# nativeRank reference

The nativeRank feature gives a reasonably good rank score which is computed at an acceptable performance, and is a good candidate for the first phase rank score. The nativeRank feature is a linear combination of the normalized scores computed by the features nativeFieldMatch, nativeProximity, and nativeAttributeMatch. All these features are described in detail below. See the configuration properties section for how to configure the features.

## nativeFieldMatch

The nativeFieldMatch feature captures how well query terms match searched index fields by looking at the number of times a term occurs in a field and how early in the field it occurs. The significance and weight of the terms are also taken into account such that unusual terms give a higher rank contribution than common ones.

The score for nativeFieldMatch is calculated as follows:

where n is the number of query terms searched in index fields, m is the number of fields searched by query term i, firstOccImpj is the firstOccurrenceImportance for field j, and firstOccBoostij, numOccBoostij and fmMaxTablej are given below. where firstOccurrenceTablej is the boost table configured for field j, typically an expdecay function (see the boost tables section below), firstOccij is the first occurrence of query term i in field j, and tableSizej is the size of the boost table. where occurrenceCountTablej is the boost table configured for field j, typically a loggrowth function (see the boost tables section below), numOccsij is the number of occurrences of query term i in field j, and tableSizej is the size of the boost table. where max(boostTablej) is the max value in that table. fmMaxTablej is 1 if table normalization is turned off (see the property nativeRank.useTableNormalization in the configuration properties section).

The default behavior for nativeFieldMatch is to consider all query terms searching in all index fields when calculating the score. The calculation can be limited to a specified set of index fields as follows:

In this case only query terms searching in index fields f1 and f2 are considered.

## nativeProximity

The nativeProximity feature captures how near the matched query terms occur in searched index fields by looking at the word distance between query terms in query term pairs. Two query terms that are close to each other should give a higher score than two terms that are far from each other.

The score for nativeProximity is calculated as follows:

where m is the number of index fields, ab is a term pair searched for in field j, proxImpj is the proximityImportance for field j, proxTablej is the forward proximity boost table for field j, distab is the minimum distance between occurrences of query terms a and b in field j, (a occurs before b), revProxTablej is the reverse proximity boost table for field j, distba is the minimum distance between occurrences of query terms b and a in field j (b occurs before a), and termPairWeightab and pMaxTablej are given below. For each field j we consider all query terms searched in this field and generate a set of term pairs. The slidingWindowSize parameter determines how many pairs that are generated. With a sliding window of size 3 over the terms a b c d we first consider the terms a b c, then the terms b c d, and finally the terms c d. The following pairs are generated: ab, ac, bc, bd, and cd. where distac is the distance between term a and c in the query. where max(boostTablej) is the max value in that table. pMaxTablej is 1 if table normalization is turned off (see the property nativeRank.useTableNormalization in the configuration properties section).

The default behavior for nativeProximity is to consider all index fields and all query terms pairs searching in these fields when calculating the score. The calculation can be limited to a specified set of index fields as follows:

In this case only query term pairs searching in index fields f1 and f2 are considered.

## nativeAttributeMatch

The nativeAttributeMatch feature captures how well query terms match searched attribute fields, and is calculated as follows:

where n is the number of query terms searched in attribute fields, weightTablej is the boost table for attribute j, max(weightTablej) is the max value in that table (1 if table normalization is turned off), sign(wij) is the sign of wij. wij is dependent on the attribute type:
• Weighted set: equals the weight associated with the key (represented by query term i) in attribute j.
• Array: equals the number of occurrences of query term i in attribute j.
• Single: equals 1.
The default behavior for nativeAttributeMatch is to consider all query terms searching in all attribute fields when calculating the score. The calculation can be limited to a specified set of attribute fields as follows: In this case only query terms searching in attribute fields a1 and a2 are considered.

## nativeRank

The nativeRank feature is just a linear combination of the three other features, and is calculated as follows:

where fmw is the fieldMatchWeight, pw is the proximityWeight, and amw is the attributeMatchWeight.

The default behavior when calculating the native rank score is to consider all query terms searching in all defined index fields and attribute fields. In many cases though only a subset of these fields are of interest for the rank score calculation. You can setup nativeRank for a subset of fields by specifying the field names in the parameter list as follows:

first-phase {
expression: nativeRank(title,body,tags)
}

In this case we have two index fields (title and body) and one attribute field (tags), and the nativeRank feature is calculated based on the features nativeFieldMatch(title,body), nativeProximity(title,body), and nativeAttributeMatch(tags). Note that the CPU cost of calculating the native rank score is also reduced when specifying a subset of the fields.

## Variables

This is a list of the common variables used in the formulas above:

VariableDescription
attributeWeightj The weight of attribute field j. See the search definitions reference for how to set this weight. The default value is 100.
connectednessab The connectedness between query terms a and b.
fieldLengthj The length of field j in number of words.
fieldWeightj The weight of index field j. See the search definitions reference for how to set this weight. The default value is 100.
termSignificancei The significance of query term i.
termWeighti The weight of query term i.

## Configuration properties

This is a comprehensive list of all the configuration properties to all native rank features:

FeatureParameterDefaultDescription
nativeFieldMatch firstOccurrenceTable expdecay(8000,12.50) The default table used when calculating boost for the first occurrence in a field.
nativeFieldMatch firstOccurrenceTable.fieldName The value of firstOccurrenceTable The table used when calculating boost for the first occurrence in the given field.
nativeFieldMatch occurrenceCountTable loggrowth(1500,4000,19) The default table used when calculating boost for the number of occurrences in a field.
nativeFieldMatch occurrenceCountTable.fieldName The value of occurrenceCountTable The table used when calculating boost for the number of occurrences in the given field.
nativeFieldMatch firstOccurrenceImportance 0.5 The default importance value used for weighting the boosts for first occurrence and number of occurrences in a field. This value should be in the interval [0, 1].
nativeFieldMatch firstOccurrenceImportance.fieldName The value of firstOccurrenceImportance The importance value used for the given field.
nativeProximity proximityTable expdecay(500,3) The default table used when calculating forward proximity boost in a field.
nativeProximity proximityTable.fieldName The value of proximityTable The table used when calculating forward proximity boost in the given field.
nativeProximity reverseProximityTable expdecay(400,3) The default table used when calculating reverse proximity boost in a field.
nativeProximity reverseProximityTable.fieldName The value of reverseProximityTable The table used when calculating reverse proximity boost in the given field.
nativeProximity proximityImportance 0.5 The default importance value used for weighting the boosts for forward and reverse proximity in a field. This value should be in the interval [0, 1].
nativeProximity proximityImportance.fieldName The value of proximityImportance The importance value used for the given field.
nativeProximity slidingWindowSize 4 The size of the sliding window used when generating term pairs.
nativeAttributeMatch weightTable linear(1,0) The default table used when calculating boost for matching in an attribute field.
nativeAttributeMatch weightTable.attributeName The value of weightTable The table used when calculating boost for matching in the given attribute.
nativeRank fieldMatchWeight 100.0 How much to weight the score from nativeFieldMatch.
nativeRank proximityWeight 25.0 How much to weight the score from nativeProximity. If table normalization is turned off the default value is 100.0.
nativeRank attributeMatchWeight 100.0 How much to weight the score from nativeAttributeMatch.
nativeRank useTableNormalization true Whether we should use table normalization for the native rank features. Set this property to false to turn off table normalization
For example, to override the occurrenceCountTable and reverseProximityTable for the index field content add the following to the rank profile in the sd file:
rank-properties {
nativeFieldMatch.occurrenceCountTable.content: "linear(0,0)"
nativeProximity.reverseProximityTable.content: "linear(0,0)"
}

See the search definitions reference for more information on rank-properties.

### Boost tables

The following boost tables are supported by the native rank features:

NameFunctionDescription
expdecay(w,t) w * exp(-x/t) Represents an exponential decay function where w is the weight controlling the amplitude and t is the tune parameter controlling the slope.
loggrowth(w,t,s) w * log(1 + (x/s)) + t Represents a logarithmic growth function where w is the weight controlling the amplitude, t is the tune parameter controlling the offset, and s is a scale parameter controlling the sensitivity to the variable x
linear(w,t) w * x + t Represents a linear function where w controls the slope and t controls the offset.
The parameters w, t, and s are floating point numbers, the same as the content of the tables. The default table size is 256 with x in the interval [0,255]. You can override this default size by specifying an optional last parameter to the table name. For instance if you use linear(1.5,0,512) you get a table with size 512 populated with the result of evaluating the function 1.5*x + 0 for all x in the interval [0,511].

### Rank types

Four predefined rank types are supported by nativeRank: about (default), identity, tags, and empty. Each type is associated with a set of boost tables that are used by the native rank features. See the rank type document for detailed information on these type.

When setting up the sd file, either use one of the predefined rank types for a field, or explicitly specify the boost tables to use for that field as a set of rank-properties. If you don't specify anything you get the boost tables associated with the about type. The about boost tables for nativeFieldMatch and nativeProximity are already optimized for textual match, while the boost table for nativeAttributeMatch is data dependent and must be optimized for each use case.

## nativeRank limitations

The nativeRank feature is a pure text match scoring feature. In particular, it does not take the following concepts into account for documents that match a query:

• Static rank or any other relevancy contribution that is based on a numeric value. Use the attribute feature in a ranking expression to get this concept into the final relevancy score.
• Geographical location of a match correlated to a location associated with the query. Use the distance or closeness feature in a ranking expression to take this into account.
• The age of the matching documents. Use the freshness feature in a ranking expression to take this into account.