Lucy::Docs::Tutorial::Analysis - How to choose and use Analyzers.
Try swapping out the PolyAnalyzer in our Schema for a RegexTokenizer:
my $tokenizer = Lucy::Analysis::RegexTokenizer->new; my $type = Lucy::Plan::FullTextType->new( analyzer => $tokenizer, );
Senator before and after making the change and re-indexing.
Under PolyAnalyzer, the results are identical for all three searches, but under RegexTokenizer, searches are case-sensitive, and the result sets for
Senator are distinct.
What's happening is that PolyAnalyzer is performing more aggressive processing than RegexTokenizer. In addition to tokenizing, it's also converting all text to lower case so that searches are case-insensitive, and using a "stemming" algorithm to reduce related words to a common stem (
senat, in this case).
PolyAnalyzer is actually multiple Analyzers wrapped up in a single package. In this case, it's three-in-one, since specifying a PolyAnalyzer with
language => 'en' is equivalent to this snippet:
my $case_folder = Lucy::Analysis::CaseFolder->new; my $tokenizer = Lucy::Analysis::RegexTokenizer->new; my $stemmer = Lucy::Analysis::SnowballStemmer->new( language => 'en' ); my $polyanalyzer = Lucy::Analysis::PolyAnalyzer->new( analyzers => [ $case_folder, $tokenizer, $stemmer ], );
You can add or subtract Analyzers from there if you like. Try adding a fourth Analyzer, a SnowballStopFilter for suppressing "stopwords" like "the", "if", and "maybe".
my $stopfilter = Lucy::Analysis::SnowballStopFilter->new( language => 'en', ); my $polyanalyzer = Lucy::Analysis::PolyAnalyzer->new( analyzers => [ $case_folder, $tokenizer, $stopfilter, $stemmer ], );
Also, try removing the SnowballStemmer.
my $polyanalyzer = Lucy::Analysis::PolyAnalyzer->new( analyzers => [ $case_folder, $tokenizer ], );
The original choice of a stock English PolyAnalyzer probably still yields the best results for this document collection, but you get the idea: sometimes you want a different Analyzer.
Sometimes you don't want an Analyzer at all. That was true for our "url" field because we didn't need it to be searchable, but it's also true for certain types of searchable fields. For instance, "category" fields are often set up to match exactly or not at all, as are fields like "last_name" (because you may not want to conflate results for "Humphrey" and "Humphries").
To specify that there should be no analysis performed at all, use StringType:
my $type = Lucy::Plan::StringType->new; $schema->spec_field( name => 'category', type => $type );
In our next tutorial chapter, Lucy::Docs::Tutorial::Highlighter, we'll add highlighted excerpts from the "content" field to our search results.