/*
 * Copyright (C) 2014 ScyllaDB
 */

/*
 * This file is part of Scylla.
 *
 * Scylla is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Scylla 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 Scylla.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <boost/range/adaptor/reversed.hpp>
#include <seastar/util/defer.hh>
#include "mutation_partition.hh"
#include "mutation_partition_applier.hh"
#include "converting_mutation_partition_applier.hh"
#include "partition_builder.hh"
#include "query-result-writer.hh"
#include "atomic_cell_hash.hh"
#include "reversibly_mergeable.hh"
#include "mutation_fragment.hh"
#include "mutation_query.hh"
#include "service/priority_manager.hh"
#include "mutation_compactor.hh"
#include "intrusive_set_external_comparator.hh"
#include "counters.hh"
#include "row_cache.hh"
#include "view_info.hh"
#include "mutation_cleaner.hh"
#include <seastar/core/execution_stage.hh>
#include "types/map.hh"

template<bool reversed>
struct reversal_traits;

template<>
struct reversal_traits<false> {
    template <typename Container>
    static auto begin(Container& c) {
        return c.begin();
    }

    template <typename Container>
    static auto end(Container& c) {
        return c.end();
    }

    template <typename Container, typename Disposer>
    static typename Container::iterator erase_and_dispose(Container& c,
        typename Container::iterator begin,
        typename Container::iterator end,
        Disposer disposer)
    {
        return c.erase_and_dispose(begin, end, std::move(disposer));
    }

    template<typename Container, typename Disposer>
    static typename Container::iterator erase_dispose_and_update_end(Container& c,
         typename Container::iterator it, Disposer&& disposer,
         typename Container::iterator&)
    {
        return c.erase_and_dispose(it, std::forward<Disposer>(disposer));
    }

    template <typename Container>
    static boost::iterator_range<typename Container::iterator> maybe_reverse(
        Container& c, boost::iterator_range<typename Container::iterator> r)
    {
        return r;
    }

    template <typename Container>
    static typename Container::iterator maybe_reverse(Container&, typename Container::iterator r) {
        return r;
    }
};

template<>
struct reversal_traits<true> {
    template <typename Container>
    static auto begin(Container& c) {
        return c.rbegin();
    }

    template <typename Container>
    static auto end(Container& c) {
        return c.rend();
    }

    template <typename Container, typename Disposer>
    static typename Container::reverse_iterator erase_and_dispose(Container& c,
        typename Container::reverse_iterator begin,
        typename Container::reverse_iterator end,
        Disposer disposer)
    {
        return typename Container::reverse_iterator(
            c.erase_and_dispose(end.base(), begin.base(), disposer)
        );
    }

    // Erases element pointed to by it and makes sure than iterator end is not
    // invalidated.
    template<typename Container, typename Disposer>
    static typename Container::reverse_iterator erase_dispose_and_update_end(Container& c,
        typename Container::reverse_iterator it, Disposer&& disposer,
        typename Container::reverse_iterator& end)
    {
        auto to_erase = std::next(it).base();
        bool update_end = end.base() == to_erase;
        auto ret = typename Container::reverse_iterator(
            c.erase_and_dispose(to_erase, std::forward<Disposer>(disposer))
        );
        if (update_end) {
            end = ret;
        }
        return ret;
    }

    template <typename Container>
    static boost::iterator_range<typename Container::reverse_iterator> maybe_reverse(
        Container& c, boost::iterator_range<typename Container::iterator> r)
    {
        using reverse_iterator = typename Container::reverse_iterator;
        return boost::make_iterator_range(reverse_iterator(r.end()), reverse_iterator(r.begin()));
    }

    template <typename Container>
    static typename Container::reverse_iterator maybe_reverse(Container&, typename Container::iterator r) {
        return typename Container::reverse_iterator(r);
    }
};

mutation_partition::mutation_partition(const schema& s, const mutation_partition& x)
        : _tombstone(x._tombstone)
        , _static_row(s, column_kind::static_column, x._static_row)
        , _static_row_continuous(x._static_row_continuous)
        , _rows()
        , _row_tombstones(x._row_tombstones) {
    auto cloner = [&s] (const auto& x) {
        return current_allocator().construct<rows_entry>(s, x);
    };
    _rows.clone_from(x._rows, cloner, current_deleter<rows_entry>());
}

mutation_partition::mutation_partition(const mutation_partition& x, const schema& schema,
        query::clustering_key_filter_ranges ck_ranges)
        : _tombstone(x._tombstone)
        , _static_row(schema, column_kind::static_column, x._static_row)
        , _static_row_continuous(x._static_row_continuous)
        , _rows()
        , _row_tombstones(x._row_tombstones, range_tombstone_list::copy_comparator_only()) {
    try {
        for(auto&& r : ck_ranges) {
            for (const rows_entry& e : x.range(schema, r)) {
                _rows.insert(_rows.end(), *current_allocator().construct<rows_entry>(schema, e), rows_entry::compare(schema));
            }
            for (auto&& rt : x._row_tombstones.slice(schema, r)) {
                _row_tombstones.apply(schema, rt);
            }
        }
    } catch (...) {
        _rows.clear_and_dispose(current_deleter<rows_entry>());
        throw;
    }
}

mutation_partition::mutation_partition(mutation_partition&& x, const schema& schema,
    query::clustering_key_filter_ranges ck_ranges)
    : _tombstone(x._tombstone)
    , _static_row(std::move(x._static_row))
    , _static_row_continuous(x._static_row_continuous)
    , _rows(std::move(x._rows))
    , _row_tombstones(std::move(x._row_tombstones))
{
    {
        auto deleter = current_deleter<rows_entry>();
        auto it = _rows.begin();
        for (auto&& range : ck_ranges.ranges()) {
            _rows.erase_and_dispose(it, lower_bound(schema, range), deleter);
            it = upper_bound(schema, range);
        }
        _rows.erase_and_dispose(it, _rows.end(), deleter);
    }
    {
        range_tombstone_list::const_iterator it = _row_tombstones.begin();
        for (auto&& range : ck_ranges.ranges()) {
            auto rt_range = _row_tombstones.slice(schema, range);
            // upper bound for previous range may be after lower bound for the next range
            // if both ranges are connected through a range tombstone. In this case the
            // erase range would be invalid.
            if (rt_range.begin() == _row_tombstones.end() || std::next(rt_range.begin()) != it) {
                _row_tombstones.erase(it, rt_range.begin());
            }
            it = rt_range.end();
        }
        _row_tombstones.erase(it, _row_tombstones.end());
    }
}

mutation_partition::~mutation_partition() {
    _rows.clear_and_dispose(current_deleter<rows_entry>());
}

mutation_partition&
mutation_partition::operator=(mutation_partition&& x) noexcept {
    if (this != &x) {
        this->~mutation_partition();
        new (this) mutation_partition(std::move(x));
    }
    return *this;
}

void mutation_partition::ensure_last_dummy(const schema& s) {
    if (_rows.empty() || !_rows.rbegin()->is_last_dummy()) {
        _rows.insert_before(_rows.end(),
            *current_allocator().construct<rows_entry>(s, rows_entry::last_dummy_tag(), is_continuous::yes));
    }
}

void mutation_partition::apply(const schema& s, const mutation_partition& p, const schema& p_schema) {
    apply_weak(s, p, p_schema);
}

void mutation_partition::apply(const schema& s, mutation_partition&& p) {
    apply_weak(s, std::move(p));
}

void mutation_partition::apply(const schema& s, mutation_partition_view p, const schema& p_schema) {
    apply_weak(s, p, p_schema);
}

struct mutation_fragment_applier {
    const schema& _s;
    mutation_partition& _mp;

    void operator()(tombstone t) {
        _mp.apply(t);
    }

    void operator()(range_tombstone rt) {
        _mp.apply_row_tombstone(_s, std::move(rt));
    }

    void operator()(const static_row& sr) {
        _mp.static_row().apply(_s, column_kind::static_column, sr.cells());
    }

    void operator()(partition_start ps) {
        _mp.apply(ps.partition_tombstone());
    }

    void operator()(partition_end ps) {
    }

    void operator()(const clustering_row& cr) {
        auto temp = clustering_row(_s, cr);
        auto& dr = _mp.clustered_row(_s, std::move(temp.key()));
        dr.apply(_s, std::move(temp));
    }
};

void deletable_row::apply(const schema& s, clustering_row cr) {
    apply(cr.tomb());
    apply(cr.marker());
    cells().apply(s, column_kind::regular_column, std::move(cr.cells()));
}

void
mutation_partition::apply(const schema& s, const mutation_fragment& mf) {
    mutation_fragment_applier applier{s, *this};
    mf.visit(applier);
}

stop_iteration mutation_partition::apply_monotonically(const schema& s, mutation_partition&& p, cache_tracker* tracker, is_preemptible preemptible) {
    _tombstone.apply(p._tombstone);
    _static_row.apply_monotonically(s, column_kind::static_column, std::move(p._static_row));
    _static_row_continuous |= p._static_row_continuous;

    if (_row_tombstones.apply_monotonically(s, std::move(p._row_tombstones), preemptible) == stop_iteration::no) {
        return stop_iteration::no;
    }

    rows_entry::compare less(s);
    auto del = current_deleter<rows_entry>();
    auto p_i = p._rows.begin();
    auto i = _rows.begin();
    while (p_i != p._rows.end()) {
      try {
        rows_entry& src_e = *p_i;
        if (i != _rows.end() && less(*i, src_e)) {
            i = _rows.lower_bound(src_e, less);
        }
        if (i == _rows.end() || less(src_e, *i)) {
            p_i = p._rows.erase(p_i);
            auto src_i = _rows.insert_before(i, src_e);
            // When falling into a continuous range, preserve continuity.
            if (i != _rows.end() && i->continuous()) {
                src_e.set_continuous(true);
                if (src_e.dummy()) {
                    if (tracker) {
                        tracker->on_remove(src_e);
                    }
                    _rows.erase_and_dispose(src_i, del);
                }
            }
        } else {
            auto continuous = i->continuous() || src_e.continuous();
            auto dummy = i->dummy() && src_e.dummy();
            i->set_continuous(continuous);
            i->set_dummy(dummy);
            // Clear continuity in the source first, so that in case of exception
            // we don't end up with the range up to src_e being marked as continuous,
            // violating exception guarantees.
            src_e.set_continuous(false);
            if (tracker) {
                tracker->on_remove(*i);
                i->_lru_link.swap_nodes(src_e._lru_link);
                // Newer evictable versions store complete rows
                i->_row = std::move(src_e._row);
            } else {
                memory::on_alloc_point();
                i->_row.apply_monotonically(s, std::move(src_e._row));
            }
            p_i = p._rows.erase_and_dispose(p_i, del);
        }
        if (preemptible && need_preempt() && p_i != p._rows.end()) {
            // We cannot leave p with the clustering range up to p_i->position()
            // marked as continuous because some of its sub-ranges may have originally been discontinuous.
            // This would result in the sum of this and p to have broader continuity after preemption,
            // also possibly violating the invariant of non-overlapping continuity between MVCC versions,
            // if that's what we're merging here.
            // It's always safe to mark the range as discontinuous.
            p_i->set_continuous(false);
            return stop_iteration::no;
        }
      } catch (...) {
          // We cannot leave p with the clustering range up to p_i->position()
          // marked as continuous because some of its sub-ranges may have originally been discontinuous.
          // This would result in the sum of this and p to have broader continuity after preemption,
          // also possibly violating the invariant of non-overlapping continuity between MVCC versions,
          // if that's what we're merging here.
          // It's always safe to mark the range as discontinuous.
          p_i->set_continuous(false);
          throw;
      }
    }
    return stop_iteration::yes;
}

stop_iteration mutation_partition::apply_monotonically(const schema& s, mutation_partition&& p, const schema& p_schema, is_preemptible preemptible) {
    if (s.version() == p_schema.version()) {
        return apply_monotonically(s, std::move(p), no_cache_tracker, preemptible);
    } else {
        mutation_partition p2(s, p);
        p2.upgrade(p_schema, s);
        return apply_monotonically(s, std::move(p2), no_cache_tracker, is_preemptible::no); // FIXME: make preemptible
    }
}

void
mutation_partition::apply_weak(const schema& s, mutation_partition_view p, const schema& p_schema) {
    // FIXME: Optimize
    mutation_partition p2(*this, copy_comparators_only{});
    partition_builder b(p_schema, p2);
    p.accept(p_schema, b);
    apply_monotonically(s, std::move(p2), p_schema);
}

void mutation_partition::apply_weak(const schema& s, const mutation_partition& p, const schema& p_schema) {
    // FIXME: Optimize
    apply_monotonically(s, mutation_partition(s, p), p_schema);
}

void mutation_partition::apply_weak(const schema& s, mutation_partition&& p) {
    apply_monotonically(s, std::move(p), no_cache_tracker);
}

tombstone
mutation_partition::range_tombstone_for_row(const schema& schema, const clustering_key& key) const {
    tombstone t = _tombstone;
    if (!_row_tombstones.empty()) {
        auto found = _row_tombstones.search_tombstone_covering(schema, key);
        t.apply(found);
    }
    return t;
}

row_tombstone
mutation_partition::tombstone_for_row(const schema& schema, const clustering_key& key) const {
    row_tombstone t = row_tombstone(range_tombstone_for_row(schema, key));

    auto j = _rows.find(key, rows_entry::compare(schema));
    if (j != _rows.end()) {
        t.apply(j->row().deleted_at(), j->row().marker());
    }

    return t;
}

row_tombstone
mutation_partition::tombstone_for_row(const schema& schema, const rows_entry& e) const {
    row_tombstone t = e.row().deleted_at();
    t.apply(range_tombstone_for_row(schema, e.key()));
    return t;
}

void
mutation_partition::apply_row_tombstone(const schema& schema, clustering_key_prefix prefix, tombstone t) {
    assert(!prefix.is_full(schema));
    auto start = prefix;
    _row_tombstones.apply(schema, {std::move(start), std::move(prefix), std::move(t)});
}

void
mutation_partition::apply_row_tombstone(const schema& schema, range_tombstone rt) {
    _row_tombstones.apply(schema, std::move(rt));
}

void
mutation_partition::apply_delete(const schema& schema, const clustering_key_prefix& prefix, tombstone t) {
    if (prefix.is_empty(schema)) {
        apply(t);
    } else if (prefix.is_full(schema)) {
        clustered_row(schema, prefix).apply(t);
    } else {
        apply_row_tombstone(schema, prefix, t);
    }
}

void
mutation_partition::apply_delete(const schema& schema, range_tombstone rt) {
    if (range_tombstone::is_single_clustering_row_tombstone(schema, rt.start, rt.start_kind, rt.end, rt.end_kind)) {
        apply_delete(schema, std::move(rt.start), std::move(rt.tomb));
        return;
    }
    apply_row_tombstone(schema, std::move(rt));
}

void
mutation_partition::apply_delete(const schema& schema, clustering_key&& prefix, tombstone t) {
    if (prefix.is_empty(schema)) {
        apply(t);
    } else if (prefix.is_full(schema)) {
        clustered_row(schema, std::move(prefix)).apply(t);
    } else {
        apply_row_tombstone(schema, std::move(prefix), t);
    }
}

void
mutation_partition::apply_delete(const schema& schema, clustering_key_prefix_view prefix, tombstone t) {
    if (prefix.is_empty(schema)) {
        apply(t);
    } else if (prefix.is_full(schema)) {
        clustered_row(schema, prefix).apply(t);
    } else {
        apply_row_tombstone(schema, prefix, t);
    }
}

void
mutation_partition::apply_insert(const schema& s, clustering_key_view key, api::timestamp_type created_at) {
    clustered_row(s, key).apply(row_marker(created_at));
}
void mutation_partition::apply_insert(const schema& s, clustering_key_view key, api::timestamp_type created_at,
        gc_clock::duration ttl, gc_clock::time_point expiry) {
    clustered_row(s, key).apply(row_marker(created_at, ttl, expiry));
}
void mutation_partition::insert_row(const schema& s, const clustering_key& key, deletable_row&& row) {
    auto e = alloc_strategy_unique_ptr<rows_entry>(
        current_allocator().construct<rows_entry>(key, std::move(row)));
    _rows.insert(_rows.end(), *e, rows_entry::compare(s));
    e.release();
}

void mutation_partition::insert_row(const schema& s, const clustering_key& key, const deletable_row& row) {
    auto e = alloc_strategy_unique_ptr<rows_entry>(
        current_allocator().construct<rows_entry>(s, key, row));
    _rows.insert(_rows.end(), *e, rows_entry::compare(s));
    e.release();
}

const row*
mutation_partition::find_row(const schema& s, const clustering_key& key) const {
    auto i = _rows.find(key, rows_entry::compare(s));
    if (i == _rows.end()) {
        return nullptr;
    }
    return &i->row().cells();
}

deletable_row&
mutation_partition::clustered_row(const schema& s, clustering_key&& key) {
    auto i = _rows.find(key, rows_entry::compare(s));
    if (i == _rows.end()) {
        auto e = alloc_strategy_unique_ptr<rows_entry>(
            current_allocator().construct<rows_entry>(std::move(key)));
        i = _rows.insert(i, *e, rows_entry::compare(s));
        e.release();
    }
    return i->row();
}

deletable_row&
mutation_partition::clustered_row(const schema& s, const clustering_key& key) {
    auto i = _rows.find(key, rows_entry::compare(s));
    if (i == _rows.end()) {
        auto e = alloc_strategy_unique_ptr<rows_entry>(
            current_allocator().construct<rows_entry>(key));
        i = _rows.insert(i, *e, rows_entry::compare(s));
        e.release();
    }
    return i->row();
}

deletable_row&
mutation_partition::clustered_row(const schema& s, clustering_key_view key) {
    auto i = _rows.find(key, rows_entry::compare(s));
    if (i == _rows.end()) {
        auto e = alloc_strategy_unique_ptr<rows_entry>(
            current_allocator().construct<rows_entry>(key));
        i = _rows.insert(i, *e, rows_entry::compare(s));
        e.release();
    }
    return i->row();
}

deletable_row&
mutation_partition::clustered_row(const schema& s, position_in_partition_view pos, is_dummy dummy, is_continuous continuous) {
    auto i = _rows.find(pos, rows_entry::compare(s));
    if (i == _rows.end()) {
        auto e = alloc_strategy_unique_ptr<rows_entry>(
            current_allocator().construct<rows_entry>(s, pos, dummy, continuous));
        i = _rows.insert(i, *e, rows_entry::compare(s));
        e.release();
    }
    return i->row();
}

mutation_partition::rows_type::const_iterator
mutation_partition::lower_bound(const schema& schema, const query::clustering_range& r) const {
    if (!r.start()) {
        return std::cbegin(_rows);
    }
    return _rows.lower_bound(position_in_partition_view::for_range_start(r), rows_entry::compare(schema));
}

mutation_partition::rows_type::const_iterator
mutation_partition::upper_bound(const schema& schema, const query::clustering_range& r) const {
    if (!r.end()) {
        return std::cend(_rows);
    }
    return _rows.lower_bound(position_in_partition_view::for_range_end(r), rows_entry::compare(schema));
}

boost::iterator_range<mutation_partition::rows_type::const_iterator>
mutation_partition::range(const schema& schema, const query::clustering_range& r) const {
    return boost::make_iterator_range(lower_bound(schema, r), upper_bound(schema, r));
}

template <typename Container>
boost::iterator_range<typename Container::iterator>
unconst(Container& c, boost::iterator_range<typename Container::const_iterator> r) {
    return boost::make_iterator_range(
        c.erase(r.begin(), r.begin()),
        c.erase(r.end(), r.end())
    );
}

template <typename Container>
typename Container::iterator
unconst(Container& c, typename Container::const_iterator i) {
    return c.erase(i, i);
}

boost::iterator_range<mutation_partition::rows_type::iterator>
mutation_partition::range(const schema& schema, const query::clustering_range& r) {
    return unconst(_rows, static_cast<const mutation_partition*>(this)->range(schema, r));
}

mutation_partition::rows_type::iterator
mutation_partition::lower_bound(const schema& schema, const query::clustering_range& r) {
    return unconst(_rows, static_cast<const mutation_partition*>(this)->lower_bound(schema, r));
}

mutation_partition::rows_type::iterator
mutation_partition::upper_bound(const schema& schema, const query::clustering_range& r) {
    return unconst(_rows, static_cast<const mutation_partition*>(this)->upper_bound(schema, r));
}

template<typename Func>
void mutation_partition::for_each_row(const schema& schema, const query::clustering_range& row_range, bool reversed, Func&& func) const
{
    auto r = range(schema, row_range);
    if (!reversed) {
        for (const auto& e : r) {
            if (func(e) == stop_iteration::yes) {
                break;
            }
        }
    } else {
        for (const auto& e : r | boost::adaptors::reversed) {
            if (func(e) == stop_iteration::yes) {
                break;
            }
        }
    }
}

template<typename RowWriter>
void write_cell(RowWriter& w, const query::partition_slice& slice, ::atomic_cell_view c) {
    assert(c.is_live());
    auto wr = w.add().write();
    auto after_timestamp = [&, wr = std::move(wr)] () mutable {
        if (slice.options.contains<query::partition_slice::option::send_timestamp>()) {
            return std::move(wr).write_timestamp(c.timestamp());
        } else {
            return std::move(wr).skip_timestamp();
        }
    }();
    auto after_value = [&, wr = std::move(after_timestamp)] () mutable {
        if (slice.options.contains<query::partition_slice::option::send_expiry>() && c.is_live_and_has_ttl()) {
            return std::move(wr).write_expiry(c.expiry());
        } else {
            return std::move(wr).skip_expiry();
        }
    }().write_fragmented_value(c.value());
    [&, wr = std::move(after_value)] () mutable {
        if (slice.options.contains<query::partition_slice::option::send_ttl>() && c.is_live_and_has_ttl()) {
            return std::move(wr).write_ttl(c.ttl());
        } else {
            return std::move(wr).skip_ttl();
        }
    }().end_qr_cell();
}

template<typename RowWriter>
void write_cell(RowWriter& w, const query::partition_slice& slice, const data_type& type, collection_mutation_view v) {
    auto ctype = static_pointer_cast<const collection_type_impl>(type);
    if (slice.options.contains<query::partition_slice::option::collections_as_maps>()) {
        ctype = map_type_impl::get_instance(ctype->name_comparator(), ctype->value_comparator(), true);
    }
    w.add().write().skip_timestamp()
        .skip_expiry()
        .write_value(ctype->to_value(v, slice.cql_format()))
        .skip_ttl()
        .end_qr_cell();
}

template<typename RowWriter>
void write_counter_cell(RowWriter& w, const query::partition_slice& slice, ::atomic_cell_view c) {
    assert(c.is_live());
  counter_cell_view::with_linearized(c, [&] (counter_cell_view ccv) {
    auto wr = w.add().write();
    [&, wr = std::move(wr)] () mutable {
        if (slice.options.contains<query::partition_slice::option::send_timestamp>()) {
            return std::move(wr).write_timestamp(c.timestamp());
        } else {
            return std::move(wr).skip_timestamp();
        }
    }().skip_expiry()
            .write_value(counter_cell_view::total_value_type()->decompose(ccv.total_value()))
            .skip_ttl()
            .end_qr_cell();
  });
}

// Used to return the timestamp of the latest update to the row
struct max_timestamp {
    api::timestamp_type max = api::missing_timestamp;

    void update(api::timestamp_type ts) {
        max = std::max(max, ts);
    }
};

template<>
struct appending_hash<row> {
    template<typename Hasher>
    void operator()(Hasher& h, const row& cells, const schema& s, column_kind kind, const query::column_id_vector& columns, max_timestamp& max_ts) const {
        for (auto id : columns) {
            const cell_and_hash* cell_and_hash = cells.find_cell_and_hash(id);
            if (!cell_and_hash) {
                return;
            }
            auto&& def = s.column_at(kind, id);
            if (def.is_atomic()) {
                max_ts.update(cell_and_hash->cell.as_atomic_cell(def).timestamp());
                if constexpr (query::using_hash_of_hash_v<Hasher>) {
                    if (cell_and_hash->hash) {
                        feed_hash(h, *cell_and_hash->hash);
                    } else {
                        query::default_hasher cellh;
                        feed_hash(cellh, cell_and_hash->cell.as_atomic_cell(def), def);
                        feed_hash(h, cellh.finalize_uint64());
                    }
                } else {
                    feed_hash(h, cell_and_hash->cell.as_atomic_cell(def), def);
                }
            } else {
                auto&& cm = cell_and_hash->cell.as_collection_mutation();
                auto&& ctype = static_pointer_cast<const collection_type_impl>(def.type);
                max_ts.update(ctype->last_update(cm));
                if constexpr (query::using_hash_of_hash_v<Hasher>) {
                    if (cell_and_hash->hash) {
                        feed_hash(h, *cell_and_hash->hash);
                    } else {
                        query::default_hasher cellh;
                        feed_hash(cellh, cm, def);
                        feed_hash(h, cellh.finalize_uint64());
                    }
                } else {
                    feed_hash(h, cm, def);
                }
            }
        }
    }
};

cell_hash_opt row::cell_hash_for(column_id id) const {
    if (_type == storage_type::vector) {
        return id < max_vector_size && _storage.vector.present.test(id) ? _storage.vector.v[id].hash : cell_hash_opt();
    }
    auto it = _storage.set.find(id, cell_entry::compare());
    if (it != _storage.set.end()) {
        return it->hash();
    }
    return cell_hash_opt();
}

void row::prepare_hash(const schema& s, column_kind kind) const {
    // const to avoid removing const qualifiers on the read path
    for_each_cell([&s, kind] (column_id id, const cell_and_hash& c_a_h) {
        if (!c_a_h.hash) {
            query::default_hasher cellh;
            feed_hash(cellh, c_a_h.cell, s.column_at(kind, id));
            c_a_h.hash = cell_hash{cellh.finalize_uint64()};
        }
    });
}

void row::clear_hash() const {
    for_each_cell([] (column_id, const cell_and_hash& c_a_h) {
        c_a_h.hash = { };
    });
}

template<typename RowWriter>
static void get_compacted_row_slice(const schema& s,
    const query::partition_slice& slice,
    column_kind kind,
    const row& cells,
    const query::column_id_vector& columns,
    RowWriter& writer)
{
    for (auto id : columns) {
        const atomic_cell_or_collection* cell = cells.find_cell(id);
        if (!cell) {
            writer.add().skip();
        } else {
            auto&& def = s.column_at(kind, id);
            if (def.is_atomic()) {
                auto c = cell->as_atomic_cell(def);
                if (!c.is_live()) {
                    writer.add().skip();
                } else if (def.is_counter()) {
                    write_counter_cell(writer, slice, cell->as_atomic_cell(def));
                } else {
                    write_cell(writer, slice, cell->as_atomic_cell(def));
                }
            } else {
                auto&& mut = cell->as_collection_mutation();
                auto&& ctype = static_pointer_cast<const collection_type_impl>(def.type);
                if (!ctype->is_any_live(mut)) {
                    writer.add().skip();
                } else {
                    write_cell(writer, slice, def.type, mut);
                }
            }
        }
    }
}

bool has_any_live_data(const schema& s, column_kind kind, const row& cells, tombstone tomb = tombstone(),
                       gc_clock::time_point now = gc_clock::time_point::min()) {
    bool any_live = false;
    cells.for_each_cell_until([&] (column_id id, const atomic_cell_or_collection& cell_or_collection) {
        const column_definition& def = s.column_at(kind, id);
        if (def.is_atomic()) {
            auto&& c = cell_or_collection.as_atomic_cell(def);
            if (c.is_live(tomb, now, def.is_counter())) {
                any_live = true;
                return stop_iteration::yes;
            }
        } else {
            auto&& cell = cell_or_collection.as_collection_mutation();
            auto&& ctype = static_pointer_cast<const collection_type_impl>(def.type);
            if (ctype->is_any_live(cell, tomb, now)) {
                any_live = true;
                return stop_iteration::yes;
            }
        }
        return stop_iteration::no;
    });
    return any_live;
}

void
mutation_partition::query_compacted(query::result::partition_writer& pw, const schema& s, uint32_t limit) const {
    const query::partition_slice& slice = pw.slice();
    max_timestamp max_ts{pw.last_modified()};

    if (limit == 0) {
        pw.retract();
        return;
    }

    auto static_cells_wr = pw.start().start_static_row().start_cells();

    if (!slice.static_columns.empty()) {
        if (pw.requested_result()) {
            get_compacted_row_slice(s, slice, column_kind::static_column, static_row(), slice.static_columns, static_cells_wr);
        }
        if (pw.requested_digest()) {
            auto pt = partition_tombstone();
            pw.digest().feed_hash(pt);
            max_ts.update(pt.timestamp);
            pw.digest().feed_hash(static_row(), s, column_kind::static_column, slice.static_columns, max_ts);
        }
    }

    auto rows_wr = std::move(static_cells_wr).end_cells()
            .end_static_row()
            .start_rows();

    uint32_t row_count = 0;

    auto is_reversed = slice.options.contains(query::partition_slice::option::reversed);
    auto send_ck = slice.options.contains(query::partition_slice::option::send_clustering_key);
    for_each_row(s, query::clustering_range::make_open_ended_both_sides(), is_reversed, [&] (const rows_entry& e) {
        if (e.dummy()) {
            return stop_iteration::no;
        }
        auto& row = e.row();
        auto row_tombstone = tombstone_for_row(s, e);

        if (pw.requested_digest()) {
            pw.digest().feed_hash(e.key(), s);
            pw.digest().feed_hash(row_tombstone);
            max_ts.update(row_tombstone.tomb().timestamp);
            pw.digest().feed_hash(row.cells(), s, column_kind::regular_column, slice.regular_columns, max_ts);
        }

        if (row.is_live(s)) {
            if (pw.requested_result()) {
                auto cells_wr = [&] {
                    if (send_ck) {
                        return rows_wr.add().write_key(e.key()).start_cells().start_cells();
                    } else {
                        return rows_wr.add().skip_key().start_cells().start_cells();
                    }
                }();
                get_compacted_row_slice(s, slice, column_kind::regular_column, row.cells(), slice.regular_columns, cells_wr);
                std::move(cells_wr).end_cells().end_cells().end_qr_clustered_row();
            }
            ++row_count;
            if (--limit == 0) {
                return stop_iteration::yes;
            }
        }
        return stop_iteration::no;
    });

    pw.last_modified() = max_ts.max;

    // If we got no rows, but have live static columns, we should only
    // give them back IFF we did not have any CK restrictions.
    // #589
    // If ck:s exist, and we do a restriction on them, we either have maching
    // rows, or return nothing, since cql does not allow "is null".
    if (row_count == 0
            && (has_ck_selector(pw.ranges())
                    || !has_any_live_data(s, column_kind::static_column, static_row()))) {
        pw.retract();
    } else {
        pw.row_count() += row_count ? : 1;
        pw.partition_count() += 1;
        std::move(rows_wr).end_rows().end_qr_partition();
    }
}

std::ostream&
operator<<(std::ostream& os, const std::pair<column_id, const atomic_cell_or_collection::printer&>& c) {
    return fmt_print(os, "{{column: {} {}}}", c.first, c.second);
}

// Transforms given range of printable into a range of strings where each element
// in the original range is prefxied with given string.
template<typename RangeOfPrintable>
static auto prefixed(const sstring& prefix, const RangeOfPrintable& r) {
    return r | boost::adaptors::transformed([&] (auto&& e) { return format("{}{}", prefix, e); });
}

std::ostream&
operator<<(std::ostream& os, const row::printer& p) {
    auto add_printer = [&] (const auto& c) {
        return std::pair<column_id, atomic_cell_or_collection::printer>(std::piecewise_construct,
            std::forward_as_tuple(c.first),
            std::forward_as_tuple(p._schema.column_at(p._kind, c.first), c.second)
        );
    };

    sstring cells;
    switch (p._row._type) {
    case row::storage_type::set:
        cells = ::join(",", prefixed("\n      ", p._row.get_range_set() | boost::adaptors::transformed(add_printer)));
        break;
    case row::storage_type::vector:
        cells = ::join(",", prefixed("\n      ", p._row.get_range_vector() | boost::adaptors::transformed(add_printer)));
        break;
    }
    return fmt_print(os, "{{row: {}}}", cells);
}

std::ostream&
operator<<(std::ostream& os, const row_marker& rm) {
    if (rm.is_missing()) {
        return fmt_print(os, "{{row_marker: }}");
    } else if (rm._ttl == row_marker::dead) {
        return fmt_print(os, "{{row_marker: dead {} {}}}", rm._timestamp, rm._expiry.time_since_epoch().count());
    } else {
        return fmt_print(os, "{{row_marker: {} {} {}}}", rm._timestamp, rm._ttl.count(),
            rm._ttl != row_marker::no_ttl ? rm._expiry.time_since_epoch().count() : 0);
    }
}

std::ostream&
operator<<(std::ostream& os, const deletable_row::printer& p) {
    auto& dr = p._deletable_row;
    os << "{deletable_row: ";
    if (!dr._marker.is_missing()) {
        os << dr._marker << " ";
    }
    if (dr._deleted_at) {
        os << dr._deleted_at << " ";
    }
    return os << row::printer(p._schema, column_kind::regular_column, dr._cells) << "}";
}

std::ostream&
operator<<(std::ostream& os, const rows_entry::printer& p) {
    auto& re = p._rows_entry;
    return fmt_print(os, "{{rows_entry: cont={} dummy={} {} {}}}", re.continuous(), re.dummy(), re.position(),
                  deletable_row::printer(p._schema, re._row));
}

std::ostream&
operator<<(std::ostream& os, const mutation_partition::printer& p) {
    auto& mp = p._mutation_partition;
    os << "{mutation_partition: ";
    if (mp._tombstone) {
        os << mp._tombstone << ",";
    }
    if (!mp._row_tombstones.empty()) {
        os << "\n range_tombstones: {" << ::join(",", prefixed("\n    ", mp._row_tombstones)) << "},";
    }
    os << "\n static: cont=" << int(mp._static_row_continuous) << " " << row::printer(p._schema, column_kind::static_column, mp._static_row) << ",";
    auto add_printer = [&] (const auto& re) {
        return rows_entry::printer(p._schema, re);
    };
    os << "\n clustered: {" << ::join(",", prefixed("\n    ", mp._rows | boost::adaptors::transformed(add_printer))) << "}}";
    return os;
}

constexpr gc_clock::duration row_marker::no_ttl;
constexpr gc_clock::duration row_marker::dead;

int compare_row_marker_for_merge(const row_marker& left, const row_marker& right) noexcept {
    if (left.timestamp() != right.timestamp()) {
        return left.timestamp() > right.timestamp() ? 1 : -1;
    }
    if (left.is_live() != right.is_live()) {
        return left.is_live() ? -1 : 1;
    }
    if (left.is_live()) {
        if (left.is_expiring() != right.is_expiring()) {
            // prefer expiring cells.
            return left.is_expiring() ? 1 : -1;
        }
        if (left.is_expiring() && left.expiry() != right.expiry()) {
            return left.expiry() < right.expiry() ? -1 : 1;
        }
    } else {
        // Both are either deleted or missing
        if (left.deletion_time() != right.deletion_time()) {
            // Origin compares big-endian serialized deletion time. That's because it
            // delegates to AbstractCell.reconcile() which compares values after
            // comparing timestamps, which in case of deleted cells will hold
            // serialized expiry.
            return (uint64_t) left.deletion_time().time_since_epoch().count()
                   < (uint64_t) right.deletion_time().time_since_epoch().count() ? -1 : 1;
        }
    }
    return 0;
}

bool
deletable_row::equal(column_kind kind, const schema& s, const deletable_row& other, const schema& other_schema) const {
    if (_deleted_at != other._deleted_at || _marker != other._marker) {
        return false;
    }
    return _cells.equal(kind, s, other._cells, other_schema);
}

void deletable_row::apply(const schema& s, deletable_row&& src) {
    apply_monotonically(s, std::move(src));
}

void deletable_row::apply_monotonically(const schema& s, deletable_row&& src) {
    _cells.apply(s, column_kind::regular_column, std::move(src._cells));
    _marker.apply(src._marker);
    _deleted_at.apply(src._deleted_at, _marker);
}

bool
rows_entry::equal(const schema& s, const rows_entry& other) const {
    return equal(s, other, s);
}

bool
rows_entry::equal(const schema& s, const rows_entry& other, const schema& other_schema) const {
    position_in_partition::equal_compare eq(s);
    return eq(position(), other.position())
           && row().equal(column_kind::regular_column, s, other.row(), other_schema);
}

bool mutation_partition::equal(const schema& s, const mutation_partition& p) const {
    return equal(s, p, s);
}

bool mutation_partition::equal(const schema& this_schema, const mutation_partition& p, const schema& p_schema) const {
    if (_tombstone != p._tombstone) {
        return false;
    }

    if (!boost::equal(non_dummy_rows(), p.non_dummy_rows(),
        [&] (const rows_entry& e1, const rows_entry& e2) {
            return e1.equal(this_schema, e2, p_schema);
        }
    )) {
        return false;
    }

    if (!std::equal(_row_tombstones.begin(), _row_tombstones.end(),
        p._row_tombstones.begin(), p._row_tombstones.end(),
        [&] (const range_tombstone& rt1, const range_tombstone& rt2) { return rt1.equal(this_schema, rt2); }
    )) {
        return false;
    }

    return _static_row.equal(column_kind::static_column, this_schema, p._static_row, p_schema);
}

bool mutation_partition::equal_continuity(const schema& s, const mutation_partition& p) const {
    return _static_row_continuous == p._static_row_continuous
        && get_continuity(s).equals(s, p.get_continuity(s));
}

mutation_partition mutation_partition::sliced(const schema& s, const query::clustering_row_ranges& ranges) const {
    auto p = mutation_partition(*this, s, ranges);
    p.row_tombstones().trim(s, ranges);
    return p;
}

static
void
apply_monotonically(const column_definition& def, cell_and_hash& dst,
                    atomic_cell_or_collection& src, cell_hash_opt src_hash) {
    // Must be run via with_linearized_managed_bytes() context, but assume it is
    // provided via an upper layer
    if (def.is_atomic()) {
        if (def.is_counter()) {
            counter_cell_view::apply(def, dst.cell, src); // FIXME: Optimize
            dst.hash = { };
        } else if (compare_atomic_cell_for_merge(dst.cell.as_atomic_cell(def), src.as_atomic_cell(def)) < 0) {
            using std::swap;
            swap(dst.cell, src);
            dst.hash = std::move(src_hash);
        }
    } else {
        auto ct = static_pointer_cast<const collection_type_impl>(def.type);
        dst.cell = ct->merge(dst.cell.as_collection_mutation(), src.as_collection_mutation());
        dst.hash = { };
    }
}

void
row::apply(const column_definition& column, const atomic_cell_or_collection& value, cell_hash_opt hash) {
    auto tmp = value.copy(*column.type);
    apply_monotonically(column, std::move(tmp), std::move(hash));
}

void
row::apply(const column_definition& column, atomic_cell_or_collection&& value, cell_hash_opt hash) {
    apply_monotonically(column, std::move(value), std::move(hash));
}

template<typename Func>
void row::consume_with(Func&& func) {
    if (_type == storage_type::vector) {
        unsigned i = 0;
        for (; i < _storage.vector.v.size(); i++) {
            if (_storage.vector.present.test(i)) {
                func(i, _storage.vector.v[i]);
                _storage.vector.present.reset(i);
                --_size;
            }
        }
    } else {
        auto del = current_deleter<cell_entry>();
        auto i = _storage.set.begin();
        while (i != _storage.set.end()) {
            func(i->id(), i->get_cell_and_hash());
            i = _storage.set.erase_and_dispose(i, del);
            --_size;
        }
    }
}

void
row::apply_monotonically(const column_definition& column, atomic_cell_or_collection&& value, cell_hash_opt hash) {
    static_assert(std::is_nothrow_move_constructible<atomic_cell_or_collection>::value
                  && std::is_nothrow_move_assignable<atomic_cell_or_collection>::value,
                  "noexcept required for atomicity");

    // our mutations are not yet immutable
    auto id = column.id;
    if (_type == storage_type::vector && id < max_vector_size) {
        if (id >= _storage.vector.v.size()) {
            _storage.vector.v.resize(id);
            _storage.vector.v.emplace_back(std::move(value), std::move(hash));
            _storage.vector.present.set(id);
            _size++;
        } else if (auto& cell_and_hash = _storage.vector.v[id]; !bool(cell_and_hash.cell)) {
            cell_and_hash = { std::move(value), std::move(hash) };
            _storage.vector.present.set(id);
            _size++;
        } else {
            ::apply_monotonically(column, cell_and_hash, value, std::move(hash));
        }
    } else {
        if (_type == storage_type::vector) {
            vector_to_set();
        }
        auto i = _storage.set.lower_bound(id, cell_entry::compare());
        if (i == _storage.set.end() || i->id() != id) {
            cell_entry* e = current_allocator().construct<cell_entry>(id);
            _storage.set.insert(i, *e);
            _size++;
            e->_cell_and_hash = { std::move(value), std::move(hash) };
        } else {
            ::apply_monotonically(column, i->_cell_and_hash, value, std::move(hash));
        }
    }
}

void
row::append_cell(column_id id, atomic_cell_or_collection value) {
    if (_type == storage_type::vector && id < max_vector_size) {
        _storage.vector.v.resize(id);
        _storage.vector.v.emplace_back(cell_and_hash{std::move(value), cell_hash_opt()});
        _storage.vector.present.set(id);
    } else {
        if (_type == storage_type::vector) {
            vector_to_set();
        }
        auto e = current_allocator().construct<cell_entry>(id, std::move(value));
        _storage.set.insert(_storage.set.end(), *e);
    }
    _size++;
}

const cell_and_hash*
row::find_cell_and_hash(column_id id) const {
    if (_type == storage_type::vector) {
        if (id >= _storage.vector.v.size() || !_storage.vector.present.test(id)) {
            return nullptr;
        }
        return &_storage.vector.v[id];
    } else {
        auto i = _storage.set.find(id, cell_entry::compare());
        if (i == _storage.set.end()) {
            return nullptr;
        }
        return &i->get_cell_and_hash();
    }
}

const atomic_cell_or_collection*
row::find_cell(column_id id) const {
    auto c_a_h = find_cell_and_hash(id);
    return c_a_h ? &c_a_h->cell : nullptr;
}

size_t row::external_memory_usage(const schema& s, column_kind kind) const {
    size_t mem = 0;
    if (_type == storage_type::vector) {
        mem += _storage.vector.v.used_space_external_memory_usage();
        column_id id = 0;
        for (auto&& c_a_h : _storage.vector.v) {
            auto& cdef = s.column_at(kind, id++);
            mem += c_a_h.cell.external_memory_usage(*cdef.type);
        }
    } else {
        for (auto&& ce : _storage.set) {
            auto& cdef = s.column_at(kind, ce.id());
            mem += sizeof(cell_entry) + ce.cell().external_memory_usage(*cdef.type);
        }
    }
    return mem;
}

size_t rows_entry::memory_usage(const schema& s) const {
    size_t size = 0;
    if (!dummy()) {
        size += key().external_memory_usage();
    }
    return size +
           row().cells().external_memory_usage(s, column_kind::regular_column) +
           sizeof(rows_entry);
}

size_t mutation_partition::external_memory_usage(const schema& s) const {
    size_t sum = 0;
    sum += static_row().external_memory_usage(s, column_kind::static_column);
    for (auto& clr : clustered_rows()) {
        sum += clr.memory_usage(s);
    }

    for (auto& rtb : row_tombstones()) {
        sum += rtb.memory_usage(s);
    }

    return sum;
}

template<bool reversed, typename Func>
void mutation_partition::trim_rows(const schema& s,
    const std::vector<query::clustering_range>& row_ranges,
    Func&& func)
{
    static_assert(std::is_same<stop_iteration, std::result_of_t<Func(rows_entry&)>>::value, "Bad func signature");

    stop_iteration stop = stop_iteration::no;
    auto last = reversal_traits<reversed>::begin(_rows);
    auto deleter = current_deleter<rows_entry>();

    auto range_begin = [this, &s] (const query::clustering_range& range) {
        return reversed ? upper_bound(s, range) : lower_bound(s, range);
    };

    auto range_end = [this, &s] (const query::clustering_range& range) {
        return reversed ? lower_bound(s, range) : upper_bound(s, range);
    };

    for (auto&& row_range : row_ranges) {
        if (stop) {
            break;
        }

        last = reversal_traits<reversed>::erase_and_dispose(_rows, last,
            reversal_traits<reversed>::maybe_reverse(_rows, range_begin(row_range)), deleter);

        auto end = reversal_traits<reversed>::maybe_reverse(_rows, range_end(row_range));
        while (last != end && !stop) {
            rows_entry& e = *last;
            stop = func(e);
            if (e.empty()) {
                last = reversal_traits<reversed>::erase_dispose_and_update_end(_rows, last, deleter, end);
            } else {
                ++last;
            }
        }
    }

    reversal_traits<reversed>::erase_and_dispose(_rows, last, reversal_traits<reversed>::end(_rows), deleter);
}

uint32_t mutation_partition::do_compact(const schema& s,
    gc_clock::time_point query_time,
    const std::vector<query::clustering_range>& row_ranges,
    bool reverse,
    uint32_t row_limit,
    can_gc_fn& can_gc)
{
    assert(row_limit > 0);

    auto gc_before = saturating_subtract(query_time, s.gc_grace_seconds());

    auto should_purge_tombstone = [&] (const tombstone& t) {
        return t.deletion_time < gc_before && can_gc(t);
    };
    auto should_purge_row_tombstone = [&] (const row_tombstone& t) {
        return t.max_deletion_time() < gc_before && can_gc(t.tomb());
    };

    bool static_row_live = _static_row.compact_and_expire(s, column_kind::static_column, row_tombstone(_tombstone),
        query_time, can_gc, gc_before);

    uint32_t row_count = 0;

    auto row_callback = [&] (rows_entry& e) {
        if (e.dummy()) {
            return stop_iteration::no;
        }
        deletable_row& row = e.row();
        row_tombstone tomb = tombstone_for_row(s, e);

        bool is_live = row.marker().compact_and_expire(tomb.tomb(), query_time, can_gc, gc_before);
        is_live |= row.cells().compact_and_expire(s, column_kind::regular_column, tomb, query_time, can_gc, gc_before, row.marker());

        if (should_purge_row_tombstone(row.deleted_at())) {
            row.remove_tombstone();
        }

        return stop_iteration(is_live && ++row_count == row_limit);
    };

    if (reverse) {
        trim_rows<true>(s, row_ranges, row_callback);
    } else {
        trim_rows<false>(s, row_ranges, row_callback);
    }

    // #589 - Do not add extra row for statics unless we did a CK range-less query.
    // See comment in query
    if (row_count == 0 && static_row_live && !has_ck_selector(row_ranges)) {
        ++row_count;
    }

    _row_tombstones.erase_where([&] (auto&& rt) {
        return should_purge_tombstone(rt.tomb) || rt.tomb <= _tombstone;
    });
    if (should_purge_tombstone(_tombstone)) {
        _tombstone = tombstone();
    }

    // FIXME: purge unneeded prefix tombstones based on row_ranges

    return row_count;
}

uint32_t
mutation_partition::compact_for_query(
    const schema& s,
    gc_clock::time_point query_time,
    const std::vector<query::clustering_range>& row_ranges,
    bool reverse,
    uint32_t row_limit)
{
    return do_compact(s, query_time, row_ranges, reverse, row_limit, always_gc);
}

void mutation_partition::compact_for_compaction(const schema& s,
    can_gc_fn& can_gc, gc_clock::time_point compaction_time)
{
    static const std::vector<query::clustering_range> all_rows = {
        query::clustering_range::make_open_ended_both_sides()
    };

    do_compact(s, compaction_time, all_rows, false, query::max_rows, can_gc);
}

// Returns true if the mutation_partition represents no writes.
bool mutation_partition::empty() const
{
    if (_tombstone.timestamp != api::missing_timestamp) {
        return false;
    }
    return !_static_row.size() && _rows.empty() && _row_tombstones.empty();
}

bool
deletable_row::is_live(const schema& s, tombstone base_tombstone, gc_clock::time_point query_time) const {
    // _created_at corresponds to the row marker cell, present for rows
    // created with the 'insert' statement. If row marker is live, we know the
    // row is live. Otherwise, a row is considered live if it has any cell
    // which is live.
    base_tombstone.apply(_deleted_at.tomb());
    return _marker.is_live(base_tombstone, query_time) || _cells.is_live(s, column_kind::regular_column, base_tombstone, query_time);
}

bool
row::is_live(const schema& s, column_kind kind, tombstone base_tombstone, gc_clock::time_point query_time) const {
    return has_any_live_data(s, kind, *this, base_tombstone, query_time);
}

bool
mutation_partition::is_static_row_live(const schema& s, gc_clock::time_point query_time) const {
    return has_any_live_data(s, column_kind::static_column, static_row(), _tombstone, query_time);
}

size_t
mutation_partition::live_row_count(const schema& s, gc_clock::time_point query_time) const {
    size_t count = 0;

    for (const rows_entry& e : non_dummy_rows()) {
        tombstone base_tombstone = range_tombstone_for_row(s, e.key());
        if (e.row().is_live(s, base_tombstone, query_time)) {
            ++count;
        }
    }

    if (count == 0 && is_static_row_live(s, query_time)) {
        return 1;
    }

    return count;
}

rows_entry::rows_entry(rows_entry&& o) noexcept
    : _link(std::move(o._link))
    , _key(std::move(o._key))
    , _row(std::move(o._row))
    , _lru_link()
    , _flags(std::move(o._flags))
{
    if (o._lru_link.is_linked()) {
        auto prev = o._lru_link.prev_;
        o._lru_link.unlink();
        cache_tracker::lru_type::node_algorithms::link_after(prev, _lru_link.this_ptr());
    }
}

row::row(const schema& s, column_kind kind, const row& o)
    : _type(o._type)
    , _size(o._size)
{
    if (_type == storage_type::vector) {
        auto& other_vec = o._storage.vector;
        auto& vec = *new (&_storage.vector) vector_storage;
        try {
            vec.present = other_vec.present;
            vec.v.reserve(other_vec.v.size());
            column_id id = 0;
            for (auto& cell : other_vec.v) {
                auto& cdef = s.column_at(kind, id++);
                vec.v.emplace_back(cell_and_hash{cell.cell.copy(*cdef.type), cell.hash});
            }
        } catch (...) {
            _storage.vector.~vector_storage();
            throw;
        }
    } else {
        auto cloner = [&] (const auto& x) {
            auto& cdef = s.column_at(kind, x.id());
            return current_allocator().construct<cell_entry>(*cdef.type, x);
        };
        new (&_storage.set) map_type;
        try {
            _storage.set.clone_from(o._storage.set, cloner, current_deleter<cell_entry>());
        } catch (...) {
            _storage.set.~map_type();
            throw;
        }
    }
}

row::~row() {
    if (_type == storage_type::vector) {
        _storage.vector.~vector_storage();
    } else {
        _storage.set.clear_and_dispose(current_deleter<cell_entry>());
        _storage.set.~map_type();
    }
}

row::cell_entry::cell_entry(const abstract_type& type, const cell_entry& o)
    : _id(o._id)
    , _cell_and_hash{ o._cell_and_hash.cell.copy(type), o._cell_and_hash.hash }
{ }

row::cell_entry::cell_entry(cell_entry&& o) noexcept
    : _link()
    , _id(o._id)
    , _cell_and_hash(std::move(o._cell_and_hash))
{
    using container_type = row::map_type;
    container_type::node_algorithms::replace_node(o._link.this_ptr(), _link.this_ptr());
    container_type::node_algorithms::init(o._link.this_ptr());
}

const atomic_cell_or_collection& row::cell_at(column_id id) const {
    auto&& cell = find_cell(id);
    if (!cell) {
        throw_with_backtrace<std::out_of_range>(format("Column not found for id = {:d}", id));
    }
    return *cell;
}

void row::vector_to_set()
{
    assert(_type == storage_type::vector);
    map_type set;
    try {
    for (auto i : bitsets::for_each_set(_storage.vector.present)) {
        auto& c_a_h = _storage.vector.v[i];
        auto e = current_allocator().construct<cell_entry>(i, std::move(c_a_h));
        set.insert(set.end(), *e);
    }
    } catch (...) {
        set.clear_and_dispose([this, del = current_deleter<cell_entry>()] (cell_entry* ce) noexcept {
            _storage.vector.v[ce->id()] = std::move(ce->get_cell_and_hash());
            del(ce);
        });
        throw;
    }
    _storage.vector.~vector_storage();
    new (&_storage.set) map_type(std::move(set));
    _type = storage_type::set;
}

void row::reserve(column_id last_column)
{
    if (_type == storage_type::vector && last_column >= internal_count) {
        if (last_column >= max_vector_size) {
            vector_to_set();
        } else {
            _storage.vector.v.reserve(last_column);
        }
    }
}

template<typename Func>
auto row::with_both_ranges(const row& other, Func&& func) const {
    if (_type == storage_type::vector) {
        if (other._type == storage_type::vector) {
            return func(get_range_vector(), other.get_range_vector());
        } else {
            return func(get_range_vector(), other.get_range_set());
        }
    } else {
        if (other._type == storage_type::vector) {
            return func(get_range_set(), other.get_range_vector());
        } else {
            return func(get_range_set(), other.get_range_set());
        }
    }
}

bool row::equal(column_kind kind, const schema& this_schema, const row& other, const schema& other_schema) const {
    if (size() != other.size()) {
        return false;
    }

    auto cells_equal = [&] (std::pair<column_id, const atomic_cell_or_collection&> c1,
                            std::pair<column_id, const atomic_cell_or_collection&> c2) {
        static_assert(schema::row_column_ids_are_ordered_by_name::value, "Relying on column ids being ordered by name");
        auto& at1 = *this_schema.column_at(kind, c1.first).type;
        auto& at2 = other_schema.column_at(kind, c2.first).type;
        return at1.equals(at2)
               && this_schema.column_at(kind, c1.first).name() == other_schema.column_at(kind, c2.first).name()
               && c1.second.equals(at1, c2.second);
    };
    return with_both_ranges(other, [&] (auto r1, auto r2) {
        return boost::equal(r1, r2, cells_equal);
    });
}

row::row() {
    new (&_storage.vector) vector_storage;
}

row::row(row&& other) noexcept
    : _type(other._type), _size(other._size) {
    if (_type == storage_type::vector) {
        new (&_storage.vector) vector_storage(std::move(other._storage.vector));
    } else {
        new (&_storage.set) map_type(std::move(other._storage.set));
    }
    other._size = 0;
}

row& row::operator=(row&& other) noexcept {
    if (this != &other) {
        this->~row();
        new (this) row(std::move(other));
    }
    return *this;
}

void row::apply(const schema& s, column_kind kind, const row& other) {
    if (other.empty()) {
        return;
    }
    if (other._type == storage_type::vector) {
        reserve(other._storage.vector.v.size() - 1);
    } else {
        reserve(other._storage.set.rbegin()->id());
    }
    other.for_each_cell([&] (column_id id, const cell_and_hash& c_a_h) {
        apply(s.column_at(kind, id), c_a_h.cell, c_a_h.hash);
    });
}

void row::apply(const schema& s, column_kind kind, row&& other) {
    apply_monotonically(s, kind, std::move(other));
}

void row::apply_monotonically(const schema& s, column_kind kind, row&& other) {
    if (other.empty()) {
        return;
    }
    if (other._type == storage_type::vector) {
        reserve(other._storage.vector.v.size() - 1);
    } else {
        reserve(other._storage.set.rbegin()->id());
    }
    other.consume_with([&] (column_id id, cell_and_hash& c_a_h) {
        apply_monotonically(s.column_at(kind, id), std::move(c_a_h.cell), std::move(c_a_h.hash));
    });
}

// When views contain a primary key column that is not part of the base table primary key,
// that column determines whether the row is live or not. We need to ensure that when that
// cell is dead, and thus the derived row marker, either by normal deletion of by TTL, so
// is the rest of the row. To ensure that none of the regular columns keep the row alive,
// we erase the live cells according to the shadowable_tombstone rules.
static bool dead_marker_shadows_row(const schema& s, column_kind kind, const row_marker& marker) {
    return s.is_view()
            && s.view_info()->base_non_pk_column_in_view_pk()
            && !marker.is_live()
            && kind == column_kind::regular_column; // not applicable to static rows
}

bool row::compact_and_expire(
        const schema& s,
        column_kind kind,
        row_tombstone tomb,
        gc_clock::time_point query_time,
        can_gc_fn& can_gc,
        gc_clock::time_point gc_before,
        const row_marker& marker)
{
    if (dead_marker_shadows_row(s, kind, marker)) {
        tomb.apply(shadowable_tombstone(api::max_timestamp, gc_clock::time_point::max()), row_marker());
    }
    bool any_live = false;
    remove_if([&] (column_id id, atomic_cell_or_collection& c) {
        bool erase = false;
        const column_definition& def = s.column_at(kind, id);
        if (def.is_atomic()) {
            atomic_cell_view cell = c.as_atomic_cell(def);
            auto can_erase_cell = [&] {
                return cell.deletion_time() < gc_before && can_gc(tombstone(cell.timestamp(), cell.deletion_time()));
            };

            if (cell.is_covered_by(tomb.regular(), def.is_counter())) {
                erase = true;
            } else if (cell.has_expired(query_time)) {
                erase = can_erase_cell();
                if (!erase) {
                    c = atomic_cell::make_dead(cell.timestamp(), cell.deletion_time());
                }
            } else if (!cell.is_live()) {
                erase = can_erase_cell();
            } else if (cell.is_covered_by(tomb.shadowable().tomb(), def.is_counter())) {
                erase = true;
            } else {
                any_live = true;
            }
        } else {
            auto&& cell = c.as_collection_mutation();
            auto&& ctype = static_pointer_cast<const collection_type_impl>(def.type);
          cell.data.with_linearized([&] (bytes_view cell_bv) {
            auto m_view = ctype->deserialize_mutation_form(cell_bv);
            collection_type_impl::mutation m = m_view.materialize(*ctype);
            any_live |= m.compact_and_expire(tomb, query_time, can_gc, gc_before);
            if (m.cells.empty() && m.tomb <= tomb.tomb()) {
                erase = true;
            } else {
                c = ctype->serialize_mutation_form(m);
            }
          });
        }
        return erase;
    });
    return any_live;
}

bool row::compact_and_expire(
        const schema& s,
        column_kind kind,
        row_tombstone tomb,
        gc_clock::time_point query_time,
        can_gc_fn& can_gc,
        gc_clock::time_point gc_before) {
    row_marker m;
    return compact_and_expire(s, kind, tomb, query_time, can_gc, gc_before, m);
}

deletable_row deletable_row::difference(const schema& s, column_kind kind, const deletable_row& other) const
{
    deletable_row dr;
    if (_deleted_at > other._deleted_at) {
        dr.apply(_deleted_at);
    }
    if (compare_row_marker_for_merge(_marker, other._marker) > 0) {
        dr.apply(_marker);
    }
    dr._cells = _cells.difference(s, kind, other._cells);
    return dr;
}

row row::difference(const schema& s, column_kind kind, const row& other) const
{
    row r;
    with_both_ranges(other, [&] (auto this_range, auto other_range) {
        auto it = other_range.begin();
        for (auto&& c : this_range) {
            while (it != other_range.end() && it->first < c.first) {
                ++it;
            }
            auto& cdef = s.column_at(kind, c.first);
            if (it == other_range.end() || it->first != c.first) {
                r.append_cell(c.first, c.second.copy(*cdef.type));
            } else if (cdef.is_counter()) {
                auto cell = counter_cell_view::difference(c.second.as_atomic_cell(cdef), it->second.as_atomic_cell(cdef));
                if (cell) {
                    r.append_cell(c.first, std::move(*cell));
                }
            } else if (s.column_at(kind, c.first).is_atomic()) {
                if (compare_atomic_cell_for_merge(c.second.as_atomic_cell(cdef), it->second.as_atomic_cell(cdef)) > 0) {
                    r.append_cell(c.first, c.second.copy(*cdef.type));
                }
            } else {
                auto ct = static_pointer_cast<const collection_type_impl>(s.column_at(kind, c.first).type);
                auto diff = ct->difference(c.second.as_collection_mutation(), it->second.as_collection_mutation());
                if (!ct->is_empty(diff)) {
                    r.append_cell(c.first, std::move(diff));
                }
            }
        }
    });
    return r;
}

mutation_partition mutation_partition::difference(schema_ptr s, const mutation_partition& other) const
{
    mutation_partition mp(s);
    if (_tombstone > other._tombstone) {
        mp.apply(_tombstone);
    }
    mp._static_row = _static_row.difference(*s, column_kind::static_column, other._static_row);

    mp._row_tombstones = _row_tombstones.difference(*s, other._row_tombstones);

    auto it_r = other._rows.begin();
    rows_entry::compare cmp_r(*s);
    for (auto&& r : _rows) {
        if (r.dummy()) {
            continue;
        }
        while (it_r != other._rows.end() && (it_r->dummy() || cmp_r(*it_r, r))) {
            ++it_r;
        }
        if (it_r == other._rows.end() || !it_r->key().equal(*s, r.key())) {
            mp.insert_row(*s, r.key(), r.row());
        } else {
            auto dr = r.row().difference(*s, column_kind::regular_column, it_r->row());
            if (!dr.empty()) {
                mp.insert_row(*s, r.key(), std::move(dr));
            }
        }
    }
    return mp;
}

void mutation_partition::accept(const schema& s, mutation_partition_visitor& v) const {
    v.accept_partition_tombstone(_tombstone);
    _static_row.for_each_cell([&] (column_id id, const atomic_cell_or_collection& cell) {
        const column_definition& def = s.static_column_at(id);
        if (def.is_atomic()) {
            v.accept_static_cell(id, cell.as_atomic_cell(def));
        } else {
            v.accept_static_cell(id, cell.as_collection_mutation());
        }
    });
    for (const range_tombstone& rt : _row_tombstones) {
        v.accept_row_tombstone(rt);
    }
    for (const rows_entry& e : _rows) {
        const deletable_row& dr = e.row();
        v.accept_row(e.position(), dr.deleted_at(), dr.marker(), e.dummy(), e.continuous());
        dr.cells().for_each_cell([&] (column_id id, const atomic_cell_or_collection& cell) {
            const column_definition& def = s.regular_column_at(id);
            if (def.is_atomic()) {
                v.accept_row_cell(id, cell.as_atomic_cell(def));
            } else {
                v.accept_row_cell(id, cell.as_collection_mutation());
            }
        });
    }
}

void
mutation_partition::upgrade(const schema& old_schema, const schema& new_schema) {
    // We need to copy to provide strong exception guarantees.
    mutation_partition tmp(new_schema.shared_from_this());
    tmp.set_static_row_continuous(_static_row_continuous);
    converting_mutation_partition_applier v(old_schema.get_column_mapping(), new_schema, tmp);
    accept(old_schema, v);
    *this = std::move(tmp);
}

// Adds mutation to query::result.
class mutation_querier {
    const schema& _schema;
    query::result_memory_accounter& _memory_accounter;
    query::result::partition_writer& _pw;
    ser::qr_partition__static_row__cells<bytes_ostream> _static_cells_wr;
    bool _live_data_in_static_row{};
    uint32_t _live_clustering_rows = 0;
    std::optional<ser::qr_partition__rows<bytes_ostream>> _rows_wr;
    bool _short_reads_allowed;
private:
    void query_static_row(const row& r, tombstone current_tombstone);
    void prepare_writers();
public:
    mutation_querier(const schema& s, query::result::partition_writer& pw,
                     query::result_memory_accounter& memory_accounter);
    void consume(tombstone) { }
    // Requires that sr.has_any_live_data()
    stop_iteration consume(static_row&& sr, tombstone current_tombstone);
    // Requires that cr.has_any_live_data()
    stop_iteration consume(clustering_row&& cr, row_tombstone current_tombstone);
    stop_iteration consume(range_tombstone&&) { return stop_iteration::no; }
    uint32_t consume_end_of_stream();
};

mutation_querier::mutation_querier(const schema& s, query::result::partition_writer& pw,
                                   query::result_memory_accounter& memory_accounter)
    : _schema(s)
    , _memory_accounter(memory_accounter)
    , _pw(pw)
    , _static_cells_wr(pw.start().start_static_row().start_cells())
    , _short_reads_allowed(pw.slice().options.contains<query::partition_slice::option::allow_short_read>())
{
}

void mutation_querier::query_static_row(const row& r, tombstone current_tombstone)
{
    const query::partition_slice& slice = _pw.slice();
    if (!slice.static_columns.empty()) {
        if (_pw.requested_result()) {
            auto start = _static_cells_wr._out.size();
            get_compacted_row_slice(_schema, slice, column_kind::static_column,
                                    r, slice.static_columns, _static_cells_wr);
            _memory_accounter.update(_static_cells_wr._out.size() - start);
        } else if (_short_reads_allowed) {
            seastar::measuring_output_stream stream;
            ser::qr_partition__static_row__cells<seastar::measuring_output_stream> out(stream, { });
            auto start = stream.size();
            get_compacted_row_slice(_schema, slice, column_kind::static_column,
                                    r, slice.static_columns, out);
            _memory_accounter.update(stream.size() - start);
        }
        if (_pw.requested_digest()) {
            max_timestamp max_ts{_pw.last_modified()};
            _pw.digest().feed_hash(current_tombstone);
            max_ts.update(current_tombstone.timestamp);
            _pw.digest().feed_hash(r, _schema, column_kind::static_column, slice.static_columns, max_ts);
            _pw.last_modified() = max_ts.max;
        }
    }
    _rows_wr.emplace(std::move(_static_cells_wr).end_cells().end_static_row().start_rows());
}

stop_iteration mutation_querier::consume(static_row&& sr, tombstone current_tombstone) {
    query_static_row(sr.cells(), current_tombstone);
    _live_data_in_static_row = true;
    return stop_iteration::no;
}

void mutation_querier::prepare_writers() {
    if (!_rows_wr) {
        row empty_row;
        query_static_row(empty_row, { });
        _live_data_in_static_row = false;
    }
}

stop_iteration mutation_querier::consume(clustering_row&& cr, row_tombstone current_tombstone) {
    prepare_writers();

    const query::partition_slice& slice = _pw.slice();

    if (_pw.requested_digest()) {
        _pw.digest().feed_hash(cr.key(), _schema);
        _pw.digest().feed_hash(current_tombstone);
        max_timestamp max_ts{_pw.last_modified()};
        max_ts.update(current_tombstone.tomb().timestamp);
        _pw.digest().feed_hash(cr.cells(), _schema, column_kind::regular_column, slice.regular_columns, max_ts);
        _pw.last_modified() = max_ts.max;
    }

    auto write_row = [&] (auto& rows_writer) {
        auto cells_wr = [&] {
            if (slice.options.contains(query::partition_slice::option::send_clustering_key)) {
                return rows_writer.add().write_key(cr.key()).start_cells().start_cells();
            } else {
                return rows_writer.add().skip_key().start_cells().start_cells();
            }
        }();
        get_compacted_row_slice(_schema, slice, column_kind::regular_column, cr.cells(), slice.regular_columns, cells_wr);
        std::move(cells_wr).end_cells().end_cells().end_qr_clustered_row();
    };

    auto stop = stop_iteration::no;
    if (_pw.requested_result()) {
        auto start = _rows_wr->_out.size();
        write_row(*_rows_wr);
        stop = _memory_accounter.update_and_check(_rows_wr->_out.size() - start);
    } else if (_short_reads_allowed) {
        seastar::measuring_output_stream stream;
        ser::qr_partition__rows<seastar::measuring_output_stream> out(stream, { });
        auto start = stream.size();
        write_row(out);
        stop = _memory_accounter.update_and_check(stream.size() - start);
    }

    _live_clustering_rows++;
    return stop && stop_iteration(_short_reads_allowed);
}

uint32_t mutation_querier::consume_end_of_stream() {
    prepare_writers();

    // If we got no rows, but have live static columns, we should only
    // give them back IFF we did not have any CK restrictions.
    // #589
    // If ck:s exist, and we do a restriction on them, we either have maching
    // rows, or return nothing, since cql does not allow "is null".
    if (!_live_clustering_rows
        && (has_ck_selector(_pw.ranges()) || !_live_data_in_static_row)) {
        _pw.retract();
        return 0;
    } else {
        auto live_rows = std::max(_live_clustering_rows, uint32_t(1));
        _pw.row_count() += live_rows;
        _pw.partition_count() += 1;
        std::move(*_rows_wr).end_rows().end_qr_partition();
        return live_rows;
    }
}

class query_result_builder {
    const schema& _schema;
    query::result::builder& _rb;
    std::optional<query::result::partition_writer> _pw;
    std::optional<mutation_querier> _mutation_consumer;
    stop_iteration _stop;
    stop_iteration _short_read_allowed;
public:
    query_result_builder(const schema& s, query::result::builder& rb)
        : _schema(s), _rb(rb)
        , _short_read_allowed(_rb.slice().options.contains<query::partition_slice::option::allow_short_read>())
    { }

    void consume_new_partition(const dht::decorated_key& dk) {
        _pw.emplace(_rb.add_partition(_schema, dk.key()));
        _mutation_consumer.emplace(mutation_querier(_schema, *_pw, _rb.memory_accounter()));
    }

    void consume(tombstone t) {
        _mutation_consumer->consume(t);
    }
    stop_iteration consume(static_row&& sr, tombstone t, bool) {
        _stop = _mutation_consumer->consume(std::move(sr), t) && _short_read_allowed;
        return _stop;
    }
    stop_iteration consume(clustering_row&& cr, row_tombstone t,  bool) {
        _stop = _mutation_consumer->consume(std::move(cr), t) && _short_read_allowed;
        return _stop;
    }
    stop_iteration consume(range_tombstone&& rt) {
        _stop = _mutation_consumer->consume(std::move(rt)) && _short_read_allowed;
        return _stop;
    }

    stop_iteration consume_end_of_partition() {
        auto live_rows_in_partition = _mutation_consumer->consume_end_of_stream();
        if (_short_read_allowed && live_rows_in_partition > 0 && !_stop) {
            _stop = _rb.memory_accounter().check();
        }
        if (_stop) {
            _rb.mark_as_short_read();
        }
        return _stop;
    }

    void consume_end_of_stream() {
    }
};

future<> data_query(
        schema_ptr s,
        const mutation_source& source,
        const dht::partition_range& range,
        const query::partition_slice& slice,
        uint32_t row_limit,
        uint32_t partition_limit,
        gc_clock::time_point query_time,
        query::result::builder& builder,
        tracing::trace_state_ptr trace_ptr,
        db::timeout_clock::time_point timeout,
        query::querier_cache_context cache_ctx)
{
    if (row_limit == 0 || slice.partition_row_limit() == 0 || partition_limit == 0) {
        return make_ready_future<>();
    }

    auto querier_opt = cache_ctx.lookup_data_querier(*s, range, slice, trace_ptr);
    auto q = querier_opt
            ? std::move(*querier_opt)
            : query::data_querier(source, s, range, slice, service::get_local_sstable_query_read_priority(), trace_ptr);

    return do_with(std::move(q), [=, &builder, trace_ptr = std::move(trace_ptr), cache_ctx = std::move(cache_ctx)] (query::data_querier& q) mutable {
        auto qrb = query_result_builder(*s, builder);
        return q.consume_page(std::move(qrb), row_limit, partition_limit, query_time, timeout).then(
                [=, &builder, &q, trace_ptr = std::move(trace_ptr), cache_ctx = std::move(cache_ctx)] () mutable {
            if (q.are_limits_reached() || builder.is_short_read()) {
                cache_ctx.insert(std::move(q), std::move(trace_ptr));
            }
        });
    });
}

void reconcilable_result_builder::consume_new_partition(const dht::decorated_key& dk) {
    _has_ck_selector = has_ck_selector(_slice.row_ranges(_schema, dk.key()));
    _static_row_is_alive = false;
    _live_rows = 0;
    auto is_reversed = _slice.options.contains(query::partition_slice::option::reversed);
    _mutation_consumer.emplace(streamed_mutation_freezer(_schema, dk.key(), is_reversed));
}

void reconcilable_result_builder::consume(tombstone t) {
    _mutation_consumer->consume(t);
}

stop_iteration reconcilable_result_builder::consume(static_row&& sr, tombstone, bool is_alive) {
    _static_row_is_alive = is_alive;
    _memory_accounter.update(sr.memory_usage(_schema));
    return _mutation_consumer->consume(std::move(sr));
}

stop_iteration reconcilable_result_builder::consume(clustering_row&& cr, row_tombstone, bool is_alive) {
    _live_rows += is_alive;
    auto stop = _memory_accounter.update_and_check(cr.memory_usage(_schema));
    if (is_alive) {
        // We are considering finishing current read only after consuming a
        // live clustering row. While sending a single live row is enough to
        // guarantee progress, not ending the result on a live row would
        // mean that the next page fetch will read all tombstones after the
        // last live row again.
        _stop = stop && stop_iteration(_short_read_allowed);
    }
    return _mutation_consumer->consume(std::move(cr)) || _stop;
}

stop_iteration reconcilable_result_builder::consume(range_tombstone&& rt) {
    _memory_accounter.update(rt.memory_usage(_schema));
    return _mutation_consumer->consume(std::move(rt));
}

stop_iteration reconcilable_result_builder::consume_end_of_partition() {
    if (_live_rows == 0 && _static_row_is_alive && !_has_ck_selector) {
        ++_live_rows;
        // Normally we count only live clustering rows, to guarantee that
        // the next page fetch won't ask for the same range. However,
        // if we return just a single static row we can stop the result as
        // well. Next page fetch will ask for the next partition and if we
        // don't do that we could end up with an unbounded number of
        // partitions with only a static row.
        _stop = _stop || (_memory_accounter.check() && stop_iteration(_short_read_allowed));
    }
    _total_live_rows += _live_rows;
    _result.emplace_back(partition { _live_rows, _mutation_consumer->consume_end_of_stream() });
    return _stop;
}

reconcilable_result reconcilable_result_builder::consume_end_of_stream() {
    return reconcilable_result(_total_live_rows, std::move(_result),
                               query::short_read(bool(_stop)),
                               std::move(_memory_accounter).done());
}

future<reconcilable_result>
static do_mutation_query(schema_ptr s,
               mutation_source source,
               const dht::partition_range& range,
               const query::partition_slice& slice,
               uint32_t row_limit,
               uint32_t partition_limit,
               gc_clock::time_point query_time,
               query::result_memory_accounter&& accounter,
               tracing::trace_state_ptr trace_ptr,
               db::timeout_clock::time_point timeout,
               query::querier_cache_context cache_ctx)
{
    if (row_limit == 0 || slice.partition_row_limit() == 0 || partition_limit == 0) {
        return make_ready_future<reconcilable_result>(reconcilable_result());
    }

    auto querier_opt = cache_ctx.lookup_mutation_querier(*s, range, slice, trace_ptr);
    auto q = querier_opt
            ? std::move(*querier_opt)
            : query::mutation_querier(source, s, range, slice, service::get_local_sstable_query_read_priority(), trace_ptr);

    return do_with(std::move(q), [=, &slice, accounter = std::move(accounter), trace_ptr = std::move(trace_ptr), cache_ctx = std::move(cache_ctx)] (
                query::mutation_querier& q) mutable {
        auto rrb = reconcilable_result_builder(*s, slice, std::move(accounter));
        return q.consume_page(std::move(rrb), row_limit, partition_limit, query_time, timeout).then(
                [=, &q, trace_ptr = std::move(trace_ptr), cache_ctx = std::move(cache_ctx)] (reconcilable_result r) mutable {
            if (q.are_limits_reached() || r.is_short_read()) {
                cache_ctx.insert(std::move(q), std::move(trace_ptr));
            }
            return r;
        });
    });
}

mutation_query_stage::mutation_query_stage()
    : _execution_stage("mutation_query", do_mutation_query)
{}

future<reconcilable_result>
mutation_query(schema_ptr s,
               mutation_source source,
               const dht::partition_range& range,
               const query::partition_slice& slice,
               uint32_t row_limit,
               uint32_t partition_limit,
               gc_clock::time_point query_time,
               query::result_memory_accounter&& accounter,
               tracing::trace_state_ptr trace_ptr,
               db::timeout_clock::time_point timeout,
               query::querier_cache_context cache_ctx)
{
    return do_mutation_query(std::move(s), std::move(source), seastar::cref(range), seastar::cref(slice),
            row_limit, partition_limit, query_time, std::move(accounter), std::move(trace_ptr), timeout, std::move(cache_ctx));
}

deletable_row::deletable_row(clustering_row&& cr)
    : _deleted_at(cr.tomb())
    , _marker(std::move(cr.marker()))
    , _cells(std::move(cr.cells()))
{ }

class counter_write_query_result_builder {
    const schema& _schema;
    mutation_opt _mutation;
public:
    counter_write_query_result_builder(const schema& s) : _schema(s) { }
    void consume_new_partition(const dht::decorated_key& dk) {
        _mutation = mutation(_schema.shared_from_this(), dk);
    }
    void consume(tombstone) { }
    stop_iteration consume(static_row&& sr, tombstone, bool) {
        _mutation->partition().static_row() = std::move(sr.cells());
        return stop_iteration::no;
    }
    stop_iteration consume(clustering_row&& cr, row_tombstone,  bool) {
        _mutation->partition().insert_row(_schema, cr.key(), deletable_row(std::move(cr)));
        return stop_iteration::no;
    }
    stop_iteration consume(range_tombstone&& rt) {
        return stop_iteration::no;
    }
    stop_iteration consume_end_of_partition() {
        return stop_iteration::no;
    }
    mutation_opt consume_end_of_stream() {
        return std::move(_mutation);
    }
};

mutation_partition::mutation_partition(mutation_partition::incomplete_tag, const schema& s, tombstone t)
    : _tombstone(t)
    , _static_row_continuous(!s.has_static_columns())
    , _rows()
    , _row_tombstones(s)
{
    _rows.insert_before(_rows.end(),
        *current_allocator().construct<rows_entry>(s, rows_entry::last_dummy_tag(), is_continuous::no));
}

bool mutation_partition::is_fully_continuous() const {
    if (!_static_row_continuous) {
        return false;
    }
    for (auto&& row : _rows) {
        if (!row.continuous()) {
            return false;
        }
    }
    return true;
}

void mutation_partition::make_fully_continuous() {
    _static_row_continuous = true;
    auto i = _rows.begin();
    while (i != _rows.end()) {
        if (i->dummy()) {
            i = _rows.erase_and_dispose(i, alloc_strategy_deleter<rows_entry>());
        } else {
            i->set_continuous(true);
            ++i;
        }
    }
}

void mutation_partition::set_continuity(const schema& s, const position_range& pr, is_continuous cont) {
    auto less = rows_entry::compare(s);

    if (!less(pr.start(), pr.end())) {
        return; // empty range
    }

    auto end = _rows.lower_bound(pr.end(), less);
    if (end == _rows.end() || less(pr.end(), end->position())) {
        end = _rows.insert_before(end, *current_allocator().construct<rows_entry>(s, pr.end(), is_dummy::yes,
            end == _rows.end() ? is_continuous::yes : end->continuous()));
    }

    auto i = _rows.lower_bound(pr.start(), less);
    if (less(pr.start(), i->position())) {
        i = _rows.insert_before(i, *current_allocator().construct<rows_entry>(s, pr.start(), is_dummy::yes, i->continuous()));
    }

    assert(i != end);
    ++i;

    while (1) {
        i->set_continuous(cont);
        if (i == end) {
            break;
        }
        if (i->dummy()) {
            i = _rows.erase_and_dispose(i, alloc_strategy_deleter<rows_entry>());
        } else {
            ++i;
        }
    }
}

clustering_interval_set mutation_partition::get_continuity(const schema& s, is_continuous cont) const {
    clustering_interval_set result;
    auto i = _rows.begin();
    auto prev_pos = position_in_partition::before_all_clustered_rows();
    while (i != _rows.end()) {
        if (i->continuous() == cont) {
            result.add(s, position_range(std::move(prev_pos), position_in_partition(i->position())));
        }
        if (i->position().is_clustering_row() && bool(i->dummy()) == !bool(cont)) {
            result.add(s, position_range(position_in_partition(i->position()),
                position_in_partition::after_key(i->position().key())));
        }
        prev_pos = i->position().is_clustering_row()
            ? position_in_partition::after_key(i->position().key())
            : position_in_partition(i->position());
        ++i;
    }
    if (cont) {
        result.add(s, position_range(std::move(prev_pos), position_in_partition::after_all_clustered_rows()));
    }
    return result;
}

stop_iteration mutation_partition::clear_gently(cache_tracker* tracker) noexcept {
    if (_row_tombstones.clear_gently() == stop_iteration::no) {
        return stop_iteration::no;
    }

    auto del = current_deleter<rows_entry>();
    auto i = _rows.begin();
    auto end = _rows.end();
    while (i != end) {
        if (tracker) {
            tracker->on_remove(*i);
        }
        i = _rows.erase_and_dispose(i, del);

        // The iterator comparison below is to not defer destruction of now empty
        // mutation_partition objects. Not doing this would cause eviction to leave garbage
        // versions behind unnecessarily.
        if (need_preempt() && i != end) {
            return stop_iteration::no;
        }
    }

    return stop_iteration::yes;
}

bool
mutation_partition::check_continuity(const schema& s, const position_range& r, is_continuous cont) const {
    auto less = rows_entry::compare(s);
    auto i = _rows.lower_bound(r.start(), less);
    auto end = _rows.lower_bound(r.end(), less);
    if (!less(r.start(), r.end())) {
        return bool(cont);
    }
    if (i != end) {
        if (no_clustering_row_between(s, r.start(), i->position())) {
            ++i;
        }
        while (i != end) {
            if (i->continuous() != cont) {
                return false;
            }
            ++i;
        }
        if (end != _rows.begin() && no_clustering_row_between(s, std::prev(end)->position(), r.end())) {
            return true;
        }
    }
    return (end == _rows.end() ? is_continuous::yes : end->continuous()) == cont;
}

bool
mutation_partition::fully_continuous(const schema& s, const position_range& r) {
    return check_continuity(s, r, is_continuous::yes);
}

bool
mutation_partition::fully_discontinuous(const schema& s, const position_range& r) {
    return check_continuity(s, r, is_continuous::no);
}

future<mutation_opt> counter_write_query(schema_ptr s, const mutation_source& source,
                                         const dht::decorated_key& dk,
                                         const query::partition_slice& slice,
                                         tracing::trace_state_ptr trace_ptr)
{
    struct range_and_reader {
        dht::partition_range range;
        flat_mutation_reader reader;

        range_and_reader(range_and_reader&&) = delete;
        range_and_reader(const range_and_reader&) = delete;

        range_and_reader(schema_ptr s, const mutation_source& source,
                         const dht::decorated_key& dk,
                         const query::partition_slice& slice,
                         tracing::trace_state_ptr trace_ptr)
            : range(dht::partition_range::make_singular(dk))
            , reader(source.make_reader(s, range, slice, service::get_local_sstable_query_read_priority(),
                                                      std::move(trace_ptr), streamed_mutation::forwarding::no,
                                                      mutation_reader::forwarding::no))
        { }
    };

    // do_with() doesn't support immovable objects
    auto r_a_r = std::make_unique<range_and_reader>(s, source, dk, slice, std::move(trace_ptr));
    auto cwqrb = counter_write_query_result_builder(*s);
    auto cfq = make_stable_flattened_mutations_consumer<compact_for_query<emit_only_live_rows::yes, counter_write_query_result_builder>>(
            *s, gc_clock::now(), slice, query::max_rows, query::max_rows, std::move(cwqrb));
    auto f = r_a_r->reader.consume(std::move(cfq), db::no_timeout, flat_mutation_reader::consume_reversed_partitions::no);
    return f.finally([r_a_r = std::move(r_a_r)] { });
}

mutation_cleaner_impl::~mutation_cleaner_impl() {
    _worker_state->done = true;
    _worker_state->cv.signal();
    _worker_state->snapshots.clear_and_dispose(typename lw_shared_ptr<partition_snapshot>::disposer());
    with_allocator(_region.allocator(), [this] {
        clear();
    });
}

void mutation_cleaner_impl::clear() noexcept {
    while (clear_gently() == stop_iteration::no) ;
}

stop_iteration mutation_cleaner_impl::clear_gently() noexcept {
    while (clear_some() == memory::reclaiming_result::reclaimed_something) {
        if (need_preempt()) {
            return stop_iteration::no;
        }
    }
    return stop_iteration::yes;
}

memory::reclaiming_result mutation_cleaner_impl::clear_some() noexcept {
    if (_versions.empty()) {
        return memory::reclaiming_result::reclaimed_nothing;
    }
    auto&& alloc = current_allocator();
    partition_version& pv = _versions.front();
    if (pv.clear_gently(_tracker) == stop_iteration::yes) {
        _versions.pop_front();
        alloc.destroy(&pv);
    }
    return memory::reclaiming_result::reclaimed_something;
}

void mutation_cleaner_impl::merge(mutation_cleaner_impl& r) noexcept {
    _versions.splice(r._versions);
    for (partition_snapshot& snp : r._worker_state->snapshots) {
        snp.migrate(&_region, _cleaner);
    }
    _worker_state->snapshots.splice(_worker_state->snapshots.end(), r._worker_state->snapshots);
    if (!_worker_state->snapshots.empty()) {
        _worker_state->cv.signal();
    }
}

void mutation_cleaner_impl::start_worker() {
    auto f = repeat([w = _worker_state, this] () mutable noexcept {
      if (w->done) {
          return make_ready_future<stop_iteration>(stop_iteration::yes);
      }
      return with_scheduling_group(_scheduling_group, [w, this] {
        return w->cv.wait([w] {
            return w->done || !w->snapshots.empty();
        }).then([this, w] () noexcept {
            if (w->done) {
                return stop_iteration::yes;
            }
            merge_some();
            return stop_iteration::no;
        });
      });
    });
    if (f.failed()) {
        f.get();
    }
}

stop_iteration mutation_cleaner_impl::merge_some(partition_snapshot& snp) noexcept {
    auto&& region = snp.region();
    return with_allocator(region.allocator(), [&] {
        return with_linearized_managed_bytes([&] {
            // Allocating sections require the region to be reclaimable
            // which means that they cannot be nested.
            // It is, however, possible, that if the snapshot is taken
            // inside an allocating section and then an exception is thrown
            // this function will be called to clean up even though we
            // still will be in the context of the allocating section.
            if (!region.reclaiming_enabled()) {
                return stop_iteration::no;
            }
            try {
                return _worker_state->alloc_section(region, [&] {
                    return snp.merge_partition_versions();
                });
            } catch (...) {
                // Merging failed, give up as there is no guarantee of forward progress.
                return stop_iteration::yes;
            }
        });
    });
}

stop_iteration mutation_cleaner_impl::merge_some() noexcept {
    if (_worker_state->snapshots.empty()) {
        return stop_iteration::yes;
    }
    partition_snapshot& snp = _worker_state->snapshots.front();
    if (merge_some(snp) == stop_iteration::yes) {
        _worker_state->snapshots.pop_front();
        lw_shared_ptr<partition_snapshot>::dispose(&snp);
    }
    return stop_iteration::no;
}

future<> mutation_cleaner_impl::drain() {
    return repeat([this] {
        return merge_some();
    }).then([this] {
        return repeat([this] {
            return with_allocator(_region.allocator(), [this] {
                return clear_gently();
            });
        });
    });
}