matcher/queryoptimiser.cc

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/* Copyright (C) 2007,2008 Olly Betts
 * Copyright (C) 2008 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 "queryoptimiser.h"

#include "andmaybepostlist.h"
#include "andnotpostlist.h"
#include "emptypostlist.h"
#include "exactphrasepostlist.h"
#include "multiandpostlist.h"
#include "multimatch.h"
#include "omassert.h"
#include "omdebug.h"
#include "omqueryinternal.h"
#include "orpostlist.h"
#include "phrasepostlist.h"
#include "postlist.h"
#include "valuegepostlist.h"
#include "valuerangepostlist.h"
#include "xorpostlist.h"

#include <algorithm>
#include <list>
#include <string>
#include <vector>

using namespace std;

PostList *
QueryOptimiser::do_subquery(const Xapian::Query::Internal * query, double factor)
{
    DEBUGCALL(MATCH, PostList *, "QueryOptimiser::do_subquery",
              query << ", " << factor);

    // Handle QueryMatchNothing.
    if (!query) RETURN(new EmptyPostList());

    switch (query->op) {
        case Xapian::Query::Internal::OP_LEAF:
            RETURN(do_leaf(query, factor));

        case Xapian::Query::OP_AND:
        case Xapian::Query::OP_FILTER:
        case Xapian::Query::OP_NEAR:
        case Xapian::Query::OP_PHRASE:
            RETURN(do_and_like(query, factor));

        case Xapian::Query::OP_OR:
        case Xapian::Query::OP_XOR:
        case Xapian::Query::OP_ELITE_SET:
            RETURN(do_or_like(query, factor));

        case Xapian::Query::OP_AND_NOT: {
            AssertEq(query->subqs.size(), 2);
            PostList * l = do_subquery(query->subqs[0], factor);
            PostList * r = do_subquery(query->subqs[1], 0.0);
            RETURN(new AndNotPostList(l, r, matcher, db_size));
        }

        case Xapian::Query::OP_AND_MAYBE: {
            AssertEq(query->subqs.size(), 2);
            PostList * l = do_subquery(query->subqs[0], factor);
            PostList * r = do_subquery(query->subqs[1], factor);
            RETURN(new AndMaybePostList(l, r, matcher, db_size));
        }

        case Xapian::Query::OP_VALUE_RANGE: {
            Xapian::valueno valno(query->parameter);
            const string & range_begin = query->tname;
            const string & range_end = query->str_parameter;
            RETURN(new ValueRangePostList(&db, valno, range_begin, range_end));
        }

        case Xapian::Query::OP_VALUE_GE: {
            Xapian::valueno valno(query->parameter);
            const string & range_begin = query->tname;
            RETURN(new ValueGePostList(&db, valno, range_begin));
        }

        case Xapian::Query::OP_VALUE_LE: {
            Xapian::valueno valno(query->parameter);
            const string & range_end = query->tname;
            RETURN(new ValueRangePostList(&db, valno, "", range_end));
        }

        case Xapian::Query::OP_SCALE_WEIGHT: {
            AssertEq(query->subqs.size(), 1);
            double sub_factor = factor;
            if (sub_factor != 0.0) sub_factor *= query->get_dbl_parameter();
            RETURN(do_subquery(query->subqs[0], sub_factor));
        }

        default:
            Assert(false);
            RETURN(NULL);
    }
}

struct PosFilter {
    PosFilter(Xapian::Query::Internal::op_t op_, size_t begin_, size_t end_,
              Xapian::termcount window_)
        : op(op_), begin(begin_), end(end_), window(window_) { }

    Xapian::Query::Internal::op_t op;

    size_t begin, end;

    Xapian::termcount window;
};

PostList *
QueryOptimiser::do_and_like(const Xapian::Query::Internal *query, double factor)
{
    DEBUGCALL(MATCH, PostList *, "QueryOptimiser::do_and_like",
              query << ", " << factor);

    list<PosFilter> pos_filters;
    vector<PostList *> plists;
    do_and_like(query, factor, plists, pos_filters);
    AssertRel(plists.size(), >=, 2);

    PostList * pl;
    pl = new MultiAndPostList(plists.begin(), plists.end(), matcher, 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>::iterator i;
    for (i = pos_filters.begin(); i != pos_filters.end(); ++i) {
        const PosFilter & filter = *i;

        // FIXME: make NearPostList, etc ctors take a pair of itors so we don't
        // need to create this temporary vector.
        vector<PostList *> terms(plists.begin() + filter.begin,
                                 plists.begin() + filter.end);

        Xapian::termcount window = filter.window;
        if (filter.op == Xapian::Query::OP_NEAR) {
            pl = new NearPostList(pl, window, terms);
        } else if (window == filter.end - filter.begin) {
            AssertEq(filter.op, Xapian::Query::OP_PHRASE);
            pl = new ExactPhrasePostList(pl, terms);
        } else {
            AssertEq(filter.op, Xapian::Query::OP_PHRASE);
            pl = new PhrasePostList(pl, window, terms);
        }
    }

    RETURN(pl);
}

inline bool is_and_like(Xapian::Query::Internal::op_t op) {
    return op == Xapian::Query::OP_AND || op == Xapian::Query::OP_FILTER ||
           op == Xapian::Query::OP_NEAR || op == Xapian::Query::OP_PHRASE;
}

void
QueryOptimiser::do_and_like(const Xapian::Query::Internal *query, double factor,
                            vector<PostList *> & and_plists,
                            list<PosFilter> & pos_filters)
{
    DEBUGCALL(MATCH, void, "QueryOptimiser::do_and_like",
              query << ", " << factor << ", [and_plists], [pos_filters]");

    Xapian::Query::Internal::op_t op = query->op;
    Assert(is_and_like(op));

    bool positional = false;
    if (op == Xapian::Query::OP_PHRASE || op == Xapian::Query::OP_NEAR) {
        // If this sub-database has no positional information, change
        // OP_PHRASE/OP_NEAR into OP_AND so that we actually return some
        // matches.
        if (!db.has_positions()) {
            op = Xapian::Query::OP_AND;
        } else {
            positional = true;
        }
    }

    const Xapian::Query::Internal::subquery_list &queries = query->subqs;
    AssertRel(queries.size(), >=, 2);

    for (size_t i = 0; i != queries.size(); ++i) {
        // The second branch of OP_FILTER is always boolean.
        if (i == 1 && op == Xapian::Query::OP_FILTER) factor = 0.0;

        const Xapian::Query::Internal * subq = queries[i];
        if (is_and_like(subq->op)) {
            do_and_like(subq, factor, and_plists, pos_filters);
        } else {
            PostList * pl = do_subquery(subq, factor);
            and_plists.push_back(pl);
        }
    }

    if (positional) {
        // Record the positional filter to apply higher up the tree.
        size_t end = and_plists.size();
        size_t begin = end - queries.size();
        Xapian::termcount window = query->parameter;

        pos_filters.push_back(PosFilter(op, begin, end, window));
    }
}

struct CmpMaxOrTerms {
    bool operator()(const PostList *a, const PostList *b) {
        if (a->get_termfreq_max() == 0) return false;
        if (b->get_termfreq_max() == 0) return true;

#if defined(__i386__) || defined(__mc68000__)
        // On some architectures, most common of which is x86, floating point
        // values are calculated and stored in registers with excess precision.
        // If the two get_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).
        //
        // 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->get_maxweight();
        volatile double b_max_wt = b->get_maxweight();
        return a_max_wt > b_max_wt;
#else
        return a->get_maxweight() > b->get_maxweight();
#endif
    }
};

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

struct ComparePostListTermFreqAscending {
    bool operator()(const PostList *a, const PostList *b) {
        return a->get_termfreq_est() > b->get_termfreq_est();
    }
};

PostList *
QueryOptimiser::do_or_like(const Xapian::Query::Internal *query, double factor)
{
    DEBUGCALL(MATCH, PostList *, "QueryOptimiser::do_or_like",
              query << ", " << factor);

    // FIXME: we could optimise by merging OP_ELITE_SET and OP_OR like we do
    // for AND-like operations.
    Xapian::Query::Internal::op_t op = query->op;
    Assert(op == Xapian::Query::OP_ELITE_SET || op == Xapian::Query::OP_OR ||
           op == Xapian::Query::OP_XOR);

    const Xapian::Query::Internal::subquery_list &queries = query->subqs;
    AssertRel(queries.size(), >=, 2);

    vector<PostList *> postlists;
    postlists.reserve(queries.size());

    Xapian::Query::Internal::subquery_list::const_iterator q;
    for (q = queries.begin(); q != queries.end(); ++q) {
        postlists.push_back(do_subquery(*q, factor));
    }

    if (op == Xapian::Query::OP_ELITE_SET) {
        // Select the best elite_set_size terms.
        Xapian::termcount elite_set_size = query->parameter;
        Assert(elite_set_size > 0);

        if (postlists.size() > elite_set_size) {
            // Call recalc_maxweight() as otherwise get_maxweight()
            // may not be valid before next() or skip_to()
            for_each(postlists.begin(), postlists.end(),
                     mem_fun(&PostList::recalc_maxweight));

            nth_element(postlists.begin(),
                        postlists.begin() + elite_set_size - 1,
                        postlists.end(), CmpMaxOrTerms());

            for_each(postlists.begin() + elite_set_size, postlists.end(),
                     delete_ptr<PostList>());

            if (elite_set_size == 1) RETURN(postlists[0]);

            postlists.resize(elite_set_size);
        }
    }

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

    // Now build a tree of binary OrPostList or XorPostList 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.
    AssertRel(postlists.size(), >=, 2);
    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 and XorPostList classes can be
        // optimised assuming that this is the case.
        PostList * r = postlists.front();
        pop_heap(postlists.begin(), postlists.end(),
                 ComparePostListTermFreqAscending());
        postlists.pop_back();
        PostList * l = postlists.front();

        PostList * pl;
        if (op == Xapian::Query::OP_XOR) {
            pl = new XorPostList(l, r, matcher, db_size);
        } else {
            pl = new OrPostList(l, r, matcher, db_size);
        }

        if (postlists.size() == 1) RETURN(pl);

        pop_heap(postlists.begin(), postlists.end(),
                 ComparePostListTermFreqAscending());
        postlists.back() = pl;
        push_heap(postlists.begin(), postlists.end(),
                  ComparePostListTermFreqAscending());
    }
}

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