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#pragma once
#include <Analyzer/IQueryTreePass.h>
namespace DB
{
/**
* This pass tries to do optimizations on logical expression:
*
* 1. Replaces chains of equality functions inside an OR with a single IN operator.
* The replacement is done if:
* - one of the operands of the equality function is a constant
* - length of chain is at least 'optimize_min_equality_disjunction_chain_length' long OR the expression has type of LowCardinality
*
* E.g. (optimize_min_equality_disjunction_chain_length = 2)
* -------------------------------
* SELECT *
* FROM table
* WHERE a = 1 OR b = 'test' OR a = 2;
*
* will be transformed into
*
* SELECT *
* FROM TABLE
* WHERE b = 'test' OR a IN (1, 2);
* -------------------------------
*
* 2. Removes duplicate OR checks
* -------------------------------
* SELECT *
* FROM table
* WHERE a = 1 OR b = 'test' OR a = 1;
*
* will be transformed into
*
* SELECT *
* FROM TABLE
* WHERE a = 1 OR b = 'test';
* -------------------------------
*
* 3. Replaces AND chains with a single constant.
* The replacement is done if:
* - one of the operands of the equality function is a constant
* - constants are different for same expression
* -------------------------------
* SELECT *
* FROM table
* WHERE a = 1 AND b = 'test' AND a = 2;
*
* will be transformed into
*
* SELECT *
* FROM TABLE
* WHERE 0;
* -------------------------------
*
* 4. Removes duplicate AND checks
* -------------------------------
* SELECT *
* FROM table
* WHERE a = 1 AND b = 'test' AND a = 1;
*
* will be transformed into
*
* SELECT *
* FROM TABLE
* WHERE a = 1 AND b = 'test';
* -------------------------------
*/
class LogicalExpressionOptimizerPass final : public IQueryTreePass
{
public:
String getName() override { return "LogicalExpressionOptimizer"; }
String getDescription() override { return "Transform equality chain to a single IN function or a constant if possible"; }
void run(QueryTreeNodePtr query_tree_node, ContextPtr context) override;
};
}
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