Sql for Lucene

Originally a side project, I've decided to put this out there in case someone else may find this useful.

Introduction

The LuceneSqlParser supports a subset of standard sql. Here are some examples:

Select last_name from '/path/to/index/' where first_name='Foo' and age <=30 and city='Boston' limit 25

Select * from 'path/index/' where age in (31, 30, 50)

Select first_name, last_name from '/path/index/' where city in ('Cincinatti', 'New York', 'Boyds')

Select first_name from '/path/index/' where age between 35 and 50 and first_name like 'Br*'
-- Also takes paths from Windows OS
Select first_name from 'C:/path/index/' where first_name='John' and (age<=45 and city not in ('New York', 'Boston', 'Atlanta'))

The LuceneSqlParser returns a BooleanQuery. The BooleanQuery will contain different types of lucene query objects depending on the predicates used. There is a class Searcher avaiable for use with the LuceneSqlParser. The Searcher abstracts away the opening of a lucene IndexSearcher, iterating over the ScoreDoc array and extracting results. Next, we'll take a look at the rules used to parse the sql.

LuceneSqlParser Functionality

At high level a sql statement is broken down and parsed in the following manner:

1. The 'Select' statement contains a comma sparated list of fields stored in a Lucene index. The parser stores fields in a Set<String> for use by the Searcher. To retreive all fields we can specify a '*' operator, or omit the 'select' clause altogether.
2. The 'From' clause takes a path in single quotes representing the location of a lucene index.
3. The 'Where' clause contains the predicates for searching the data.
4. The parser analyzes values used for searching in a similar fashion as the StandardAnalyzer (lower cased, whitespace and special characters removed). There are exceptions to this rule. The PrefixQuery, RegexQuery and the WildcardQuery are special cases and only removes characters not defined as special characters used by Lucene.
5. Predicates can be nested to an arbitrary depth. For example: where field='1' and (field2='2' and field3='3' and (field4='4' and (field5='5' and field='6')))
6. The 'Select' and 'From' clauses are optional.

The LuceneQueryParser defines two static methods parseQuery and parseToBooleanQuery. The parseQuery method is intended to be used in conjuction with the Searcher. The parseQuery returns a QueryParseResults object that contains the path for the index, the set of fields to retrieve and the BooleanQuery to execute. The parseToBooleanQuery is intended to be used for parsing only and returns a BooleanQuery.

SQL to Lucene Query Functionality Mapping

We now will list the supported Lucene query objects and how they are mapped from the input sql.

• <field name> ='Foo' converts to a TermQuery.
• <field name> ='Bar Baz' converts to a PhraseQuery
• <field name> like 'Fo*' converts to a PrefixQuery
• <field name> like 'B?l?' converts to a WildcardQuery
• <field name> matches('[Bb].*[hH]?') converts to a RegexpQuery
• <field name> in ('foo', 'bar', 'baz') converts to a BooleanQuery consisting of 3 BooleanClause objects. Each clause is a TermQuery with a BooleanClause.Occur of SHOULD.
• PhraseQuery and TermQuery objects can be combined in the 'in' operator. For example city in ('New York', 'Boston', 'Los Angeles')
• between 'Foo' AND 'Bar' converts to a TermRangeQuery with both items being inclusive.
• between 25 and 40 converts to a NumericRangeQuery again inclusive.
• The >, <, >= <= operators are converted to either TermRangeQuery or NumericRangeQuery objects with one side of the range being unbounded. The < and > operators are exclusive. The <= and >= operators are inclusive.

The AND,OR & NOT operators are mappped in the following manner.

• AND converts to BooleanClause.Occur.MUST
• OR converts to BooleanClause.Occur.SHOULD
• NOT converts to BooleanClause.Occur.MUST_NOT

Optimizations

In two cases the query is converted to something different from the mappings shown previously. The first case is a query that contains a single predicate that must not match. For example:

Select * from /index/path/ where last_name != 'Smith'
Select first_name, last_name where city not in ('Foo', 'Bar')

A query submitted in this format will not work in lucene. This fix for this query is simple. The parser takes the original BooleanQuery and adds an addtional clause. The underlying query object in the new clause is a MatchAllDocsQuery. The MatchAllDocsQuery returns all documents in the index and the orginal predicate will fiter out the unwanted results. The second case is when searching for a numeric field in the TermQuery format. For example:

Select * from '/index/path' where age = 49

Normally a 'field=value' or 'field != value' predicate is converted to a TermQuery. But the way lucene searches for values it will not find a field if it is searching for a number versus a string. In this case the parser constructs a NumericRangeQuery where the low and high value are equal and inclusive.

Limitations

There are a several limitations at this point.

• Converting to a PhraseQuery does not allow for specifying any slop. For a match with a PhraseQuery all terms must be located adjacent to each other.
• Numeric type queries only support the Int type for now.
• Range querries are inclusive when both a high and low value are specified.
• If no limit clause is specified in the query, a default limit of 10,000 records is used.

Searcher

The second component of sql for lucene is the Searcherclass. The Searcher could be thought of as a convenience method for performing a lucene search and extracting the results. The Searcher has one method search that takes a sql query and returns a List<Map<String,Object>> containing the search results. Each map in the list represents a document with the keys being the field names and the values are the values stored in the retrieved fields.

String query = "Select age,city from '/path/to/index/' where first_name='john' and age='50'";
List<Map<String,Object>> results = searcher.search(query);

It's worth noting the list and map returned from the searcher are of type ImmutableList and ImmutableMap respectively. The Searcher has 4 constructors:

    public Searcher() {}

    public Searcher(String indexPath){
        Preconditions.checkArgument(indexPath != null && !indexPath.trim().isEmpty(), "Index Path is can't be null or empty");
        this.indexSearcher = createIndexSearcherFromStringPath.apply(indexPath);
    }

    public Searcher(Path indexPath){
        Preconditions.checkNotNull(indexPath,"Index Path can't be null");
        this.indexSearcher = createIndexSearcherFromPath.apply(indexPath);
    }

    public Searcher(Directory directory) {
        Preconditions.checkNotNull(directory,"Directory can't be null");
        this.indexSearcher = fromDirectoryToIndexSearcher.apply(directory);
    }

If the searcher is instantiated with the no-arg constructor, then the path for the index will be extracted from the query and used to open the IndexSearcher. All subsequent queries can safely omit the from clause. If the searcher is instantiated with any of the other 3 constructors, the 'from' clause will be ignored and can be omitted from the query.

Features To Be Added

• A JDBC Driver.
• Insert, Update and Delete support.
• Support all the numeric types supported by Lucene.
• Syntax to support FuzzyQuery searches.
• Support for filters.
• Ability to specify slop for PhraseQuery searches.