document IDs.
+ This cache stores '''unordered''' sets of document IDs. This cache can be for three different purposes:
+ 
+ First, the filter cache stores the results of any filter queries ("fq" parameters) that Solr is explicitly asked to execute. (Each filter is executed and cached separately. When it's time to use them to limit the number of results returned by a query, this is done using set intersections.)
+ 
+ Second, the filter cache is used for faceting in some cases. (In particular, when the TermEnum method is used, a filterCache entry is added for each term tested.)
+ 
+ Finally, the filter cache may be used for sorting if the <useFilterForSortedQuery/> config option is set to true in solfconfig.xml.
  
  If you use faceting with the fieldCache method (see SolrFacetingOverview), it is recommended that you set the filterCache size to be greater than the number of unique values in all of your faceted fields.
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