src/trace_processor/db/view.cc - third_party/perfetto - Git at Google

 /*
  * Copyright (C) 2022 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include "src/trace_processor/db/view.h"

 #include <stddef.h>
 #include <stdint.h>
 #include <algorithm>
 #include <iterator>
 #include <limits>
 #include <map>
 #include <memory>
 #include <string>
 #include <vector>

 #include "perfetto/base/compiler.h"
 #include "perfetto/base/flat_set.h"
 #include "perfetto/ext/base/flat_hash_map.h"
 #include "perfetto/ext/base/string_view.h"
 #include "perfetto/trace_processor/basic_types.h"
 #include "src/trace_processor/containers/row_map.h"
 #include "src/trace_processor/db/column.h"
 #include "src/trace_processor/db/table.h"
 #include "src/trace_processor/db/typed_column.h"

 namespace perfetto {
 namespace trace_processor {

 View::View() = default;

 View::View(std::unique_ptr<TableNode> root,
            std::vector<SourceColumn> source_col_by_output_idx,
            Table::Schema schema)
     : root_node_(std::move(root)),
       source_col_by_output_idx_(std::move(source_col_by_output_idx)),
       schema_(std::move(schema)) {}

 View::~View() = default;

 base::Status View::Create(Table* root_table,
                           const char* root_table_name,
                           std::initializer_list<JoinTable> joins,
                           std::initializer_list<OutputColumn> cols,
                           View* view) {
   // Insert the node for the root table; the column indices being std::nullopt
   // indicates this is the root.
   std::unique_ptr<TableNode> root_node(
       new TableNode{root_table, std::nullopt, std::nullopt, JoinFlag::kNoFlag,
                     TableNode::Children{}});
   base::FlatHashMap<base::StringView, TableNode*> node_map;
   node_map.Insert(root_table_name, root_node.get());

   // Verify that all the joins are well-formed and build the join-tree
   // structure.
   for (const JoinTable& join : joins) {
     // Verify that the previous table was previously defined (either by
     // the root or prior join).
     TableNode** prev_node_it = node_map.Find(join.prev_table_name);
     if (!prev_node_it) {
       return base::ErrStatus(
           "View has table %s joining with table %s which was not "
           "previously defined",
           join.table_name, join.prev_table_name);
     }
     TableNode* prev_node = *prev_node_it;

     // Verify that the previous table's column exists.
     std::optional<uint32_t> opt_prev_col_idx =
         prev_node->table->GetColumnIndexByName(join.prev_col);
     if (!opt_prev_col_idx) {
       return base::ErrStatus(
           "View references column %s in table %s which does not exist",
           join.prev_col, join.prev_table_name);
     }

     // Verify that the current table's column exists.
     std::optional<uint32_t> opt_col_idx =
         join.table->GetColumnIndexByName(join.col);
     if (!opt_col_idx) {
       return base::ErrStatus(
           "View references column %s in table %s which does not exist",
           join.col, join.table_name);
     }

     // TODO(lalitm): add some extra checks about the columns being joined here
     // (i.e. right column being an id, left column being non-nullable, neither
     // column being a dummy column etc, neither column is hidden etc.).

     // Build the node and insert it into the map.
     prev_node->children.emplace_back(
         new TableNode{join.table, *opt_col_idx, *opt_prev_col_idx,
                       join.join_flags, TableNode::Children{}});

     auto it_and_inserted =
         node_map.Insert(join.table_name, prev_node->children.back().get());
     if (!it_and_inserted.second) {
       return base::ErrStatus("View has duplicate table name %s",
                              join.table_name);
     }
   }

   // Verify that there all the output columns are well formed.
   {
     base::FlatSet<base::StringView> col_names;
     for (const OutputColumn& col : cols) {
       auto it_and_inserted = col_names.insert(col.col_name);
       if (!it_and_inserted.second) {
         return base::ErrStatus("View has duplicate column %s", col.col_name);
       }

       TableNode** node_it = node_map.Find(col.source_table_name);
       if (!node_it) {
         return base::ErrStatus(
             "View references table %s as source for column %s which does "
             "not exist",
             col.source_table_name, col.col_name);
       }

       const TableNode* node = *node_it;
       if (!node->table->GetColumnIndexByName(col.source_col_name)) {
         return base::ErrStatus(
             "View references column %s in table %s as source for column %s "
             "which does not exist",
             col.source_col_name, col.source_table_name, col.col_name);
       }
     }
   }

   // Build the schema of the output table and a mapping from each output column
   // to the source column which generates it.
   std::vector<SourceColumn> source_col_by_output_idx(cols.size());
   Table::Schema schema;
   for (const OutputColumn& col : cols) {
     const TableNode* node = *node_map.Find(col.source_table_name);
     uint32_t table_col_idx =
         *node->table->GetColumnIndexByName(col.source_col_name);

     const Column& table_col = node->table->GetColumn(table_col_idx);
     PERFETTO_DCHECK(!table_col.IsHidden());

     // TODO(lalitm): if the view specifies the right hand side table as
     // the source for a joined column, we should be able to use the left hand
     // side instead. Add this as a future optimization or detect it and
     // error out.

     base::StringView source_table_name(col.source_table_name);
     schema.columns.emplace_back(Table::Schema::Column{
         col.col_name, table_col.type(),
         source_table_name == root_table_name ? table_col.IsId() : false,
         source_table_name == root_table_name ? table_col.IsSorted() : false,
         table_col.IsHidden(),
         source_table_name == root_table_name ? table_col.IsSetId() : false});

     uint32_t output_idx = static_cast<uint32_t>(schema.columns.size() - 1);
     source_col_by_output_idx[output_idx] = {node, table_col_idx};
   }

   *view = View(std::move(root_node), std::move(source_col_by_output_idx),
                std::move(schema));
   return base::OkStatus();
 }

 View::View(Table* root_table,
            const char* root_table_name,
            std::initializer_list<JoinTable> joins,
            std::initializer_list<OutputColumn> columns) {
   base::Status status = Create(root_table, root_table_name, std::move(joins),
                                std::move(columns), this);
   if (!status.ok()) {
     PERFETTO_FATAL("Failed building view: %s", status.c_message());
   }
 }

 Table View::Query(const std::vector<Constraint>& cs,
                   const std::vector<Order>& ob,
                   const BitVector& cols_used) const {
   PERFETTO_DCHECK(cols_used.size() == schema_.columns.size());

   TableNode* root = root_node_.get();

   // Below is the core algorithm which does joining and querying simultaneously.
   // We do this to allow optimizations on which way to order the join and
   // filter based on the join type, constraints, row counters etc.
   //
   // The algorithm is implemented by the |QueryHelper| class for the purposes
   // of sharing a bunch of temporary state between the different stages of the
   // algorithm.

   // The constructor for query helper builds all the temporary state:
   // essentially a copy of the join tree with metadata about which tables are
   // used, which tables remove rows from parents and generates the initial
   // output tables and RowMaps.
   QueryHelper helper(root, source_col_by_output_idx_, cs, cols_used);

   // |FilterAndJoinRecursive| is responsible for filtering all relevant tables
   // which have a constraint necessary for them, materializing any tables
   // participating in the join and computing the "child" table and "parent"
   // RowMap.
   //
   // It does *not* propogate the RowMap downwards: this is done by
   // |ApplyRowMapRecursive|. We don't do this because it would be very
   // inefficient to constantly propogate the RowMap at every level in the middle
   // of a DFS (at its heart, this function is a post-order DFS).
   helper.FilterAndJoinRecursive(root);

   // |ApplyRowMapRecursive| is responsible for recursively propogating the
   // join RowMaps downwards. This is necessary because if you have
   //
   // A JOIN B JOIN C
   //
   // |FilterAndJoinRecursive| will compute the final state of A but only
   // intermediate states for B and C: for B, it will filter out all rows which
   // don't exist in C and for C it will simply leave as-is. The fact that
   // every row in A now has a corresponding row in B and similarily with C
   // is the job of this function.
   //
   // ApplyRowMapRecursive then pushes down the RowMap representing the join A
   // and B and applies that to B. Finally, it selects the B-C RowMap with the
   // A-B RowMap and applies this to C's table.
   helper.ApplyRowMapRecursive(root);

   // |BuildTable| converts the intermediate tables from the above and generates
   // a cohesive table matching the schema of this view. Any "not used" columns
   // are simply replaced with dummy columns who cannot be queried which saves
   // the cost of doing unnecessary joins.
   Table filtered =
       helper.BuildTable(root, schema_, source_col_by_output_idx_, cols_used);

   // The final step is simply to sort the table resulting from filtering.
   //
   // TODO(lalitm): we could be more efficient about this and sort the source
   // tables *before* we join. However, given sorts are relatively rare, we don't
   // do this yet.
   return filtered.Sort(ob);
 }

 View::QueryHelper::QueryHelper(
     TableNode* root_node,
     const std::vector<SourceColumn>& source_col_by_output_idx,
     const std::vector<Constraint>& cs,
     const BitVector& cols_used)
     : state_(BuildNodeStateMap(root_node,
                                source_col_by_output_idx,
                                cs,
                                cols_used)) {}

 void View::QueryHelper::FilterAndJoinRecursive(TableNode* node) {
   NodeState& state = *state_.Find(node);

   // TODO(lalitm): instead of computing the left table straight away here, we
   // could more intelligently figure out whether doing the join first is more
   // efficient.
   state.output = state.output.Filter(state.cs);

   const Table& left_table = state.output;
   RowMap left_rm(0, left_table.row_count());
   for (const auto& child : node->children) {
     NodeState* child_state = state_.Find(child.get());

     // If we have no rows, just bail out to minimize work done.
     if (left_rm.empty())
       break;

     // If the table is not used and doesn't remove any rows in the parent, we
     // can just rely on the default RowMap.
     if (!child_state->is_used && !child_state->removes_parent_rows)
       break;

     // Recurse on the child table so we now the contents of the right table
     // before we filter any further.
     FilterAndJoinRecursive(child.get());

     // If the right table is empty, the left table cannot possibly join
     // without removing rows.
     const Table& right_table = child_state->output;
     if (right_table.row_count() == 0) {
       left_rm = RowMap();
       break;
     }

     const auto& left_col = *TypedColumn<BaseId>::FromColumn(
         &state.output.GetColumn(*child->parent_join_col_idx));
     const auto& right_col = *IdColumn<BaseId>::FromColumn(
         &child_state->output.GetColumn(*child->join_col_idx));

     // The core join loop. This function iterates through every row in
     // the left table and figures out whether to keep it if the row
     // also exists in the right table. While doing this, it also figures
     // out the row number in the right table for every row in the left table.
     std::vector<uint32_t> right_rm_iv;
     right_rm_iv.reserve(left_rm.size());
     left_col.overlay().FilterInto(&left_rm, [&](uint32_t idx) {
       // Check if the right table has the value from the left table.
       std::optional<uint32_t> opt_idx =
           right_col.IndexOf(left_col.GetAtIdx(idx));

       // If it doesn't, return false indicating that this row should be
       // removed from the left table.
       if (!opt_idx)
         return false;

       // If the row does exist, then keep track of the index of the row
       // for applying to the right table and return true to also keep this
       // row in the left table.
       right_rm_iv.emplace_back(*opt_idx);
       return true;
     });
     child_state->parent_join_rm = RowMap(std::move(right_rm_iv));
   }
   state.output = state.output.Apply(std::move(left_rm));
 }

 void View::QueryHelper::ApplyRowMapRecursive(TableNode* node, RowMap rm) {
   NodeState& state = *state_.Find(node);
   for (const auto& child : node->children) {
     const NodeState& child_state = *state_.Find(child.get());
     // If the child table is not used, then we don't need to recurse any
     // further.
     if (!child_state.is_used)
       break;
     ApplyRowMapRecursive(child.get(),
                          child_state.parent_join_rm.SelectRows(rm));
   }
   state.output = state.output.Apply(std::move(rm));
 }

 View::QueryHelper::NodeStateMap View::QueryHelper::BuildNodeStateMap(
     TableNode* root_node,
     const std::vector<SourceColumn>& source_col_by_output_idx,
     const std::vector<Constraint>& cs,
     const BitVector& cols_used) {
   // Populate the map contains all the nodes in the tree.
   base::FlatHashMap<const TableNode*, QueryHelper::NodeState> node_state;
   PostOrderDfs(root_node, [&node_state](TableNode* node) {
     node_state.Insert(node,
                       QueryHelper::NodeState{
                           {}, false, false, node->table->Copy(), RowMap()});
   });

   // For each constraint, add the translated constraint to the relevant table's
   // constraint set.
   for (const Constraint& c : cs) {
     const auto& source_col = source_col_by_output_idx[c.col_idx];
     auto& metadata = *node_state.Find(source_col.first);
     metadata.cs.emplace_back(Constraint{source_col.second, c.op, c.value});
   }

   // For each used column, mark the associated table as being used.
   for (auto it = cols_used.IterateSetBits(); it; it.Next()) {
     const auto& source = source_col_by_output_idx[it.index()];
     node_state.Find(source.first)->is_used = true;
   }

   // For each node, figure out whether ti will cause parent rows
   // to be removed.
   for (auto it = node_state.GetIterator(); it; ++it) {
     // The below logic doesn't make sense on the root node.
     if (it.key() == root_node)
       continue;

     // A join will retain (i.e. *not* remove parent rows) if one of the
     // following is true:
     //  a) the child (right-side of join) table contains every id
     //     which could exist in the parent (left-side) table.
     // TODO(lalitm): add more conditions here.
     bool join_retains_parent_rows =
         (it.key()->join_flags & JoinFlag::kIdAlwaysPresent) != 0;

     // However, if this table has constraints, then we could always remove the
     // parents rows even if the join would normally retain all rows.
     it.value().removes_parent_rows =
         !it.value().cs.empty() || !join_retains_parent_rows;
   }

   // Do a DFS on the node tree and propogate up the is_used and
   // removes_parent_rows boolean. In other words, if a table is used by SQLite,
   // every ancestor must also be used as we need to join with every table on the
   // path between the root and used table. Similarily, if a table removes parent
   // rows, then it does this recursively upwards.
   PostOrderDfs(root_node, [&node_state](TableNode* node) {
     NodeState& state = *node_state.Find(node);
     for (const auto& child : node->children) {
       const NodeState& child_metadata = *node_state.Find(child.get());
       state.is_used |= child_metadata.is_used;
       state.removes_parent_rows |= child_metadata.removes_parent_rows;
     }
   });

   return node_state;
 }

 Table View::QueryHelper::BuildTable(
     TableNode* root,
     const Table::Schema& schema,
     const std::vector<SourceColumn>& source_col_by_output_idx,
     const BitVector& cols_used) {
   NodeState& root_state = *state_.Find(root);

   Table output(root->table->string_pool_);
   output.row_count_ = root_state.output.row_count();
   output.overlays_.emplace_back(ColumnStorageOverlay(output.row_count_));

   std::map<std::pair<const TableNode*, uint32_t>, uint32_t> cached_rm;
   for (auto it = cols_used.IterateAllBits(); it; it.Next()) {
     const char* col_name = schema.columns[it.index()].name.c_str();
     if (!it.IsSet()) {
       output.columns_.emplace_back(
           Column::DummyColumn(col_name, &output, it.index()));
       continue;
     }

     const auto& source_col = source_col_by_output_idx[it.index()];

     Table& node_table = state_.Find(source_col.first)->output;
     const Column& table_col = node_table.GetColumn(source_col.second);

     auto it_and_inserted = cached_rm.emplace(
         std::make_pair(source_col.first, table_col.overlay_index()),
         static_cast<uint32_t>(output.overlays_.size()));
     if (it_and_inserted.second) {
       output.overlays_.emplace_back(
           std::move(node_table.overlays_[table_col.overlay_index()]));
     }

     uint32_t rm_idx = it_and_inserted.first->second;
     output.columns_.emplace_back(
         Column(table_col, &output, it.index(), rm_idx, col_name));
   }
   return output;
 }

 uint32_t View::GetColumnCount() const {
   return static_cast<uint32_t>(schema_.columns.size());
 }

 uint32_t View::EstimateRowCount() const {
   uint32_t count = 0;
   PostOrderDfs(root_node_.get(), [&count](TableNode* node) {
     count = std::max(node->table->row_count(), count);
   });
   return count;
 }

 }  // namespace trace_processor
 }  // namespace perfetto
	/*
	* Copyright (C) 2022 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "src/trace_processor/db/view.h"

	#include <stddef.h>
	#include <stdint.h>
	#include <algorithm>
	#include <iterator>
	#include <limits>
	#include <map>
	#include <memory>
	#include <string>
	#include <vector>

	#include "perfetto/base/compiler.h"
	#include "perfetto/base/flat_set.h"
	#include "perfetto/ext/base/flat_hash_map.h"
	#include "perfetto/ext/base/string_view.h"
	#include "perfetto/trace_processor/basic_types.h"
	#include "src/trace_processor/containers/row_map.h"
	#include "src/trace_processor/db/column.h"
	#include "src/trace_processor/db/table.h"
	#include "src/trace_processor/db/typed_column.h"

	namespace perfetto {
	namespace trace_processor {

	View::View() = default;

	View::View(std::unique_ptr<TableNode> root,
	std::vector<SourceColumn> source_col_by_output_idx,
	Table::Schema schema)
	: root_node_(std::move(root)),
	source_col_by_output_idx_(std::move(source_col_by_output_idx)),
	schema_(std::move(schema)) {}

	View::~View() = default;

	base::Status View::Create(Table* root_table,
	const char* root_table_name,
	std::initializer_list<JoinTable> joins,
	std::initializer_list<OutputColumn> cols,
	View* view) {
	// Insert the node for the root table; the column indices being std::nullopt
	// indicates this is the root.
	std::unique_ptr<TableNode> root_node(
	new TableNode{root_table, std::nullopt, std::nullopt, JoinFlag::kNoFlag,
	TableNode::Children{}});
	base::FlatHashMap<base::StringView, TableNode*> node_map;
	node_map.Insert(root_table_name, root_node.get());

	// Verify that all the joins are well-formed and build the join-tree
	// structure.
	for (const JoinTable& join : joins) {
	// Verify that the previous table was previously defined (either by
	// the root or prior join).
	TableNode** prev_node_it = node_map.Find(join.prev_table_name);
	if (!prev_node_it) {
	return base::ErrStatus(
	"View has table %s joining with table %s which was not "
	"previously defined",
	join.table_name, join.prev_table_name);
	}
	TableNode* prev_node = *prev_node_it;

	// Verify that the previous table's column exists.
	std::optional<uint32_t> opt_prev_col_idx =
	prev_node->table->GetColumnIndexByName(join.prev_col);
	if (!opt_prev_col_idx) {
	return base::ErrStatus(
	"View references column %s in table %s which does not exist",
	join.prev_col, join.prev_table_name);
	}

	// Verify that the current table's column exists.
	std::optional<uint32_t> opt_col_idx =
	join.table->GetColumnIndexByName(join.col);
	if (!opt_col_idx) {
	return base::ErrStatus(
	"View references column %s in table %s which does not exist",
	join.col, join.table_name);
	}

	// TODO(lalitm): add some extra checks about the columns being joined here
	// (i.e. right column being an id, left column being non-nullable, neither
	// column being a dummy column etc, neither column is hidden etc.).

	// Build the node and insert it into the map.
	prev_node->children.emplace_back(
	new TableNode{join.table, opt_col_idx, opt_prev_col_idx,
	join.join_flags, TableNode::Children{}});

	auto it_and_inserted =
	node_map.Insert(join.table_name, prev_node->children.back().get());
	if (!it_and_inserted.second) {
	return base::ErrStatus("View has duplicate table name %s",
	join.table_name);
	}
	}

	// Verify that there all the output columns are well formed.
	{
	base::FlatSet<base::StringView> col_names;
	for (const OutputColumn& col : cols) {
	auto it_and_inserted = col_names.insert(col.col_name);
	if (!it_and_inserted.second) {
	return base::ErrStatus("View has duplicate column %s", col.col_name);
	}

	TableNode** node_it = node_map.Find(col.source_table_name);
	if (!node_it) {
	return base::ErrStatus(
	"View references table %s as source for column %s which does "
	"not exist",
	col.source_table_name, col.col_name);
	}

	const TableNode* node = *node_it;
	if (!node->table->GetColumnIndexByName(col.source_col_name)) {
	return base::ErrStatus(
	"View references column %s in table %s as source for column %s "
	"which does not exist",
	col.source_col_name, col.source_table_name, col.col_name);
	}
	}
	}

	// Build the schema of the output table and a mapping from each output column
	// to the source column which generates it.
	std::vector<SourceColumn> source_col_by_output_idx(cols.size());
	Table::Schema schema;
	for (const OutputColumn& col : cols) {
	const TableNode* node = *node_map.Find(col.source_table_name);
	uint32_t table_col_idx =
	*node->table->GetColumnIndexByName(col.source_col_name);

	const Column& table_col = node->table->GetColumn(table_col_idx);
	PERFETTO_DCHECK(!table_col.IsHidden());

	// TODO(lalitm): if the view specifies the right hand side table as
	// the source for a joined column, we should be able to use the left hand
	// side instead. Add this as a future optimization or detect it and
	// error out.

	base::StringView source_table_name(col.source_table_name);
	schema.columns.emplace_back(Table::Schema::Column{
	col.col_name, table_col.type(),
	source_table_name == root_table_name ? table_col.IsId() : false,
	source_table_name == root_table_name ? table_col.IsSorted() : false,
	table_col.IsHidden(),
	source_table_name == root_table_name ? table_col.IsSetId() : false});

	uint32_t output_idx = static_cast<uint32_t>(schema.columns.size() - 1);
	source_col_by_output_idx[output_idx] = {node, table_col_idx};
	}

	*view = View(std::move(root_node), std::move(source_col_by_output_idx),
	std::move(schema));
	return base::OkStatus();
	}

	View::View(Table* root_table,
	const char* root_table_name,
	std::initializer_list<JoinTable> joins,
	std::initializer_list<OutputColumn> columns) {
	base::Status status = Create(root_table, root_table_name, std::move(joins),
	std::move(columns), this);
	if (!status.ok()) {
	PERFETTO_FATAL("Failed building view: %s", status.c_message());
	}
	}

	Table View::Query(const std::vector<Constraint>& cs,
	const std::vector<Order>& ob,
	const BitVector& cols_used) const {
	PERFETTO_DCHECK(cols_used.size() == schema_.columns.size());

	TableNode* root = root_node_.get();

	// Below is the core algorithm which does joining and querying simultaneously.
	// We do this to allow optimizations on which way to order the join and
	// filter based on the join type, constraints, row counters etc.
	//
	// The algorithm is implemented by the \|QueryHelper\| class for the purposes
	// of sharing a bunch of temporary state between the different stages of the
	// algorithm.

	// The constructor for query helper builds all the temporary state:
	// essentially a copy of the join tree with metadata about which tables are
	// used, which tables remove rows from parents and generates the initial
	// output tables and RowMaps.
	QueryHelper helper(root, source_col_by_output_idx_, cs, cols_used);

	// \|FilterAndJoinRecursive\| is responsible for filtering all relevant tables
	// which have a constraint necessary for them, materializing any tables
	// participating in the join and computing the "child" table and "parent"
	// RowMap.
	//
	// It does not propogate the RowMap downwards: this is done by
	// \|ApplyRowMapRecursive\|. We don't do this because it would be very
	// inefficient to constantly propogate the RowMap at every level in the middle
	// of a DFS (at its heart, this function is a post-order DFS).
	helper.FilterAndJoinRecursive(root);

	// \|ApplyRowMapRecursive\| is responsible for recursively propogating the
	// join RowMaps downwards. This is necessary because if you have
	//
	// A JOIN B JOIN C
	//
	// \|FilterAndJoinRecursive\| will compute the final state of A but only
	// intermediate states for B and C: for B, it will filter out all rows which
	// don't exist in C and for C it will simply leave as-is. The fact that
	// every row in A now has a corresponding row in B and similarily with C
	// is the job of this function.
	//
	// ApplyRowMapRecursive then pushes down the RowMap representing the join A
	// and B and applies that to B. Finally, it selects the B-C RowMap with the
	// A-B RowMap and applies this to C's table.
	helper.ApplyRowMapRecursive(root);

	// \|BuildTable\| converts the intermediate tables from the above and generates
	// a cohesive table matching the schema of this view. Any "not used" columns
	// are simply replaced with dummy columns who cannot be queried which saves
	// the cost of doing unnecessary joins.
	Table filtered =
	helper.BuildTable(root, schema_, source_col_by_output_idx_, cols_used);

	// The final step is simply to sort the table resulting from filtering.
	//
	// TODO(lalitm): we could be more efficient about this and sort the source
	// tables before we join. However, given sorts are relatively rare, we don't
	// do this yet.
	return filtered.Sort(ob);
	}

	View::QueryHelper::QueryHelper(
	TableNode* root_node,
	const std::vector<SourceColumn>& source_col_by_output_idx,
	const std::vector<Constraint>& cs,
	const BitVector& cols_used)
	: state_(BuildNodeStateMap(root_node,
	source_col_by_output_idx,
	cs,
	cols_used)) {}

	void View::QueryHelper::FilterAndJoinRecursive(TableNode* node) {
	NodeState& state = *state_.Find(node);

	// TODO(lalitm): instead of computing the left table straight away here, we
	// could more intelligently figure out whether doing the join first is more
	// efficient.
	state.output = state.output.Filter(state.cs);

	const Table& left_table = state.output;
	RowMap left_rm(0, left_table.row_count());
	for (const auto& child : node->children) {
	NodeState* child_state = state_.Find(child.get());

	// If we have no rows, just bail out to minimize work done.
	if (left_rm.empty())
	break;

	// If the table is not used and doesn't remove any rows in the parent, we
	// can just rely on the default RowMap.
	if (!child_state->is_used && !child_state->removes_parent_rows)
	break;

	// Recurse on the child table so we now the contents of the right table
	// before we filter any further.
	FilterAndJoinRecursive(child.get());

	// If the right table is empty, the left table cannot possibly join
	// without removing rows.
	const Table& right_table = child_state->output;
	if (right_table.row_count() == 0) {
	left_rm = RowMap();
	break;
	}

	const auto& left_col = *TypedColumn<BaseId>::FromColumn(
	&state.output.GetColumn(*child->parent_join_col_idx));
	const auto& right_col = *IdColumn<BaseId>::FromColumn(
	&child_state->output.GetColumn(*child->join_col_idx));

	// The core join loop. This function iterates through every row in
	// the left table and figures out whether to keep it if the row
	// also exists in the right table. While doing this, it also figures
	// out the row number in the right table for every row in the left table.
	std::vector<uint32_t> right_rm_iv;
	right_rm_iv.reserve(left_rm.size());
	left_col.overlay().FilterInto(&left_rm, [&](uint32_t idx) {
	// Check if the right table has the value from the left table.
	std::optional<uint32_t> opt_idx =
	right_col.IndexOf(left_col.GetAtIdx(idx));

	// If it doesn't, return false indicating that this row should be
	// removed from the left table.
	if (!opt_idx)
	return false;

	// If the row does exist, then keep track of the index of the row
	// for applying to the right table and return true to also keep this
	// row in the left table.
	right_rm_iv.emplace_back(*opt_idx);
	return true;
	});
	child_state->parent_join_rm = RowMap(std::move(right_rm_iv));
	}
	state.output = state.output.Apply(std::move(left_rm));
	}

	void View::QueryHelper::ApplyRowMapRecursive(TableNode* node, RowMap rm) {
	NodeState& state = *state_.Find(node);
	for (const auto& child : node->children) {
	const NodeState& child_state = *state_.Find(child.get());
	// If the child table is not used, then we don't need to recurse any
	// further.
	if (!child_state.is_used)
	break;
	ApplyRowMapRecursive(child.get(),
	child_state.parent_join_rm.SelectRows(rm));
	}
	state.output = state.output.Apply(std::move(rm));
	}

	View::QueryHelper::NodeStateMap View::QueryHelper::BuildNodeStateMap(
	TableNode* root_node,
	const std::vector<SourceColumn>& source_col_by_output_idx,
	const std::vector<Constraint>& cs,
	const BitVector& cols_used) {
	// Populate the map contains all the nodes in the tree.
	base::FlatHashMap<const TableNode*, QueryHelper::NodeState> node_state;
	PostOrderDfs(root_node, [&node_state](TableNode* node) {
	node_state.Insert(node,
	QueryHelper::NodeState{
	{}, false, false, node->table->Copy(), RowMap()});
	});

	// For each constraint, add the translated constraint to the relevant table's
	// constraint set.
	for (const Constraint& c : cs) {
	const auto& source_col = source_col_by_output_idx[c.col_idx];
	auto& metadata = *node_state.Find(source_col.first);
	metadata.cs.emplace_back(Constraint{source_col.second, c.op, c.value});
	}

	// For each used column, mark the associated table as being used.
	for (auto it = cols_used.IterateSetBits(); it; it.Next()) {
	const auto& source = source_col_by_output_idx[it.index()];
	node_state.Find(source.first)->is_used = true;
	}

	// For each node, figure out whether ti will cause parent rows
	// to be removed.
	for (auto it = node_state.GetIterator(); it; ++it) {
	// The below logic doesn't make sense on the root node.
	if (it.key() == root_node)
	continue;

	// A join will retain (i.e. not remove parent rows) if one of the
	// following is true:
	// a) the child (right-side of join) table contains every id
	// which could exist in the parent (left-side) table.
	// TODO(lalitm): add more conditions here.
	bool join_retains_parent_rows =
	(it.key()->join_flags & JoinFlag::kIdAlwaysPresent) != 0;

	// However, if this table has constraints, then we could always remove the
	// parents rows even if the join would normally retain all rows.
	it.value().removes_parent_rows =
	!it.value().cs.empty() \|\| !join_retains_parent_rows;
	}

	// Do a DFS on the node tree and propogate up the is_used and
	// removes_parent_rows boolean. In other words, if a table is used by SQLite,
	// every ancestor must also be used as we need to join with every table on the
	// path between the root and used table. Similarily, if a table removes parent
	// rows, then it does this recursively upwards.
	PostOrderDfs(root_node, [&node_state](TableNode* node) {
	NodeState& state = *node_state.Find(node);
	for (const auto& child : node->children) {
	const NodeState& child_metadata = *node_state.Find(child.get());
	state.is_used \|= child_metadata.is_used;
	state.removes_parent_rows \|= child_metadata.removes_parent_rows;
	}
	});

	return node_state;
	}

	Table View::QueryHelper::BuildTable(
	TableNode* root,
	const Table::Schema& schema,
	const std::vector<SourceColumn>& source_col_by_output_idx,
	const BitVector& cols_used) {
	NodeState& root_state = *state_.Find(root);

	Table output(root->table->string_pool_);
	output.row_count_ = root_state.output.row_count();
	output.overlays_.emplace_back(ColumnStorageOverlay(output.row_count_));

	std::map<std::pair<const TableNode*, uint32_t>, uint32_t> cached_rm;
	for (auto it = cols_used.IterateAllBits(); it; it.Next()) {
	const char* col_name = schema.columns[it.index()].name.c_str();
	if (!it.IsSet()) {
	output.columns_.emplace_back(
	Column::DummyColumn(col_name, &output, it.index()));
	continue;
	}

	const auto& source_col = source_col_by_output_idx[it.index()];

	Table& node_table = state_.Find(source_col.first)->output;
	const Column& table_col = node_table.GetColumn(source_col.second);

	auto it_and_inserted = cached_rm.emplace(
	std::make_pair(source_col.first, table_col.overlay_index()),
	static_cast<uint32_t>(output.overlays_.size()));
	if (it_and_inserted.second) {
	output.overlays_.emplace_back(
	std::move(node_table.overlays_[table_col.overlay_index()]));
	}

	uint32_t rm_idx = it_and_inserted.first->second;
	output.columns_.emplace_back(
	Column(table_col, &output, it.index(), rm_idx, col_name));
	}
	return output;
	}

	uint32_t View::GetColumnCount() const {
	return static_cast<uint32_t>(schema_.columns.size());
	}

	uint32_t View::EstimateRowCount() const {
	uint32_t count = 0;
	PostOrderDfs(root_node_.get(), [&count](TableNode* node) {
	count = std::max(node->table->row_count(), count);
	});
	return count;
	}

	} // namespace trace_processor
	} // namespace perfetto