Xapian::Query Class Reference

Class representing a query. More...

Classes
class	Internal

Public Types
enum	op {   OP_AND = 0, OP_OR = 1, OP_AND_NOT = 2, OP_XOR = 3,   OP_AND_MAYBE = 4, OP_FILTER = 5, OP_NEAR = 6, OP_PHRASE = 7,   OP_VALUE_RANGE = 8, OP_SCALE_WEIGHT = 9, OP_ELITE_SET = 10, OP_VALUE_GE = 11,   OP_VALUE_LE = 12, OP_SYNONYM = 13, OP_MAX = 14, OP_WILDCARD = 15,   OP_INVALID = 99, LEAF_TERM = 100, LEAF_POSTING_SOURCE, LEAF_MATCH_ALL,   LEAF_MATCH_NOTHING }
	Query operators. More...
enum	{ WILDCARD_LIMIT_ERROR, WILDCARD_LIMIT_FIRST, WILDCARD_LIMIT_MOST_FREQUENT }

Public Member Functions
	Query ()
	Default constructor.
	~Query ()
	Destructor.
	Query (const Query &o)
	Copying is allowed. More...
Query &	operator= (const Query &o)
	Copying is allowed. More...
	Query (const std::string &term, Xapian::termcount wqf=1, Xapian::termpos pos=0)
	Construct a Query object for a term. More...
	Query (Xapian::PostingSource *source)
	Construct a Query object for a PostingSource. More...
	Query (double factor, const Xapian::Query &subquery)
	Query (op op_, const Xapian::Query &subquery, double factor)
	Query (op op_, const Xapian::Query &a, const Xapian::Query &b)
	Query (op op_, const std::string &a, const std::string &b)
	Query (op op_, Xapian::valueno slot, const std::string &limit)
	Query (op op_, Xapian::valueno slot, const std::string &begin, const std::string &end)
	Query (op op_, const std::string &pattern, Xapian::termcount max_expansion=0, int max_type=WILDCARD_LIMIT_ERROR, op combiner=OP_SYNONYM)
	Query constructor for OP_WILDCARD queries. More...
template<typename I >
	Query (op op_, I begin, I end, Xapian::termcount window=0)
const TermIterator	get_terms_begin () const
const TermIterator	get_terms_end () const
const TermIterator	get_unique_terms_begin () const
Xapian::termcount	get_length () const
bool	empty () const
std::string	serialise () const
op	get_type () const
	Get the type of the top level of the query. More...
size_t	get_num_subqueries () const
	Get the number of subqueries of the top level query. More...
const Query	get_subquery (size_t n) const
	Read a top level subquery. More...
std::string	get_description () const
const Query	operator&= (const Query &o)
const Query	operator|= (const Query &o)
const Query	operator^= (const Query &o)
const Query	operator*= (double factor)
const Query	operator/= (double factor)
	Query (Query::op op_)

Static Public Member Functions
static const Query	unserialise (const std::string &serialised, const Registry &reg=Registry())

Static Public Attributes
static const Xapian::Query	MatchNothing
static const Xapian::Query	MatchAll

Detailed Description

Class representing a query.

Member Enumeration Documentation

anonymous enum

Enumerator
WILDCARD_LIMIT_ERROR	Throw an error if OP_WILDCARD exceeds its expansion limit. Xapian::WildcardError will be thrown when the query is actually run.
WILDCARD_LIMIT_FIRST	Stop expanding when OP_WILDCARD reaches its expansion limit. This makes the wildcard expand to only the first N terms (sorted by byte order).
WILDCARD_LIMIT_MOST_FREQUENT	Limit OP_WILDCARD expansion to the most frequent terms. If OP_WILDCARD would expand to more than its expansion limit, the most frequent terms are taken. This approach works well for cases such as expanding a partial term at the end of a query string which the user hasn't finished typing yet - as well as being less expense to evaluate than the full expansion, using only the most frequent terms tends to give better results too.

enum Xapian::Query::op

Query operators.

Enumerator

OP_ELITE_SET

Pick the best N subqueries and combine with OP_OR. If you want to implement a feature which finds documents similar to a piece of text, an obvious approach is to build an "OR" query from all the terms in the text, and run this query against a database containing the documents. However such a query can contain a lots of terms and be quite slow to perform, yet many of these terms don't contribute usefully to the results. The OP_ELITE_SET operator can be used instead of OP_OR in this situation. OP_ELITE_SET selects the most important ''N'' terms and then acts as an OP_OR query with just these, ignoring any other terms. This will usually return results just as good as the full OP_OR query, but much faster. In general, the OP_ELITE_SET operator can be used when you have a large OR query, but it doesn't matter if the search completely ignores some of the less important terms in the query. The subqueries don't have to be terms, but if they aren't then OP_ELITE_SET will look at the estimated frequencies of the subqueries and so could pick a subset which don't actually match any documents even if the full OR would match some. You can specify a parameter to the query constructor which control the number of terms which OP_ELITE_SET will pick. If not specified, this defaults to 10 (Xapian used to default to ceil(sqrt(number_of_subqueries)) if there are more than 100 subqueries, but this rather arbitrary special case was dropped in 1.3.0). For example, this will pick the best 7 terms: Xapian::Query query(Xapian::Query::OP_ELITE_SET, subqs.begin(), subqs.end(), 7); If the number of subqueries is less than this threshold, OP_ELITE_SET behaves identically to OP_OR.

Constructor & Destructor Documentation

Xapian::Query::Query ( const Query &  o )

inline

Copying is allowed.

The internals are reference counted, so copying is cheap.

Xapian::Query::Query	(	const std::string &	term,
		Xapian::termcount	wqf = 1,
		Xapian::termpos	pos = 0
	)

Construct a Query object for a term.

Xapian::Query::Query ( Xapian::PostingSource *  source )

explicit

Construct a Query object for a PostingSource.

Xapian::Query::Query	(	op	op_,
		const std::string &	pattern,
		Xapian::termcount	max_expansion = 0,
		int	max_type = WILDCARD_LIMIT_ERROR,
		op	combiner = OP_SYNONYM
	)

Query constructor for OP_WILDCARD queries.

Parameters

op	Must be OP_WILDCARD
pattern	The wildcard pattern - currently this is just a string and the wildcard expands to terms which start with exactly this string.
max_expansion	The maximum number of terms to expand to (default: 0, which means no limit)
max_type	How to enforce max_expansion - one of WILDCARD_LIMIT_ERROR (the default), WILDCARD_LIMIT_FIRST or WILDCARD_LIMIT_MOST_FREQUENT. When searching multiple databases, the expansion limit is currently applied independently for each database, so the total number of terms may be higher than the limit. This is arguably a bug, and may change in future versions.
combiner	The to combine the terms with - one of OP_SYNONYM (the default), OP_OR or OP_MAX.

Member Function Documentation

size_t Xapian::Query::get_num_subqueries

(

)

const

Get the number of subqueries of the top level query.

const Query Xapian::Query::get_subquery

(

size_t

n

)

const

Read a top level subquery.

Parameters

n	Return the n-th subquery (starting from 0) - only valid when 0 <= n < get_num_subqueries().

op Xapian::Query::get_type

(

)

const

Get the type of the top level of the query.

Query& Xapian::Query::operator= ( const Query &  o )

inline

Copying is allowed.

The internals are reference counted, so assignment is cheap.

References XAPIAN_PURE_FUNCTION.

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The documentation for this class was generated from the following file:

• xapian/query.h