ClickHouse/dbms/src/Storages/MergeTree/MergeTreeWhereOptimizer.cpp

#include <DB/Storages/MergeTree/MergeTreeWhereOptimizer.h>
#include <DB/Storages/MergeTree/MergeTreeData.h>
#include <DB/Storages/MergeTree/PKCondition.h>
#include <DB/Parsers/ASTSelectQuery.h>
#include <DB/Parsers/ASTFunction.h>
#include <DB/Parsers/ASTIdentifier.h>
#include <DB/Parsers/ASTLiteral.h>
#include <DB/Parsers/ASTExpressionList.h>
#include <DB/Parsers/ASTSubquery.h>
#include <DB/Parsers/ASTSet.h>
#include <DB/Parsers/formatAST.h>
#include <DB/Common/escapeForFileName.h>
#include <DB/DataTypes/DataTypeNested.h>
#include <ext/scope_guard.hpp>
#include <ext/map.hpp>
#include <memory>
#include <unordered_map>
#include <map>
#include <limits>
#include <cstddef>


namespace DB
{

static constexpr auto threshold = 10;
/// We decided to remove the restriction due to the absence of a penalty for the transfer in PREWHERE
static constexpr auto max_columns_relative_size = 1.0f;
static constexpr auto and_function_name = "and";
static constexpr auto equals_function_name = "equals";
static constexpr auto array_join_function_name = "arrayJoin";
static constexpr auto global_in_function_name = "globalIn";
static constexpr auto global_not_in_function_name = "globalNotIn";


MergeTreeWhereOptimizer::MergeTreeWhereOptimizer(
	ASTPtr & query, const Context & context, const MergeTreeData & data, const Names & column_names,
	Logger * log)
	: primary_key_columns{ext::map<std::unordered_set>(data.getSortDescription(),
		[] (const SortColumnDescription & col) { return col.column_name; })
		},
		table_columns{ext::map<std::unordered_set>(data.getColumnsList(),
		[] (const NameAndTypePair & col) { return col.name; })
		}, block_with_constants{PKCondition::getBlockWithConstants(query, context, data.getColumnsList())},
		log{log}
{
	calculateColumnSizes(data, column_names);
	auto & select = typeid_cast<ASTSelectQuery &>(*query);
	determineArrayJoinedNames(select);
	optimize(select);
}


void MergeTreeWhereOptimizer::optimize(ASTSelectQuery & select) const
{
	if (!select.where_expression || select.prewhere_expression)
		return;

	const auto function = typeid_cast<ASTFunction *>(select.where_expression.get());
	if (function && function->name == and_function_name)
		optimizeConjunction(select, function);
	else
		optimizeArbitrary(select);
}


void MergeTreeWhereOptimizer::calculateColumnSizes(const MergeTreeData & data, const Names & column_names)
{
	for (const auto & column_name : column_names)
	{
		const auto column_size = data.getColumnCompressedSize(column_name);

		column_sizes[column_name] = column_size;
		total_column_size += column_size;
	}
}


void MergeTreeWhereOptimizer::optimizeConjunction(ASTSelectQuery & select, ASTFunction * const fun) const
{
	/// used as max possible size and indicator that appropriate condition has not been found
	const auto no_such_condition = std::numeric_limits<std::size_t>::max();

	/// { first: condition index, second: summary column size }
	std::pair<std::size_t, std::size_t> lightest_good_condition{no_such_condition, no_such_condition};
	std::pair<std::size_t, std::size_t> lightest_viable_condition{no_such_condition, no_such_condition};

	auto & conditions = fun->arguments->children;

	/// remove condition by swapping it with the last one and calling ::pop_back()
	const auto remove_condition_at_index = [&conditions] (const std::size_t idx) {
		if (idx < conditions.size() - 1)
			std::swap(conditions[idx], conditions.back());
		conditions.pop_back();
	};

	/// linearize conjunction and divide conditions into "good" and not-"good" ones
	for (std::size_t idx = 0; idx < conditions.size();)
	{
		const auto condition = conditions[idx].get();

		/// linearize sub-conjunctions
		if (const auto function = typeid_cast<ASTFunction *>(condition))
		{
			if (function->name == and_function_name)
			{
				for (auto & child : function->arguments->children)
					conditions.emplace_back(std::move(child));

				/// remove the condition corresponding to conjunction
				remove_condition_at_index(idx);

				/// continue iterating without increment to ensure the just added conditions are processed
				continue;
			}
		}

		SCOPE_EXIT(++idx);

		if (cannotBeMoved(condition))
			continue;

		IdentifierNameSet identifiers{};
		collectIdentifiersNoSubqueries(condition, identifiers);

		/// do not take into consideration the conditions consisting only of primary key columns
		if (!hasPrimaryKeyAtoms(condition) && isSubsetOfTableColumns(identifiers))
		{
			/// calculate size of columns involved in condition
			const auto cond_columns_size = getIdentifiersColumnSize(identifiers);

			/// place condition either in good or viable conditions set
			auto & good_or_viable_condition = isConditionGood(condition) ? lightest_good_condition : lightest_viable_condition;
			if (good_or_viable_condition.second > cond_columns_size)
			{
				good_or_viable_condition.first = idx;
				good_or_viable_condition.second = cond_columns_size;
			}
		}
	}

	const auto move_condition_to_prewhere = [&] (const std::size_t idx) {
		select.prewhere_expression = conditions[idx];
		select.children.push_back(select.prewhere_expression);
		LOG_DEBUG(log, "MergeTreeWhereOptimizer: condition `" << select.prewhere_expression << "` moved to PREWHERE");

		/** Replace conjunction with the only remaining argument if only two conditions were present,
			*  remove selected condition from conjunction otherwise. */
		if (conditions.size() == 2)
		{
			/// find old where_expression in children of select
			const auto it = std::find(std::begin(select.children), std::end(select.children), select.where_expression);
			/// replace where_expression with the remaining argument
			select.where_expression = std::move(conditions[idx == 0 ? 1 : 0]);
			/// overwrite child entry with the new where_expression
			*it = select.where_expression;
		}
		else
			remove_condition_at_index(idx);
	};

	/// if there is a "good" condition - move it to PREWHERE
	if (lightest_good_condition.first != no_such_condition)
	{
		move_condition_to_prewhere(lightest_good_condition.first);
	}
	else if (lightest_viable_condition.first != no_such_condition)
	{
		/// check that the relative column size is less than max
		if (total_column_size != 0)
		{
			/// calculate relative size of condition's columns
			const auto cond_columns_size = lightest_viable_condition.second;
			const auto columns_relative_size = static_cast<float>(cond_columns_size) / total_column_size;

			/// do nothing if it exceeds max relative size
			if (columns_relative_size > max_columns_relative_size)
				return;
		}

		move_condition_to_prewhere(lightest_viable_condition.first);
	}
}


void MergeTreeWhereOptimizer::optimizeArbitrary(ASTSelectQuery & select) const
{
	auto & condition = select.where_expression;

	/// do not optimize restricted expressions
	if (cannotBeMoved(select.where_expression.get()))
		return;

	IdentifierNameSet identifiers{};
	collectIdentifiersNoSubqueries(condition.get(), identifiers);

	if (hasPrimaryKeyAtoms(condition.get()) || !isSubsetOfTableColumns(identifiers))
		return;

	/// if condition is not "good" - check that it can be moved
	if (!isConditionGood(condition.get()) && total_column_size != 0)
	{
		const auto cond_columns_size = getIdentifiersColumnSize(identifiers);
		const auto columns_relative_size = static_cast<float>(cond_columns_size) / total_column_size;

		if (columns_relative_size > max_columns_relative_size)
			return;
	}

	/// add the condition to PREWHERE, remove it from WHERE
	std::swap(select.prewhere_expression, condition);
	LOG_DEBUG(log, "MergeTreeWhereOptimizer: condition `" << select.prewhere_expression << "` moved to PREWHERE");
}


std::size_t MergeTreeWhereOptimizer::getIdentifiersColumnSize(const IdentifierNameSet & identifiers) const
{
	/** for expressions containing no columns (or where columns could not be determined otherwise) assume maximum
		*	possible size so they do not have priority in eligibility over other expressions. */
	if (identifiers.empty())
		return std::numeric_limits<std::size_t>::max();

	std::size_t size{};

	for (const auto & identifier : identifiers)
		if (column_sizes.count(identifier))
			size += column_sizes.find(identifier)->second;

	return size;
}


bool MergeTreeWhereOptimizer::isConditionGood(const IAST * condition) const
{
	const auto function = typeid_cast<const ASTFunction *>(condition);
	if (!function)
		return false;

	/** we are only considering conditions of form `equals(one, another)` or `one = another`,
		* especially if either `one` or `another` is ASTIdentifier */
	if (function->name != equals_function_name)
		return false;

	auto left_arg = function->arguments->children.front().get();
	auto right_arg = function->arguments->children.back().get();

	/// try to ensure left_arg points to ASTIdentifier
	if (!typeid_cast<const ASTIdentifier *>(left_arg) && typeid_cast<const ASTIdentifier *>(right_arg))
		std::swap(left_arg, right_arg);

	if (typeid_cast<const ASTIdentifier *>(left_arg))
	{
		/// condition may be "good" if only right_arg is a constant and its value is outside the threshold
		if (const auto literal = typeid_cast<const ASTLiteral *>(right_arg))
		{
			const auto & field = literal->value;
			const auto type = field.getType();

			/// check the value with respect to threshold
			if (type == Field::Types::UInt64)
			{
				const auto value = field.get<UInt64>();
				return value > threshold;
			}
			else if (type == Field::Types::Int64)
			{
				const auto value = field.get<Int64>();
				return value < -threshold || threshold < value;
			}
			else if (type == Field::Types::Float64)
			{
				const auto value = field.get<Float64>();
				return value < threshold || threshold < value;
			}
		}
	}

	return false;
}


void MergeTreeWhereOptimizer::collectIdentifiersNoSubqueries(const IAST * const ast, IdentifierNameSet & set)
{
	if (const auto identifier = typeid_cast<const ASTIdentifier *>(ast))
		return (void) set.insert(identifier->name);

	if (typeid_cast<const ASTSubquery *>(ast))
		return;

	for (const auto & child : ast->children)
		collectIdentifiersNoSubqueries(child.get(), set);
}


bool MergeTreeWhereOptimizer::hasPrimaryKeyAtoms(const IAST * ast) const
{
	if (const auto func = typeid_cast<const ASTFunction *>(ast))
	{
		const auto & args = func->arguments->children;

		if ((func->name == "not" && 1 == args.size()) || func->name == "and" || func->name == "or")
		{
			for (const auto & arg : args)
				if (hasPrimaryKeyAtoms(arg.get()))
					return true;

			return false;
		}
	}

	return isPrimaryKeyAtom(ast);
}


bool MergeTreeWhereOptimizer::isPrimaryKeyAtom(const IAST * const ast) const
{
	if (const auto func = typeid_cast<const ASTFunction *>(ast))
	{
		if (!PKCondition::atom_map.count(func->name))
			return false;

		const auto & args = func->arguments->children;
		if (args.size() != 2)
			return false;

		const auto & first_arg_name = args.front()->getColumnName();
		const auto & second_arg_name = args.back()->getColumnName();

		if ((primary_key_columns.count(first_arg_name) && isConstant(args[1])) ||
			(primary_key_columns.count(second_arg_name) && isConstant(args[0])) ||
			(primary_key_columns.count(first_arg_name)
				&& (typeid_cast<const ASTSet *>(args[1].get()) || typeid_cast<const ASTSubquery *>(args[1].get()))))
			return true;
	}

	return false;
}


bool MergeTreeWhereOptimizer::isConstant(const ASTPtr & expr) const
{
	const auto column_name = expr->getColumnName();

	if (typeid_cast<const ASTLiteral *>(expr.get()) ||
		(block_with_constants.has(column_name) && block_with_constants.getByName(column_name).column->isConst()))
		return true;

	return false;
}


bool MergeTreeWhereOptimizer::isSubsetOfTableColumns(const IdentifierNameSet & identifiers) const
{
	for (const auto & identifier : identifiers)
		if (table_columns.count(identifier) == 0)
			return false;

	return true;
}


bool MergeTreeWhereOptimizer::cannotBeMoved(const IAST * ptr) const
{
	if (const auto function_ptr = typeid_cast<const ASTFunction *>(ptr))
	{
		/// disallow arrayJoin expressions to be moved to PREWHERE for now
		if (array_join_function_name == function_ptr->name)
			return true;

		/// disallow GLOBAL IN, GLOBAL NOT IN
		if (global_in_function_name == function_ptr->name
			|| global_not_in_function_name == function_ptr->name)
			return true;

		/// indexHint is a special function that it does not make sense to transfer to PREWHERE
		if ("indexHint" == function_ptr->name)
			return true;
	}
	else if (const auto identifier_ptr = typeid_cast<const ASTIdentifier *>(ptr))
	{
		/// disallow moving result of ARRAY JOIN to PREWHERE
		if (identifier_ptr->kind == ASTIdentifier::Column)
			if (array_joined_names.count(identifier_ptr->name) ||
				array_joined_names.count(DataTypeNested::extractNestedTableName(identifier_ptr->name)))
				return true;
	}

	for (const auto & child : ptr->children)
		if (cannotBeMoved(child.get()))
			return true;

	return false;
}


void MergeTreeWhereOptimizer::determineArrayJoinedNames(ASTSelectQuery & select)
{
	auto array_join_expression_list = select.array_join_expression_list();

	/// much simplified code from ExpressionAnalyzer::getArrayJoinedColumns()
	if (!array_join_expression_list)
		return;

	for (const auto & ast : array_join_expression_list->children)
		array_joined_names.emplace(ast->getAliasOrColumnName());
}

}