SQLite full-text search relevance ranking - search

Question

I am using the fts4 extension of sqlite3 to enable full-text indexing and searching of text data. This it working great, but I've noticed that the results are not relevance-ranked at all. I guess I am too used to Lucene. I've seen some brief suggestions to write a custom rank method using the matchinfo() results, but it's not clear to me how this is done, or whether there are any sophisticated examples out there. How have others dealt with this?

Answer 1

There's a complete example in the documentation, look at the end of appendix a. You'll need to do slightly more work to get a good relevance ranking as the function provided is good only for getting started. For example, with matchinfo(table,'pcnalx') there's enough information to implement Okapi BM25.

Answer 2

There seems to be a distinct lack of documentation on how to implement Okapi BM25 in C and it seems it is an unspoken thing that the implementation is left as an exercise for the user.
Well I found the bro of a programmer "Radford 'rads' Smith" who chucked this up on GitHub
https://github.com/rads/sqlite-okapi-bm25
It only implements BM25 although I'm looking into BM25F tweaks now....
....and here it is.
https://github.com/neozenith/sqlite-okapi-bm25

Answer 3

For FTS5, according to SQLite FTS5 Extension,
FTS5 has no matchinfo().
FTS5 supports ORDER BY rank
So very simply, something like
SELECT * FROM email WHERE email MATCH 'fts5' ORDER BY rank;
without DESC works.

Answer 4

Here is an implementation of Okapi BM25. Using this in combination with the suggestions at SQLite.org will help you generate a relevance-ranked MATCH query. This was written all in VB.Net and the query was called using System.Data.SQLite functions. The custom SQLiteFunction at the end can be called from the SQL code without issue, as long as the SQL code is called using System.Data.SQLite functions.
Public Class MatchInfo
Property matchablePhrases As Integer
Property userDefinedColumns As Integer
Property totalDocuments As Integer
Private _int32HitData As List(Of Integer)
Private _longestSubsequencePhraseMatches As New List(Of Integer)
Private _tokensInDocument As New List(Of Integer)
Private _averageTokensInDocument As New List(Of Integer)
Private _max_hits_this_row As Integer?
Public ReadOnly Property max_hits_this_row As Integer
Get
If _max_hits_this_row Is Nothing Then
_max_hits_this_row = 0
For p = 0 To matchablePhrases - 1
For c = 0 To userDefinedColumns - 1
Dim myHitsThisRow As Integer = hits_this_row(p, c)
If myHitsThisRow > _max_hits_this_row Then
_max_hits_this_row = myHitsThisRow
End If
Next
Next
End If
Return _max_hits_this_row
End Get
End Property
Private _max_hits_all_rows As Integer?
Public ReadOnly Property max_hits_all_rows As Integer
Get
If _max_hits_all_rows Is Nothing Then
_max_hits_all_rows = 0
For p = 0 To matchablePhrases - 1
For c = 0 To userDefinedColumns - 1
Dim myHitsAllRows As Integer = hits_all_rows(p, c)
If myHitsAllRows > _max_hits_all_rows Then
_max_hits_all_rows = myHitsAllRows
End If
Next
Next
End If
Return _max_hits_all_rows
End Get
End Property
Private _max_docs_with_hits As Integer?
Public ReadOnly Property max_docs_with_hits As Integer
Get
If _max_docs_with_hits Is Nothing Then
_max_docs_with_hits = 0
For p = 0 To matchablePhrases - 1
For c = 0 To userDefinedColumns - 1
Dim myDocsWithHits As Integer = docs_with_hits(p, c)
If myDocsWithHits > _max_docs_with_hits Then
_max_docs_with_hits = myDocsWithHits
End If
Next
Next
End If
Return _max_docs_with_hits
End Get
End Property
Private _BM25Rank As Double?
Public ReadOnly Property BM25Rank As Double
Get
If _BM25Rank Is Nothing Then
_BM25Rank = 0
'calculate BM25 Rank
'http://en.wikipedia.org/wiki/Okapi_BM25
'k1, calibrates the document term frequency scaling. Having k1 as 0 corresponds to a binary model – no term frequency. Increasing k1 will give rare words more boost.
'b, calibrates the scaling by document length, and can take values from 0 to 1, where having 0 means no length normalization and having 1 corresponds to fully scaling the term weight by the document length.
Dim k1 As Double = 1.2
Dim b As Double = 0.75
For column = 0 To userDefinedColumns - 1
For phrase = 0 To matchablePhrases - 1
Dim IDF As Double = Math.Log((totalDocuments - hits_all_rows(phrase, column) + 0.5) / (hits_all_rows(phrase, column) + 0.5))
Dim score As Double = (IDF * ((hits_this_row(phrase, column) * (k1 + 1)) / (hits_this_row(phrase, column) + k1 * (1 - b + b * _tokensInDocument(column) / _averageTokensInDocument(column)))))
If score < 0 Then
score = 0
End If
_BM25Rank += score
Next
Next
End If
Return _BM25Rank
End Get
End Property
Public Sub New(raw_pcnalsx_MatchInfo As Byte())
Dim int32_pcsx_MatchInfo As New List(Of Integer)
For i = 0 To raw_pcnalsx_MatchInfo.Length - 1 Step 4
int32_pcsx_MatchInfo.Add(BitConverter.ToUInt32(raw_pcnalsx_MatchInfo, i))
Next
'take the raw data and parse it out
Me.matchablePhrases = int32_pcsx_MatchInfo(0)
int32_pcsx_MatchInfo.RemoveAt(0)
Me.userDefinedColumns = int32_pcsx_MatchInfo(0)
int32_pcsx_MatchInfo.RemoveAt(0)
Me.totalDocuments = int32_pcsx_MatchInfo(0)
int32_pcsx_MatchInfo.RemoveAt(0)
'remember that the columns are 0-based
For i = 0 To userDefinedColumns - 1
_averageTokensInDocument.Add(int32_pcsx_MatchInfo(0))
int32_pcsx_MatchInfo.RemoveAt(0)
Next
For i = 0 To userDefinedColumns - 1
_tokensInDocument.Add(int32_pcsx_MatchInfo(0))
int32_pcsx_MatchInfo.RemoveAt(0)
Next
For i = 0 To userDefinedColumns - 1
_longestSubsequencePhraseMatches.Add(int32_pcsx_MatchInfo(0))
int32_pcsx_MatchInfo.RemoveAt(0)
Next
_int32HitData = New List(Of Integer)(int32_pcsx_MatchInfo)
End Sub
Public Function hits_this_row(phrase As Integer, column As Integer) As Integer
Return _int32HitData(3 * (column + phrase * userDefinedColumns) + 0)
End Function
Public Function hits_all_rows(phrase As Integer, column As Integer) As Integer
Return _int32HitData(3 * (column + phrase * userDefinedColumns) + 1)
End Function
Public Function docs_with_hits(phrase As Integer, column As Integer) As Integer
Return _int32HitData(3 * (column + phrase * userDefinedColumns) + 2)
End Function
End Class
<SQLiteFunction("Rank", 1, FunctionType.Scalar)>
Public Class Rank
Inherits SQLiteFunction
Public Overrides Function Invoke(args() As Object) As Object
Return New MatchInfo(args(0)).BM25Rank
End Function
End Class