Class: Ferret::Search::Sort

Inherits:
Object
  • Object
show all
Defined in:
lib/ferret/search/sort.rb

Overview

Encapsulates sort criteria for returned hits.

The fields used to determine sort order must be carefully chosen. Documents must contain a single term in such a field, and the value of the term should indicate the document’s relative position in a given sort order. The field must be indexed, but should not be tokenized, and does not need to be stored (unless you happen to want it back with the rest of your document data). In other words:

document << Field.new("by_number",
                      x.to_s,
                      Field::Store::NO,
                      Field::Index::UN_TOKENIZED))

Valid Types of Values

There are three possible kinds of term values which may be put into sorting fields: Integers, Floats, or Strings. Unless SortField objects are specified, the type of value in the field is determined by parsing the first term in the field.

Integer term values should contain only digits and an optional preceeding negative sign. Values must be base 10. Documents which should appear first in the sort should have low value integers, later documents high values (i.e. the documents should be numbered 1..n where 1 is the first and n the last).

Float term values should conform to values accepted by String#to_f. Documents which should appear first in the sort should have low values, later documents high values.

String term values can contain any valid String, but should not be tokenized. The values are sorted according to their Comparable natural order. Note that using this type of term value has higher memory requirements than the other two types.

Object Reuse

One of these objects can be used multiple times and the sort order changed between usages.

This class is thread safe.

Memory Usage

Sorting uses caches of term values maintained by the internal HitQueue(s). The cache is static and contains an integer or float array of length IndexReader#max_doc for each field name for which a sort is performed. In other words, the size of the cache in bytes is:

4 * IndexReader#max_doc * (# of different fields actually used to sort)

For String fields, the cache is larger: in addition to the above array, the value of every term in the field is kept in memory. If there are many unique terms in the field, this could be quite large.

Note that the size of the cache is not affected by how many fields are in the index and might be used to sort - only by the ones actually used to sort a result set.

The cache is cleared each time a new IndexReader is passed in, or if the value returned by max_doc() changes for the current IndexReader. This class is not set up to be able to efficiently sort hits from more than one index simultaneously.

Constant Summary collapse

RELEVANCE =

Represents sorting by computed relevance. Using this sort criteria returns the same results as calling Searcher#search(Query) Searcher#search() without a sort criteria, only with slightly more overhead.

Sort.new()
INDEX_ORDER =

Represents sorting by index order.

Sort.new(SortField::FIELD_DOC)

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(fields = [SortField::FIELD_SCORE, SortField::FIELD_DOC], reverse = false) ⇒ Sort

Sorts by computed relevance. You can pass a string representing the name of the field you want to sort on, a SortField, or an array of either (but not a mixed array). If you pass a string or and array of strings you can also pass a reverse flag. If you pass a SortField the reverse is handled by it.

fields

The fields you want to sort on. See also SortField

reverse

pass true if you want the sort order to be reversed. Only

works if you pass the field names.



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# File 'lib/ferret/search/sort.rb', line 80

def initialize(fields = [SortField::FIELD_SCORE, SortField::FIELD_DOC],
               reverse = false)
  fields = [fields] unless fields.is_a?(Array)
  @fields = fields
  if fields[0].is_a?(String)
    @fields = fields.map do |field|
      SortField.new(field, {:sort_type => SortField::SortType::AUTO,
                            :reverse => reverse})
    end
    @fields << SortField::FIELD_DOC if @fields.size == 1
  end
end

Instance Attribute Details

#fields ⇒ Object

Returns the value of attribute fields.



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# File 'lib/ferret/search/sort.rb', line 69

def fields
  @fields
end

Instance Method Details

#to_s ⇒ Object 



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# File 'lib/ferret/search/sort.rb', line 101

def to_s() 
  return @fields.map {|field| "#{field}"}.join(", ")
end