ClickHouse/dbms/src/Interpreters/LogicalExpressionsOptimizer.h
2017-06-03 19:21:00 +03:00

107 lines
3.3 KiB
C++

#pragma once
#include <Parsers/IAST.h>
#include <string>
#include <vector>
#include <map>
#include <unordered_map>
#include <unordered_set>
namespace DB
{
struct Settings;
class ASTFunction;
class ASTSelectQuery;
/** This class provides functions for optimizing boolean expressions within queries.
*
* For simplicity, we call a homogeneous OR-chain any expression having the following structure:
* expr = x1 OR ... OR expr = xN
* where `expr` is an arbitrary expression and x1, ..., xN are literals of the same type
*/
class LogicalExpressionsOptimizer final
{
public:
/// Constructor. Accepts the root of the query DAG.
LogicalExpressionsOptimizer(ASTSelectQuery * select_query_, const Settings & settings_);
/** Replace all rather long homogeneous OR-chains expr = x1 OR ... OR expr = xN
* on the expressions `expr` IN (x1, ..., xN).
*/
void perform();
LogicalExpressionsOptimizer(const LogicalExpressionsOptimizer &) = delete;
LogicalExpressionsOptimizer & operator=(const LogicalExpressionsOptimizer &) = delete;
private:
/** The OR function with the expression.
*/
struct OrWithExpression
{
OrWithExpression(ASTFunction * or_function_, const IAST::Hash & expression_,
const std::string & alias_);
bool operator<(const OrWithExpression & rhs) const;
ASTFunction * or_function;
const IAST::Hash expression;
const std::string alias;
};
struct Equalities
{
std::vector<ASTFunction *> functions;
bool is_processed = false;
};
using DisjunctiveEqualityChainsMap = std::map<OrWithExpression, Equalities>;
using DisjunctiveEqualityChain = DisjunctiveEqualityChainsMap::value_type;
private:
/** Collect information about all the equations in the OR chains (not necessarily homogeneous).
* This information is grouped by the expression that is on the left side of the equation.
*/
void collectDisjunctiveEqualityChains();
/** Check that the set of equalities expr = x1, ..., expr = xN fulfills the following two requirements:
* 1. It's not too small
* 2. x1, ... xN have the same type
*/
bool mayOptimizeDisjunctiveEqualityChain(const DisjunctiveEqualityChain & chain) const;
/// Insert the IN expression into the OR chain.
void addInExpression(const DisjunctiveEqualityChain & chain);
/// Delete the equalities that were replaced by the IN expressions.
void cleanupOrExpressions();
/// Delete OR expressions that have only one operand.
void fixBrokenOrExpressions();
/// Restore the original column order after optimization.
void reorderColumns();
private:
using ParentNodes = std::vector<IAST *>;
using FunctionParentMap = std::unordered_map<IAST *, ParentNodes>;
using ColumnToPosition = std::unordered_map<IAST *, size_t>;
private:
ASTSelectQuery * select_query;
const Settings & settings;
/// Information about the OR-chains inside the query.
DisjunctiveEqualityChainsMap disjunctive_equality_chains_map;
/// Number of processed OR-chains.
size_t processed_count = 0;
/// Parents of OR functions.
FunctionParentMap or_parent_map;
/// The position of each column.
ColumnToPosition column_to_position;
/// Set of nodes, that was visited.
std::unordered_set<void *> visited_nodes;
};
}