Detect common wdf-disjoint OP_SYNONYM subqueries

[xapian.git] / xapian-core / api / queryinternal.cc

/** @file queryinternal.cc
 * @brief Xapian::Query internals
 */
/* Copyright (C) 2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018 Olly Betts
 * Copyright (C) 2008,2009 Lemur Consulting Ltd
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 */

#include <config.h>

#include "queryinternal.h"

#include "xapian/error.h"
#include "xapian/postingsource.h"
#include "xapian/query.h"

#include "heap.h"
#include "leafpostlist.h"
#include "matcher/andmaybepostlist.h"
#include "matcher/andnotpostlist.h"
#include "emptypostlist.h"
#include "matcher/exactphrasepostlist.h"
#include "matcher/externalpostlist.h"
#include "matcher/maxpostlist.h"
#include "matcher/multiandpostlist.h"
#include "matcher/multixorpostlist.h"
#include "matcher/nearpostlist.h"
#include "matcher/orpospostlist.h"
#include "matcher/orpostlist.h"
#include "matcher/phrasepostlist.h"
#include "matcher/queryoptimiser.h"
#include "matcher/valuerangepostlist.h"
#include "matcher/valuegepostlist.h"
#include "net/length.h"
#include "serialise-double.h"
#include "stringutils.h"
#include "termlist.h"

#include "debuglog.h"
#include "omassert.h"
#include "str.h"
#include "unicode/description_append.h"

#include <algorithm>
#include <functional>
#include <list>
#include <memory>
#include <string>
#include <unordered_set>
#include <vector>

using namespace std;

template<class CLASS> struct delete_ptr {
    void operator()(CLASS *p) const { delete p; }
};

using Xapian::Internal::AndContext;
using Xapian::Internal::OrContext;
using Xapian::Internal::XorContext;

namespace Xapian {

namespace Internal {

/** Class providing an operator which sorts postlists to select max or terms.
 *  This returns true if a has a (strictly) greater termweight than b.
 *
 *  Using this comparator will tend to result in multiple calls to
 *  recalc_maxweight() for each of the subqueries (we use it with nth_element
 *  which should be O(n)) - perhaps it'd be better to call recalc_maxweight()
 *  once and then sort on that.
 */
struct CmpMaxOrTerms {
    /** Return true if and only if a has a strictly greater termweight than b. */
    bool operator()(PostList *a, PostList *b) {
#if (defined(__i386__) && !defined(__SSE_MATH__)) || \
    defined(__mc68000__) || defined(__mc68010__) || \
    defined(__mc68020__) || defined(__mc68030__)
        // On some architectures, most common of which is x86, floating point
        // values are calculated and stored in registers with excess precision.
        // If the two recalc_maxweight() calls below return identical values in a
        // register, the excess precision may be dropped for one of them but
        // not the other (e.g. because the compiler saves the first calculated
        // weight to memory while calculating the second, then reloads it to
        // compare).  This leads to both a > b and b > a being true, which
        // violates the antisymmetry property of the strict weak ordering
        // required by nth_element().  This can have serious consequences (e.g.
        // segfaults).
        //
        // Note that m68k only has excess precision in earlier models - 68040
        // and later are OK:
        // http://gcc.gnu.org/ml/gcc-patches/2008-11/msg00105.html
        //
        // To avoid this, we store each result in a volatile double prior to
        // comparing them.  This means that the result of this test should
        // match that on other architectures with the same double format (which
        // is desirable), and actually has less overhead than rounding both
        // results to float (which is another approach which works).
        volatile double a_max_wt = a->recalc_maxweight();
        volatile double b_max_wt = b->recalc_maxweight();
        return a_max_wt > b_max_wt;
#else
        return a->recalc_maxweight() > b->recalc_maxweight();
#endif
    }
};

/// Comparison functor which orders PostList* by descending get_termfreq_est().
struct ComparePostListTermFreqAscending {
    /// Order by descending get_termfreq_est().
    bool operator()(const PostList *a, const PostList *b) const {
        return a->get_termfreq_est() > b->get_termfreq_est();
    }
};

class Context {
  protected:
    QueryOptimiser* qopt;

    vector<PostList*> pls;

  public:
    Context(QueryOptimiser* qopt_, size_t reserve);

    ~Context();

    void add_postlist(PostList * pl) {
        pls.push_back(pl);
    }

    bool empty() const {
        return pls.empty();
    }

    size_t size() const {
        return pls.size();
    }

    void shrink(size_t new_size);
};

Context::Context(QueryOptimiser* qopt_, size_t reserve)
    : qopt(qopt_)
{
    pls.reserve(reserve);
}

void
Context::shrink(size_t new_size)
{
    AssertRel(new_size, <=, pls.size());
    if (new_size >= pls.size())
        return;

    const PostList * hint_pl = qopt->get_hint_postlist();
    for (auto&& i = pls.begin() + new_size; i != pls.end(); ++i) {
        const PostList * pl = *i;
        if (rare(pl == hint_pl)) {
            // We were about to delete qopt's hint - instead tell qopt to take
            // ownership.
            qopt->take_hint_ownership();
            hint_pl = NULL;
        } else {
            delete pl;
        }
    }
    pls.resize(new_size);
}

Context::~Context()
{
    shrink(0);
}

class OrContext : public Context {
  public:
    OrContext(QueryOptimiser* qopt_, size_t reserve)
        : Context(qopt_, reserve) { }

    /// Select the best set_size postlists from the last out_of added.
    void select_elite_set(size_t set_size, size_t out_of);

    /// Select the set_size postlists with the highest term frequency.
    void select_most_frequent(size_t set_size);

    PostList * postlist();
    PostList * postlist_max();
};

void
OrContext::select_elite_set(size_t set_size, size_t out_of)
{
    vector<PostList*>::iterator begin = pls.begin() + pls.size() - out_of;
    nth_element(begin, begin + set_size - 1, pls.end(), CmpMaxOrTerms());
    shrink(pls.size() - out_of + set_size);
}

void
OrContext::select_most_frequent(size_t set_size)
{
    vector<PostList*>::iterator begin = pls.begin();
    nth_element(begin, begin + set_size - 1, pls.end(),
                ComparePostListTermFreqAscending());
    shrink(set_size);
}

PostList *
OrContext::postlist()
{
    Assert(!pls.empty());

    if (pls.size() == 1) {
        PostList * pl = pls[0];
        pls.clear();
        return pl;
    }

    // Make postlists into a heap so that the postlist with the greatest term
    // frequency is at the top of the heap.
    Heap::make(pls.begin(), pls.end(), ComparePostListTermFreqAscending());

    // Now build a tree of binary OrPostList objects.
    //
    // The algorithm used to build the tree is like that used to build an
    // optimal Huffman coding tree.  If we called next() repeatedly, this
    // arrangement would minimise the number of method calls.  Generally we
    // don't actually do that, but this arrangement is still likely to be a
    // good one, and it does minimise the work in the worst case.
    while (true) {
        // We build the tree such that at each branch:
        //
        //   l.get_termfreq_est() >= r.get_termfreq_est()
        //
        // We do this so that the OrPostList class can be optimised assuming
        // that this is the case.
        PostList * r = pls.front();
        Heap::pop(pls.begin(), pls.end(), ComparePostListTermFreqAscending());
        pls.pop_back();
        PostList * pl;
        pl = new OrPostList(pls.front(), r, qopt->matcher, qopt->db_size);

        if (pls.size() == 1) {
            pls.clear();
            return pl;
        }

        pls[0] = pl;
        Heap::replace(pls.begin(), pls.end(),
                      ComparePostListTermFreqAscending());
    }
}

PostList *
OrContext::postlist_max()
{
    Assert(!pls.empty());

    if (pls.size() == 1) {
        PostList * pl = pls[0];
        pls.clear();
        return pl;
    }

    // Sort the postlists so that the postlist with the greatest term frequency
    // is first.
    sort(pls.begin(), pls.end(), ComparePostListTermFreqAscending());

    PostList * pl;
    pl = new MaxPostList(pls.begin(), pls.end(), qopt->matcher, qopt->db_size);

    pls.clear();
    return pl;
}

class XorContext : public Context {
  public:
    XorContext(QueryOptimiser* qopt_, size_t reserve)
        : Context(qopt_, reserve) { }

    PostList * postlist();
};

PostList *
XorContext::postlist()
{
    Xapian::doccount db_size = qopt->db_size;
    PostList * pl;
    pl = new MultiXorPostList(pls.begin(), pls.end(), qopt->matcher, db_size);

    // Empty pls so our destructor doesn't delete them all!
    pls.clear();
    return pl;
}

class AndContext : public Context {
    class PosFilter {
        Xapian::Query::op op_;

        /// Start and end indices for the PostLists this positional filter uses.
        size_t begin, end;

        Xapian::termcount window;

      public:
        PosFilter(Xapian::Query::op op__, size_t begin_, size_t end_,
                  Xapian::termcount window_)
            : op_(op__), begin(begin_), end(end_), window(window_) { }

        PostList * postlist(PostList* pl,
                            const vector<PostList*>& pls,
                            PostListTree* pltree) const;
    };

    list<PosFilter> pos_filters;

  public:
    AndContext(QueryOptimiser* qopt_, size_t reserve)
        : Context(qopt_, reserve) { }

    void add_pos_filter(Query::op op_,
                        size_t n_subqs,
                        Xapian::termcount window);

    PostList * postlist();
};

PostList *
AndContext::PosFilter::postlist(PostList* pl,
                                const vector<PostList*>& pls,
                                PostListTree* pltree) const
try {
    vector<PostList *>::const_iterator terms_begin = pls.begin() + begin;
    vector<PostList *>::const_iterator terms_end = pls.begin() + end;

    if (op_ == Xapian::Query::OP_NEAR) {
        pl = new NearPostList(pl, window, terms_begin, terms_end, pltree);
    } else if (window == end - begin) {
        AssertEq(op_, Xapian::Query::OP_PHRASE);
        pl = new ExactPhrasePostList(pl, terms_begin, terms_end, pltree);
    } else {
        AssertEq(op_, Xapian::Query::OP_PHRASE);
        pl = new PhrasePostList(pl, window, terms_begin, terms_end, pltree);
    }
    return pl;
} catch (...) {
    delete pl;
    throw;
}

void
AndContext::add_pos_filter(Query::op op_,
                           size_t n_subqs,
                           Xapian::termcount window)
{
    Assert(n_subqs > 1);
    size_t end = pls.size();
    size_t begin = end - n_subqs;
    pos_filters.push_back(PosFilter(op_, begin, end, window));
}

PostList *
AndContext::postlist()
{
    if (pls.empty()) {
        // This case only happens if this sub-database has no positional data
        // (but another sub-database does).
        Assert(pos_filters.empty());
        return new EmptyPostList;
    }

    unique_ptr<PostList> pl(new MultiAndPostList(pls.begin(), pls.end(),
                                                 qopt->matcher, qopt->db_size));

    // Sort the positional filters to try to apply them in an efficient order.
    // FIXME: We need to figure out what that is!  Try applying lowest cf/tf
    // first?

    // Apply any positional filters.
    list<PosFilter>::const_iterator i;
    for (i = pos_filters.begin(); i != pos_filters.end(); ++i) {
        const PosFilter & filter = *i;
        pl.reset(filter.postlist(pl.release(), pls, qopt->matcher));
    }

    // Empty pls so our destructor doesn't delete them all!
    pls.clear();
    return pl.release();
}

}

Query::Internal::~Internal() { }

size_t
Query::Internal::get_num_subqueries() const XAPIAN_NOEXCEPT
{
    return 0;
}

const Query
Query::Internal::get_subquery(size_t) const
{
    throw Xapian::InvalidArgumentError("get_subquery() not meaningful for this Query object");
}

Xapian::termcount
Query::Internal::get_wqf() const
{
    throw Xapian::InvalidArgumentError("get_wqf() not meaningful for this Query object");
}

Xapian::termpos
Query::Internal::get_pos() const
{
    throw Xapian::InvalidArgumentError("get_pos() not meaningful for this Query object");
}

void
Query::Internal::gather_terms(void *) const
{
}

Xapian::termcount
Query::Internal::get_length() const XAPIAN_NOEXCEPT
{
    return 0;
}

Query::Internal *
Query::Internal::unserialise(const char ** p, const char * end,
                             const Registry & reg)
{
    if (*p == end)
        return NULL;
    unsigned char ch = *(*p)++;
    switch (ch >> 5) {
        case 4: case 5: case 6: case 7: {
            // Multi-way branch
            //
            // 1ccccnnn where:
            //   nnn -> n_subqs (0 means encoded value follows)
            //   cccc -> code (which OP_XXX)
            size_t n_subqs = ch & 0x07;
            if (n_subqs == 0) {
                decode_length(p, end, n_subqs);
                n_subqs += 8;
            }
            unsigned char code = (ch >> 3) & 0x0f;
            Xapian::termcount parameter = 0;
            if (code >= 13)
                decode_length(p, end, parameter);
            Xapian::Internal::QueryBranch * result;
            switch (code) {
                case 0: // OP_AND
                    result = new Xapian::Internal::QueryAnd(n_subqs);
                    break;
                case 1: // OP_OR
                    result = new Xapian::Internal::QueryOr(n_subqs);
                    break;
                case 2: // OP_AND_NOT
                    result = new Xapian::Internal::QueryAndNot(n_subqs);
                    break;
                case 3: // OP_XOR
                    result = new Xapian::Internal::QueryXor(n_subqs);
                    break;
                case 4: // OP_AND_MAYBE
                    result = new Xapian::Internal::QueryAndMaybe(n_subqs);
                    break;
                case 5: // OP_FILTER
                    result = new Xapian::Internal::QueryFilter(n_subqs);
                    break;
                case 6: // OP_SYNONYM
                    result = new Xapian::Internal::QuerySynonym(n_subqs);
                    break;
                case 7: // OP_MAX
                    result = new Xapian::Internal::QueryMax(n_subqs);
                    break;
                case 13: // OP_ELITE_SET
                    result = new Xapian::Internal::QueryEliteSet(n_subqs,
                                                                 parameter);
                    break;
                case 14: // OP_NEAR
                    result = new Xapian::Internal::QueryNear(n_subqs,
                                                             parameter);
                    break;
                case 15: // OP_PHRASE
                    result = new Xapian::Internal::QueryPhrase(n_subqs,
                                                               parameter);
                    break;
                default:
                    // 8 to 12 are currently unused.
                    throw SerialisationError("Unknown multi-way branch Query operator");
            }
            do {
                result->add_subquery(Xapian::Query(unserialise(p, end, reg)));
            } while (--n_subqs);
            result->done();
            return result;
        }
        case 2: case 3: { // Term
            // Term
            //
            // 01ccLLLL where:
            //   LLLL -> length (0 means encoded value follows)
            //   cc -> code:
            //     0: wqf = 0; pos = 0
            //     1: wqf = 1; pos = 0
            //     2: wqf = 1; pos -> encoded value follows
            //     3: wqf -> encoded value follows; pos -> encoded value follows
            size_t len = ch & 0x0f;
            if (len == 0) {
                decode_length(p, end, len);
                len += 16;
            }
            if (size_t(end - *p) < len)
                throw SerialisationError("Not enough data");
            string term(*p, len);
            *p += len;

            int code = ((ch >> 4) & 0x03);

            Xapian::termcount wqf = static_cast<Xapian::termcount>(code > 0);
            if (code == 3)
                decode_length(p, end, wqf);

            Xapian::termpos pos = 0;
            if (code >= 2)
                decode_length(p, end, pos);

            return new Xapian::Internal::QueryTerm(term, wqf, pos);
        }
        case 1: {
            // OP_VALUE_RANGE or OP_VALUE_GE or OP_VALUE_LE
            //
            // 001tssss where:
            //   ssss -> slot number (15 means encoded value follows)
            //   t -> op:
            //     0: OP_VALUE_RANGE (or OP_VALUE_LE if begin empty)
            //     1: OP_VALUE_GE
            Xapian::valueno slot = ch & 15;
            if (slot == 15) {
                decode_length(p, end, slot);
                slot += 15;
            }
            size_t len;
            decode_length_and_check(p, end, len);
            string begin(*p, len);
            *p += len;
            if (ch & 0x10) {
                // OP_VALUE_GE
                return new Xapian::Internal::QueryValueGE(slot, begin);
            }

            // OP_VALUE_RANGE
            decode_length_and_check(p, end, len);
            string end_(*p, len);
            *p += len;
            if (begin.empty()) // FIXME: is this right?
                return new Xapian::Internal::QueryValueLE(slot, end_);
            return new Xapian::Internal::QueryValueRange(slot, begin, end_);
        }
        case 0: {
            // Other operators
            //
            //   000ttttt where:
            //     ttttt -> encodes which OP_XXX
            switch (ch & 0x1f) {
                case 0x00: // OP_INVALID
                    return new Xapian::Internal::QueryInvalid();
                case 0x0b: { // Wildcard
                    if (*p == end)
                        throw SerialisationError("not enough data");
                    Xapian::termcount max_expansion;
                    decode_length(p, end, max_expansion);
                    if (end - *p < 2)
                        throw SerialisationError("not enough data");
                    int max_type = static_cast<unsigned char>(*(*p)++);
                    op combiner = static_cast<op>(*(*p)++);
                    size_t len;
                    decode_length_and_check(p, end, len);
                    string pattern(*p, len);
                    *p += len;
                    return new Xapian::Internal::QueryWildcard(pattern,
                                                               max_expansion,
                                                               max_type,
                                                               combiner);
                }
                case 0x0c: { // PostingSource
                    size_t len;
                    decode_length_and_check(p, end, len);
                    string name(*p, len);
                    *p += len;

                    const PostingSource * reg_source = reg.get_posting_source(name);
                    if (!reg_source) {
                        string m = "PostingSource ";
                        m += name;
                        m += " not registered";
                        throw SerialisationError(m);
                    }

                    decode_length_and_check(p, end, len);
                    PostingSource * source =
                        reg_source->unserialise_with_registry(string(*p, len),
                                                              reg);
                    *p += len;
                    return new Xapian::Internal::QueryPostingSource(source->release());
                }
                case 0x0d: {
                    using Xapian::Internal::QueryScaleWeight;
                    double scale_factor = unserialise_double(p, end);
                    return new QueryScaleWeight(scale_factor,
                                                Query(unserialise(p, end, reg)));
                }
                case 0x0e: {
                    Xapian::termcount wqf;
                    Xapian::termpos pos;
                    decode_length(p, end, wqf);
                    decode_length(p, end, pos);
                    return new Xapian::Internal::QueryTerm(string(), wqf, pos);
                }
                case 0x0f:
                    return new Xapian::Internal::QueryTerm();
                default: // Others currently unused.
                    break;
            }
            break;
        }
    }
    string msg = "Unknown Query serialisation: ";
    msg += str(ch);
    throw SerialisationError(msg);
}

void
Query::Internal::postlist_sub_and_like(AndContext& ctx,
                                       QueryOptimiser * qopt,
                                       double factor) const
{
    ctx.add_postlist(postlist(qopt, factor));
}

void
Query::Internal::postlist_sub_or_like(OrContext& ctx,
                                      QueryOptimiser * qopt,
                                      double factor) const
{
    ctx.add_postlist(postlist(qopt, factor));
}

void
Query::Internal::postlist_sub_xor(XorContext& ctx,
                                  QueryOptimiser * qopt,
                                  double factor) const
{
    ctx.add_postlist(postlist(qopt, factor));
}

namespace Internal {

Query::op
QueryTerm::get_type() const XAPIAN_NOEXCEPT
{
    return term.empty() ? Query::LEAF_MATCH_ALL : Query::LEAF_TERM;
}

string
QueryTerm::get_description() const
{
    string desc;
    if (term.empty()) {
        desc = "<alldocuments>";
    } else {
        description_append(desc, term);
    }
    if (wqf != 1) {
        desc += '#';
        desc += str(wqf);
    }
    if (pos) {
        desc += '@';
        desc += str(pos);
    }
    return desc;
}

QueryPostingSource::QueryPostingSource(PostingSource * source_)
    : source(source_)
{
    if (!source_)
        throw Xapian::InvalidArgumentError("source parameter can't be NULL");
    if (source->_refs == 0) {
        // source_ isn't reference counted, so try to clone it.  If clone()
        // isn't implemented, just use the object provided and it's the
        // caller's responsibility to ensure it stays valid while in use.
        PostingSource * cloned_source = source->clone();
        if (cloned_source) source = cloned_source->release();
    }
}

Query::op
QueryPostingSource::get_type() const XAPIAN_NOEXCEPT
{
    return Query::LEAF_POSTING_SOURCE;
}

string
QueryPostingSource::get_description() const
{
    string desc = "PostingSource(";
    desc += source->get_description();
    desc += ')';
    return desc;
}

QueryScaleWeight::QueryScaleWeight(double factor, const Query & subquery_)
    : scale_factor(factor), subquery(subquery_)
{
    if (rare(scale_factor < 0.0))
        throw Xapian::InvalidArgumentError("OP_SCALE_WEIGHT requires factor >= 0");
}

Query::op
QueryScaleWeight::get_type() const XAPIAN_NOEXCEPT
{
    return Query::OP_SCALE_WEIGHT;
}

size_t
QueryScaleWeight::get_num_subqueries() const XAPIAN_NOEXCEPT
{
    return 1;
}

const Query
QueryScaleWeight::get_subquery(size_t) const
{
    return subquery;
}

string
QueryScaleWeight::get_description() const
{
    Assert(subquery.internal.get());
    string desc = str(scale_factor);
    desc += " * ";
    desc += subquery.internal->get_description();
    return desc;
}

PostList*
QueryTerm::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryTerm::postlist", qopt | factor);
    if (factor != 0.0)
        qopt->inc_total_subqs();
    RETURN(qopt->open_post_list(term, wqf, factor));
}

PostList*
QueryPostingSource::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryPostingSource::postlist", qopt | factor);
    Assert(source.get());
    if (factor != 0.0)
        qopt->inc_total_subqs();
    // Casting away const on the Database::Internal here is OK, as we wrap
    // them in a const Xapian::Database so non-const methods can't actually
    // be called on the Database::Internal object.
    const Xapian::Database wrappeddb(
            const_cast<Xapian::Database::Internal*>(&(qopt->db)));
    RETURN(new ExternalPostList(wrappeddb, source.get(), factor, qopt->matcher));
}

PostList*
QueryScaleWeight::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryScaleWeight::postlist", qopt | factor);
    RETURN(subquery.internal->postlist(qopt, factor * scale_factor));
}

void
QueryTerm::gather_terms(void * void_terms) const
{
    // Skip Xapian::Query::MatchAll (aka Xapian::Query("")).
    if (!term.empty()) {
        vector<pair<Xapian::termpos, string>> &terms =
            *static_cast<vector<pair<Xapian::termpos, string>>*>(void_terms);
        terms.push_back(make_pair(pos, term));
    }
}

PostList*
QueryValueRange::postlist(QueryOptimiser *qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryValueRange::postlist", qopt | factor);
    if (factor != 0.0)
        qopt->inc_total_subqs();
    const Xapian::Database::Internal & db = qopt->db;
    const string & lb = db.get_value_lower_bound(slot);
    if (lb.empty()) {
        // This should only happen if there are no values in this slot (which
        // could be because the backend just doesn't support values at all).
        // If there were values in the slot, the backend should have a
        // non-empty lower bound, even if it isn't a tight one.
        AssertEq(db.get_value_freq(slot), 0);
        RETURN(new EmptyPostList);
    }
    if (end < lb) {
        RETURN(new EmptyPostList);
    }
    const string & ub = db.get_value_upper_bound(slot);
    if (begin > ub) {
        RETURN(new EmptyPostList);
    }
    if (end >= ub) {
        // If begin <= lb too, then the range check isn't needed, but we do
        // still need to consider which documents have a value set in this
        // slot.  If this value is set for all documents, we can replace it
        // with the MatchAll postlist, which is especially efficient if
        // there are no gaps in the docids.
        if (begin <= lb && db.get_value_freq(slot) == db.get_doccount()) {
            RETURN(db.open_post_list(string()));
        }
        RETURN(new ValueGePostList(&db, slot, begin));
    }
    RETURN(new ValueRangePostList(&db, slot, begin, end));
}

void
QueryValueRange::serialise(string & result) const
{
    if (slot < 15) {
        result += static_cast<char>(0x20 | slot);
    } else {
        result += static_cast<char>(0x20 | 15);
        result += encode_length(slot - 15);
    }
    result += encode_length(begin.size());
    result += begin;
    result += encode_length(end.size());
    result += end;
}

Query::op
QueryValueRange::get_type() const XAPIAN_NOEXCEPT
{
    return Query::OP_VALUE_RANGE;
}

string
QueryValueRange::get_description() const
{
    string desc = "VALUE_RANGE ";
    desc += str(slot);
    desc += ' ';
    description_append(desc, begin);
    desc += ' ';
    description_append(desc, end);
    return desc;
}

PostList*
QueryValueLE::postlist(QueryOptimiser *qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryValueLE::postlist", qopt | factor);
    if (factor != 0.0)
        qopt->inc_total_subqs();
    const Xapian::Database::Internal & db = qopt->db;
    const string & lb = db.get_value_lower_bound(slot);
    // If lb.empty(), the backend doesn't provide value bounds.
    if (!lb.empty()) {
        if (limit < lb) {
            RETURN(new EmptyPostList);
        }
        if (limit >= db.get_value_upper_bound(slot)) {
            // The range check isn't needed, but we do still need to consider
            // which documents have a value set in this slot.  If this value is
            // set for all documents, we can replace it with the MatchAll
            // postlist, which is especially efficient if there are no gaps in
            // the docids.
            if (db.get_value_freq(slot) == db.get_doccount()) {
                RETURN(db.open_post_list(string()));
            }
        }
    }
    RETURN(new ValueRangePostList(&db, slot, string(), limit));
}

void
QueryValueLE::serialise(string & result) const
{
    // Encode as a range with an empty start (which only takes a single byte to
    // encode).
    if (slot < 15) {
        result += static_cast<char>(0x20 | slot);
    } else {
        result += static_cast<char>(0x20 | 15);
        result += encode_length(slot - 15);
    }
    result += encode_length(0);
    result += encode_length(limit.size());
    result += limit;
}

Query::op
QueryValueLE::get_type() const XAPIAN_NOEXCEPT
{
    return Query::OP_VALUE_LE;
}

string
QueryValueLE::get_description() const
{
    string desc = "VALUE_LE ";
    desc += str(slot);
    desc += ' ';
    description_append(desc, limit);
    return desc;
}

PostList*
QueryValueGE::postlist(QueryOptimiser *qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryValueGE::postlist", qopt | factor);
    if (factor != 0.0)
        qopt->inc_total_subqs();
    const Xapian::Database::Internal & db = qopt->db;
    const string & lb = db.get_value_lower_bound(slot);
    // If lb.empty(), the backend doesn't provide value bounds.
    if (!lb.empty()) {
        if (limit > db.get_value_upper_bound(slot)) {
            RETURN(new EmptyPostList);
        }
        if (limit < lb) {
            // The range check isn't needed, but we do still need to consider
            // which documents have a value set in this slot.  If this value is
            // set for all documents, we can replace it with the MatchAll
            // postlist, which is especially efficient if there are no gaps in
            // the docids.
            if (db.get_value_freq(slot) == db.get_doccount()) {
                RETURN(db.open_post_list(string()));
            }
        }
    }
    RETURN(new ValueGePostList(&db, slot, limit));
}

void
QueryValueGE::serialise(string & result) const
{
    if (slot < 15) {
        result += static_cast<char>(0x20 | 0x10 | slot);
    } else {
        result += static_cast<char>(0x20 | 0x10 | 15);
        result += encode_length(slot - 15);
    }
    result += encode_length(limit.size());
    result += limit;
}

Query::op
QueryValueGE::get_type() const XAPIAN_NOEXCEPT
{
    return Query::OP_VALUE_GE;
}

string
QueryValueGE::get_description() const
{
    string desc = "VALUE_GE ";
    desc += str(slot);
    desc += ' ';
    description_append(desc, limit);
    return desc;
}

PostList*
QueryWildcard::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryWildcard::postlist", qopt | factor);
    Query::op op = combiner;
    double or_factor = 0.0;
    if (factor == 0.0) {
        // If we have a factor of 0, we don't care about the weights, so
        // we're just like a normal OR query.
        op = Query::OP_OR;
    } else if (op != Query::OP_SYNONYM) {
        or_factor = factor;
    }

    bool old_in_synonym = qopt->in_synonym;
    if (!old_in_synonym) {
        qopt->in_synonym = (op == Query::OP_SYNONYM);
    }

    OrContext ctx(qopt, 0);
    unique_ptr<TermList> t(qopt->db.open_allterms(pattern));
    Xapian::termcount expansions_left = max_expansion;
    // If there's no expansion limit, set expansions_left to the maximum
    // value Xapian::termcount can hold.
    if (expansions_left == 0)
        --expansions_left;
    while (true) {
        t->next();
        if (t->at_end())
            break;
        if (max_type < Xapian::Query::WILDCARD_LIMIT_MOST_FREQUENT) {
            if (expansions_left-- == 0) {
                if (max_type == Xapian::Query::WILDCARD_LIMIT_FIRST)
                    break;
                string msg("Wildcard ");
                msg += pattern;
                msg += "* expands to more than ";
                msg += str(max_expansion);
                msg += " terms";
                throw Xapian::WildcardError(msg);
            }
        }
        const string & term = t->get_termname();
        ctx.add_postlist(qopt->open_lazy_post_list(term, 1, or_factor));
    }

    if (max_type == Xapian::Query::WILDCARD_LIMIT_MOST_FREQUENT) {
        // FIXME: open_lazy_post_list() results in the term getting registered
        // for stats, so we still incur an avoidable cost from the full
        // expansion size of the wildcard, which is most likely to be visible
        // with the remote backend.  Perhaps we should split creating the lazy
        // postlist from registering the term for stats.
        if (ctx.size() > max_expansion)
            ctx.select_most_frequent(max_expansion);
    }

    if (factor != 0.0) {
        if (op != Query::OP_SYNONYM) {
            qopt->set_total_subqs(qopt->get_total_subqs() + ctx.size());
        } else {
            qopt->inc_total_subqs();
        }
    }

    qopt->in_synonym = old_in_synonym;

    if (ctx.empty())
        RETURN(new EmptyPostList);

    if (op == Query::OP_MAX)
        RETURN(ctx.postlist_max());

    PostList * pl = ctx.postlist();
    if (op == Query::OP_OR)
        RETURN(pl);

    // We build an OP_OR tree for OP_SYNONYM and then wrap it in a
    // SynonymPostList, which supplies the weights.
    //
    // We know the subqueries from a wildcard expansion are wdf-disjoint
    // (i.e. each wdf from the document contributes at most itself to the
    // wdf of the subquery).
    RETURN(qopt->make_synonym_postlist(pl, factor, true));
}

termcount
QueryWildcard::get_length() const XAPIAN_NOEXCEPT
{
    // We currently assume wqf is 1 for calculating the synonym's weight
    // since conceptually the synonym is one "virtual" term.  If we were
    // to combine multiple occurrences of the same synonym expansion into
    // a single instance with wqf set, we would want to track the wqf.
    return 1;
}

void
QueryWildcard::serialise(string & result) const
{
    result += static_cast<char>(0x0b);
    result += encode_length(max_expansion);
    result += static_cast<unsigned char>(max_type);
    result += static_cast<unsigned char>(combiner);
    result += encode_length(pattern.size());
    result += pattern;
}

Query::op
QueryWildcard::get_type() const XAPIAN_NOEXCEPT
{
    return Query::OP_WILDCARD;
}

string
QueryWildcard::get_description() const
{
    string desc = "WILDCARD ";
    switch (combiner) {
        case Query::OP_SYNONYM:
            desc += "SYNONYM ";
            break;
        case Query::OP_MAX:
            desc += "MAX ";
            break;
        case Query::OP_OR:
            desc += "OR ";
            break;
        default:
            desc += "BAD ";
            break;
    }
    description_append(desc, pattern);
    return desc;
}

Xapian::termcount
QueryBranch::get_length() const XAPIAN_NOEXCEPT
{
    // Sum results from all subqueries.
    Xapian::termcount result = 0;
    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done(), but we
        // can't use Assert in a XAPIAN_NOEXCEPT function.  But we'll get a
        // segfault anyway.
        result += (*i).internal->get_length();
    }
    return result;
}

#define MULTIWAY(X) static_cast<unsigned char>(0x80 | (X) << 3)
#define MISC(X) static_cast<unsigned char>(X)
void
QueryBranch::serialise_(string & result, Xapian::termcount parameter) const
{
    static const unsigned char first_byte[] = {
        MULTIWAY(0),    // OP_AND
        MULTIWAY(1),    // OP_OR
        MULTIWAY(2),    // OP_AND_NOT
        MULTIWAY(3),    // OP_XOR
        MULTIWAY(4),    // OP_AND_MAYBE
        MULTIWAY(5),    // OP_FILTER
        MULTIWAY(14),   // OP_NEAR
        MULTIWAY(15),   // OP_PHRASE
        0,              // OP_VALUE_RANGE
        MISC(3),        // OP_SCALE_WEIGHT
        MULTIWAY(13),   // OP_ELITE_SET
        0,              // OP_VALUE_GE
        0,              // OP_VALUE_LE
        MULTIWAY(6),    // OP_SYNONYM
        MULTIWAY(7)     // OP_MAX
    };
    Xapian::Query::op op_ = get_op();
    AssertRel(size_t(op_),<,sizeof(first_byte));
    unsigned char ch = first_byte[op_];
    if (ch & 0x80) {
        // Multi-way operator.
        if (subqueries.size() < 8)
            ch |= subqueries.size();
        result += ch;
        if (subqueries.size() >= 8)
            result += encode_length(subqueries.size() - 8);
        if (ch >= MULTIWAY(13))
            result += encode_length(parameter);
    } else {
        result += ch;
    }

    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*i).internal.get());
        (*i).internal->serialise(result);
    }

    // For OP_NEAR, OP_PHRASE, and OP_ELITE_SET, the window/set size gets
    // appended next by an overloaded serialise() method in the subclass.
}

void
QueryBranch::serialise(string & result) const
{
    QueryBranch::serialise_(result);
}

void
QueryNear::serialise(string & result) const
{
    // FIXME: window - subqueries.size() ?
    QueryBranch::serialise_(result, window);
}

void
QueryPhrase::serialise(string & result) const
{
    // FIXME: window - subqueries.size() ?
    QueryBranch::serialise_(result, window);
}

void
QueryEliteSet::serialise(string & result) const
{
    // FIXME: set_size - subqueries.size() ?
    QueryBranch::serialise_(result, set_size);
}

void
QueryBranch::gather_terms(void * void_terms) const
{
    // Gather results from all subqueries.
    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*i).internal.get());
        (*i).internal->gather_terms(void_terms);
    }
}

void
QueryBranch::do_or_like(OrContext& ctx, QueryOptimiser * qopt, double factor,
                        Xapian::termcount elite_set_size, size_t first) const
{
    LOGCALL_VOID(MATCH, "QueryBranch::do_or_like", ctx | qopt | factor | elite_set_size);

    // FIXME: we could optimise by merging OP_ELITE_SET and OP_OR like we do
    // for AND-like operations.

    // OP_SYNONYM with a single subquery is only simplified by
    // QuerySynonym::done() if the single subquery is a term or MatchAll.
    Assert(subqueries.size() >= 2 || get_op() == Query::OP_SYNONYM);

    vector<PostList *> postlists;
    postlists.reserve(subqueries.size() - first);

    QueryVector::const_iterator q;
    for (q = subqueries.begin() + first; q != subqueries.end(); ++q) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*q).internal.get());
        (*q).internal->postlist_sub_or_like(ctx, qopt, factor);
    }

    if (elite_set_size && elite_set_size < subqueries.size()) {
        ctx.select_elite_set(elite_set_size, subqueries.size());
        // FIXME: not right!
    }
}

PostList *
QueryBranch::do_synonym(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(MATCH, PostList *, "QueryBranch::do_synonym", qopt | factor);
    OrContext ctx(qopt, subqueries.size());
    if (factor == 0.0) {
        // If we have a factor of 0, we don't care about the weights, so
        // we're just like a normal OR query.
        do_or_like(ctx, qopt, 0.0);
        return ctx.postlist();
    }

    bool old_in_synonym = qopt->in_synonym;
    qopt->in_synonym = true;
    do_or_like(ctx, qopt, 0.0);
    PostList * pl = ctx.postlist();
    qopt->in_synonym = old_in_synonym;

    bool wdf_disjoint = false;
    Assert(!subqueries.empty());
    auto type = subqueries.front().get_type();
    if (type == Query::OP_WILDCARD) {
        // Detect common easy case where all subqueries are OP_WILDCARD whose
        // constant prefixes form a prefix-free set.
        wdf_disjoint = true;
        vector<string> prefixes;
        for (auto&& q : subqueries) {
            if (q.get_type() != Query::OP_WILDCARD) {
                wdf_disjoint = false;
                break;
            }
            auto qw = static_cast<const QueryWildcard*>(q.internal.get());
            prefixes.push_back(qw->get_pattern());
        }

        if (wdf_disjoint) {
            sort(prefixes.begin(), prefixes.end());
            const string* prev = nullptr;
            for (const auto& i : prefixes) {
                if (prev) {
                    if (startswith(i, *prev)) {
                        wdf_disjoint = false;
                        break;
                    }
                }
                prev = &i;
            }
        }
    } else if (type == Query::LEAF_TERM) {
        // Detect common easy case where all subqueries are terms, none of
        // which are the same.
        wdf_disjoint = true;
        unordered_set<string> terms;
        for (auto&& q : subqueries) {
            if (q.get_type() != Query::LEAF_TERM) {
                wdf_disjoint = false;
                break;
            }
            auto qt = static_cast<const QueryTerm*>(q.internal.get());
            if (!terms.insert(qt->get_term()).second) {
                wdf_disjoint = false;
                break;
            }
        }
    }

    // We currently assume wqf is 1 for calculating the synonym's weight
    // since conceptually the synonym is one "virtual" term.  If we were
    // to combine multiple occurrences of the same synonym expansion into
    // a single instance with wqf set, we would want to track the wqf.

    // We build an OP_OR tree for OP_SYNONYM and then wrap it in a
    // SynonymPostList, which supplies the weights.
    RETURN(qopt->make_synonym_postlist(pl, factor, wdf_disjoint));
}

PostList *
QueryBranch::do_max(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(MATCH, PostList *, "QueryBranch::do_max", qopt | factor);
    OrContext ctx(qopt, subqueries.size());
    do_or_like(ctx, qopt, factor);
    if (factor == 0.0) {
        // If we have a factor of 0, we don't care about the weights, so
        // we're just like a normal OR query.
        RETURN(ctx.postlist());
    }

    // We currently assume wqf is 1 for calculating the OP_MAX's weight
    // since conceptually the OP_MAX is one "virtual" term.  If we were
    // to combine multiple occurrences of the same OP_MAX expansion into
    // a single instance with wqf set, we would want to track the wqf.
    RETURN(ctx.postlist_max());
}

Xapian::Query::op
QueryBranch::get_type() const XAPIAN_NOEXCEPT
{
    return get_op();
}

size_t
QueryBranch::get_num_subqueries() const XAPIAN_NOEXCEPT
{
    return subqueries.size();
}

const Query
QueryBranch::get_subquery(size_t n) const
{
    return subqueries[n];
}

const string
QueryBranch::get_description_helper(const char * op,
                                    Xapian::termcount parameter) const
{
    string desc = "(";
    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        if (desc.size() > 1) {
            desc += op;
            if (parameter) {
                desc += str(parameter);
                desc += ' ';
            }
        }
        Assert((*i).internal.get());
        // MatchNothing subqueries should have been removed by done(), and we
        // shouldn't get called before done() is, since that happens at the
        // end of the Xapian::Query constructor.
        desc += (*i).internal->get_description();
    }
    desc += ')';
    return desc;
}

Query::Internal *
QueryWindowed::done()
{
    // If window size not specified, default it.
    if (window == 0)
        window = subqueries.size();
    return QueryAndLike::done();
}

void
QueryScaleWeight::gather_terms(void * void_terms) const
{
    subquery.internal->gather_terms(void_terms);
}

void QueryTerm::serialise(string & result) const
{
    size_t len = term.size();
    if (len == 0) {
        if (wqf == 1 && pos == 0) {
            // Query::MatchAll
            result += '\x0f';
        } else {
            // Weird mutant versions of MatchAll
            result += '\x0e';
            result += encode_length(wqf);
            result += encode_length(pos);
        }
    } else if (wqf == 1) {
        if (pos == 0) {
            // Single occurrence free-text term without position set.
            if (len >= 16) {
                result += static_cast<char>(0x40 | 0x10);
                result += encode_length(term.size() - 16);
            } else {
                result += static_cast<char>(0x40 | 0x10 | len);
            }
            result += term;
        } else {
            // Single occurrence free-text term with position set.
            if (len >= 16) {
                result += static_cast<char>(0x40 | 0x20);
                result += encode_length(term.size() - 16);
            } else {
                result += static_cast<char>(0x40 | 0x20 | len);
            }
            result += term;
            result += encode_length(pos);
        }
    } else if (wqf > 1 || pos > 0) {
        // General case.
        if (len >= 16) {
            result += static_cast<char>(0x40 | 0x30);
            result += encode_length(term.size() - 16);
        } else if (len) {
            result += static_cast<char>(0x40 | 0x30 | len);
        }
        result += term;
        result += encode_length(wqf);
        result += encode_length(pos);
    } else {
        // Typical boolean term.
        AssertEq(wqf, 0);
        AssertEq(pos, 0);
        if (len >= 16) {
            result += static_cast<char>(0x40);
            result += encode_length(term.size() - 16);
        } else {
            result += static_cast<char>(0x40 | len);
        }
        result += term;
    }
}

void QueryPostingSource::serialise(string & result) const
{
    result += static_cast<char>(0x0c);

    const string & n = source->name();
    result += encode_length(n.size());
    result += n;

    const string & s = source->serialise();
    result += encode_length(s.size());
    result += s;
}

void QueryScaleWeight::serialise(string & result) const
{
    Assert(subquery.internal.get());
    const string & s = serialise_double(scale_factor);
    result += '\x0d';
    result += s;
    subquery.internal->serialise(result);
}

struct is_matchnothing {
    bool operator()(const Xapian::Query & q) const {
        return q.internal.get() == NULL;
    }
};

void
QueryAndLike::add_subquery(const Xapian::Query & subquery)
{
    // If the AndLike is already MatchNothing, do nothing.
    if (subqueries.size() == 1 && subqueries[0].internal.get() == NULL)
        return;
    // If we're adding MatchNothing, discard any previous subqueries.
    if (subquery.internal.get() == NULL)
        subqueries.clear();
    subqueries.push_back(subquery);
}

Query::Internal *
QueryAndLike::done()
{
    // Empty AndLike gives MatchNothing.
    if (subqueries.empty())
        return NULL;
    // We handle any subquery being MatchNothing in add_subquery() by leaving
    // a single MatchNothing subquery, and so this check results in AndLike
    // giving MatchNothing.
    if (subqueries.size() == 1)
        return subqueries[0].internal.get();
    return this;
}

PostList*
QueryAndLike::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryAndLike::postlist", qopt | factor);
    AndContext ctx(qopt, subqueries.size());
    postlist_sub_and_like(ctx, qopt, factor);
    RETURN(ctx.postlist());
}

void
QueryAndLike::postlist_sub_and_like(AndContext& ctx, QueryOptimiser * qopt, double factor) const
{
    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*i).internal.get());
        (*i).internal->postlist_sub_and_like(ctx, qopt, factor);
    }
}

void
QueryOrLike::add_subquery(const Xapian::Query & subquery)
{
    // Drop any subqueries which are MatchNothing.
    if (subquery.internal.get() != NULL)
        subqueries.push_back(subquery);
}

Query::Internal *
QueryOrLike::done()
{
    // An empty OrLike gives MatchNothing.  Note that add_subquery() drops any
    // subqueries which are MatchNothing.
    if (subqueries.empty())
        return NULL;
    if (subqueries.size() == 1)
        return subqueries[0].internal.get();
    return this;
}

void
QueryAndNot::add_subquery(const Xapian::Query & subquery)
{
    if (!subqueries.empty()) {
        // We're adding the 2nd or subsequent subquery, so this subquery is
        // negated.
        if (subqueries[0].internal.get() == NULL) {
            // The left side is already MatchNothing so drop any right side.
            //
            // MatchNothing AND_NOT X == MatchNothing
            return;
        }
        if (subquery.internal.get() == NULL) {
            // Drop MatchNothing on the right of AndNot.
            //
            // X AND_NOT MatchNothing == X
            return;
        }
        if (subquery.get_type() == subquery.OP_SCALE_WEIGHT) {
            // Strip OP_SCALE_WEIGHT wrapping from queries on the right of
            // AndNot as no weight is taken from them.
            subqueries.push_back(subquery.get_subquery(0));
            // The Query constructor for OP_SCALE_WEIGHT constructor should
            // eliminate OP_SCALE_WEIGHT applied to MatchNothing.
            Assert(subquery.get_subquery(0).internal.get() != NULL);
            return;
        }
    }
    subqueries.push_back(subquery);
}

Query::Internal *
QueryAndNot::done()
{
    // Any MatchNothing right subqueries get discarded by add_subquery() - if
    // that leaves just the left subquery, return that.
    //
    // If left subquery is MatchNothing, then add_subquery() discards all right
    // subqueries, so this check also gives MatchNothing for this case.
    if (subqueries.size() == 1)
        return subqueries[0].internal.get();
    return this;
}

void
QueryAndMaybe::add_subquery(const Xapian::Query & subquery)
{
    // If the left side of AndMaybe is already MatchNothing, do nothing.
    if (subqueries.size() == 1 && subqueries[0].internal.get() == NULL)
        return;
    // Drop any 2nd or subsequent subqueries which are MatchNothing.
    if (subquery.internal.get() != NULL || subqueries.empty())
        subqueries.push_back(subquery);
}

Query::Internal *
QueryAndMaybe::done()
{
    // Any MatchNothing right subqueries get discarded by add_subquery() - if
    // that leaves just the left subquery, return that.
    //
    // If left subquery is MatchNothing, then add_subquery() discards all right
    // subqueries, so this check also gives MatchNothing for this case.
    if (subqueries.size() == 1)
        return subqueries[0].internal.get();
    return this;
}

PostList*
QueryOr::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryOr::postlist", qopt | factor);
    OrContext ctx(qopt, subqueries.size());
    do_or_like(ctx, qopt, factor);
    RETURN(ctx.postlist());
}

void
QueryOr::postlist_sub_or_like(OrContext& ctx, QueryOptimiser * qopt, double factor) const
{
    do_or_like(ctx, qopt, factor);
}

PostList*
QueryAndNot::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryAndNot::postlist", qopt | factor);
    // FIXME: Combine and-like side with and-like stuff above.
    unique_ptr<PostList> l(subqueries[0].internal->postlist(qopt, factor));
    OrContext ctx(qopt, subqueries.size() - 1);
    do_or_like(ctx, qopt, 0.0, 0, 1);
    unique_ptr<PostList> r(ctx.postlist());
    RETURN(new AndNotPostList(l.release(), r.release(),
                              qopt->matcher, qopt->db_size));
}

PostList*
QueryXor::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryXor::postlist", qopt | factor);
    XorContext ctx(qopt, subqueries.size());
    postlist_sub_xor(ctx, qopt, factor);
    RETURN(ctx.postlist());
}

void
QueryXor::postlist_sub_xor(XorContext& ctx, QueryOptimiser * qopt, double factor) const
{
    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*i).internal.get());
        (*i).internal->postlist_sub_xor(ctx, qopt, factor);
    }
}

PostList*
QueryAndMaybe::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryAndMaybe::postlist", qopt | factor);
    // FIXME: Combine and-like side with and-like stuff above.
    unique_ptr<PostList> l(subqueries[0].internal->postlist(qopt, factor));
    if (factor == 0.0) {
        // An unweighted OP_AND_MAYBE can be replaced with its left branch.
        RETURN(l.release());
    }
    OrContext ctx(qopt, subqueries.size() - 1);
    do_or_like(ctx, qopt, factor, 0, 1);
    unique_ptr<PostList> r(ctx.postlist());
    RETURN(new AndMaybePostList(l.release(), r.release(),
                                qopt->matcher, qopt->db_size));
}

PostList*
QueryFilter::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryFilter::postlist", qopt | factor);
    // FIXME: Combine and-like stuff, like QueryOptimiser.
    AssertEq(subqueries.size(), 2);
    PostList * pls[2];
    unique_ptr<PostList> l(subqueries[0].internal->postlist(qopt, factor));
    pls[1] = subqueries[1].internal->postlist(qopt, 0.0);
    pls[0] = l.release();
    RETURN(new MultiAndPostList(pls, pls + 2, qopt->matcher, qopt->db_size));
}

void
QueryFilter::postlist_sub_and_like(AndContext& ctx, QueryOptimiser * qopt, double factor) const
{
    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*i).internal.get());
        (*i).internal->postlist_sub_and_like(ctx, qopt, factor);
        // Second and subsequent subqueries are unweighted.
        factor = 0.0;
    }
}

void
QueryWindowed::postlist_windowed(Query::op op, AndContext& ctx, QueryOptimiser * qopt, double factor) const
{
    if (!qopt->full_db_has_positions) {
        // No positional data anywhere, so just handle as AND.
        QueryAndLike::postlist_sub_and_like(ctx, qopt, factor);
        return;
    }

    if (!qopt->db.has_positions()) {
        // No positions in this subdatabase so this matches nothing, which
        // means the whole andcontext matches nothing.
        //
        // Bailing out here means we don't recurse deeper and that means we
        // don't call QueryOptimiser::inc_total_subqs() for leaf postlists in
        // the phrase, but at least one shard will count them, and the matcher
        // takes the highest answer (since 1.4.6).
        ctx.shrink(0);
        return;
    }

    bool old_need_positions = qopt->need_positions;
    qopt->need_positions = true;

    QueryVector::const_iterator i;
    for (i = subqueries.begin(); i != subqueries.end(); ++i) {
        // MatchNothing subqueries should have been removed by done().
        Assert((*i).internal.get());
        bool is_term = ((*i).internal->get_type() == Query::LEAF_TERM);
        PostList* pl = (*i).internal->postlist(qopt, factor);
        if (!is_term)
            pl = new OrPosPostList(pl);
        ctx.add_postlist(pl);
    }
    // Record the positional filter to apply higher up the tree.
    ctx.add_pos_filter(op, subqueries.size(), window);

    qopt->need_positions = old_need_positions;
}

void
QueryPhrase::postlist_sub_and_like(AndContext & ctx, QueryOptimiser * qopt, double factor) const
{
    QueryWindowed::postlist_windowed(Query::OP_PHRASE, ctx, qopt, factor);
}

void
QueryNear::postlist_sub_and_like(AndContext & ctx, QueryOptimiser * qopt, double factor) const
{
    QueryWindowed::postlist_windowed(Query::OP_NEAR, ctx, qopt, factor);
}

PostList*
QueryEliteSet::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryEliteSet::postlist", qopt | factor);
    OrContext ctx(qopt, subqueries.size());
    do_or_like(ctx, qopt, factor, set_size);
    RETURN(ctx.postlist());
}

void
QueryEliteSet::postlist_sub_or_like(OrContext& ctx, QueryOptimiser * qopt, double factor) const
{
    do_or_like(ctx, qopt, factor, set_size);
}

PostList*
QuerySynonym::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QuerySynonym::postlist", qopt | factor);
    // Save and restore total_subqs so we only add one for the whole
    // OP_SYNONYM subquery (or none if we're not weighted).
    Xapian::termcount save_total_subqs = qopt->get_total_subqs();
    if (factor != 0.0)
        ++save_total_subqs;
    PostList * pl = do_synonym(qopt, factor);
    qopt->set_total_subqs(save_total_subqs);
    RETURN(pl);
}

Query::Internal *
QuerySynonym::done()
{
    // An empty Synonym gives MatchNothing.  Note that add_subquery() drops any
    // subqueries which are MatchNothing.
    if (subqueries.empty())
        return NULL;
    if (subqueries.size() == 1) {
        Query::op sub_type = subqueries[0].get_type();
        // Synonym of a single subquery should only be simplified if that
        // subquery is a term (or MatchAll), or if it's also OP_SYNONYM.  Note
        // that MatchNothing subqueries are dropped, so we'd never get here
        // with a single MatchNothing subquery.
        if (sub_type == Query::LEAF_TERM || sub_type == Query::LEAF_MATCH_ALL ||
            sub_type == Query::OP_SYNONYM) {
            return subqueries[0].internal.get();
        }
        if (sub_type == Query::OP_WILDCARD) {
            auto q = static_cast<QueryWildcard*>(subqueries[0].internal.get());
            // SYNONYM over WILDCARD X -> WILDCARD SYNONYM for any combiner X.
            return q->change_combiner(Query::OP_SYNONYM);
        }
    }
    return this;
}

PostList*
QueryMax::postlist(QueryOptimiser * qopt, double factor) const
{
    LOGCALL(QUERY, PostList*, "QueryMax::postlist", qopt | factor);
    // Save and restore total_subqs so we only add one for the whole
    // OP_MAX subquery (or none if we're not weighted).
    Xapian::termcount save_total_subqs = qopt->get_total_subqs();
    if (factor != 0.0)
        ++save_total_subqs;
    PostList * pl = do_max(qopt, factor);
    qopt->set_total_subqs(save_total_subqs);
    RETURN(pl);
}

Xapian::Query::op
QueryAnd::get_op() const
{
    return Xapian::Query::OP_AND;
}

Xapian::Query::op
QueryOr::get_op() const
{
    return Xapian::Query::OP_OR;
}

Xapian::Query::op
QueryAndNot::get_op() const
{
    return Xapian::Query::OP_AND_NOT;
}

Xapian::Query::op
QueryXor::get_op() const
{
    return Xapian::Query::OP_XOR;
}

Xapian::Query::op
QueryAndMaybe::get_op() const
{
    return Xapian::Query::OP_AND_MAYBE;
}

Xapian::Query::op
QueryFilter::get_op() const
{
    return Xapian::Query::OP_FILTER;
}

Xapian::Query::op
QueryNear::get_op() const
{
    return Xapian::Query::OP_NEAR;
}

Xapian::Query::op
QueryPhrase::get_op() const
{
    return Xapian::Query::OP_PHRASE;
}

Xapian::Query::op
QueryEliteSet::get_op() const
{
    return Xapian::Query::OP_ELITE_SET;
}

Xapian::Query::op
QuerySynonym::get_op() const
{
    return Xapian::Query::OP_SYNONYM;
}

Xapian::Query::op
QueryMax::get_op() const
{
    return Xapian::Query::OP_MAX;
}

Xapian::Query::op
QueryWildcard::get_op() const
{
    return Xapian::Query::OP_WILDCARD;
}

string
QueryAnd::get_description() const
{
    return get_description_helper(" AND ");
}

string
QueryOr::get_description() const
{
    return get_description_helper(" OR ");
}

string
QueryAndNot::get_description() const
{
    return get_description_helper(" AND_NOT ");
}

string
QueryXor::get_description() const
{
    return get_description_helper(" XOR ");
}

string
QueryAndMaybe::get_description() const
{
    return get_description_helper(" AND_MAYBE ");
}

string
QueryFilter::get_description() const
{
    return get_description_helper(" FILTER ");
}

string
QueryNear::get_description() const
{
    return get_description_helper(" NEAR ", window);
}

string
QueryPhrase::get_description() const
{
    return get_description_helper(" PHRASE ", window);
}

string
QueryEliteSet::get_description() const
{
    return get_description_helper(" ELITE_SET ", set_size);
}

string
QuerySynonym::get_description() const
{
    if (subqueries.size() == 1) {
        string d = "(SYNONYM ";
        d += subqueries[0].internal->get_description();
        d += ")";
        return d;
    }
    return get_description_helper(" SYNONYM ");
}

string
QueryMax::get_description() const
{
    return get_description_helper(" MAX ");
}

Xapian::Query::op
QueryInvalid::get_type() const XAPIAN_NOEXCEPT
{
    return Xapian::Query::OP_INVALID;
}

PostList*
QueryInvalid::postlist(QueryOptimiser *, double) const
{
    throw Xapian::InvalidOperationError("Query is invalid");
}

void
QueryInvalid::serialise(std::string & result) const
{
    result += static_cast<char>(0x00);
}

string
QueryInvalid::get_description() const
{
    return "<INVALID>";
}

}
}