#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';
 * -------------------------------
 *
 * 5. Replaces chains of inequality functions inside an AND with a single NOT IN operator.
 * The replacement is done if:
 *  - one of the operands of the inequality function is a constant
 *  - length of chain is at least 'optimize_min_inequality_conjunction_chain_length' long OR the expression has type of LowCardinality
 *
 * E.g. (optimize_min_inequality_conjunction_chain_length = 2)
 * -------------------------------
 * SELECT *
 * FROM table
 * WHERE a <> 1 AND a <> 2;
 *
 * will be transformed into
 *
 * SELECT *
 * FROM TABLE
 * WHERE a NOT IN (1, 2);
 * -------------------------------
 *
 * 6. Remove unnecessary IS NULL checks in JOIN ON clause
 *   - equality check with explicit IS NULL check replaced with <=> operator
 * -------------------------------
 * SELECT * FROM t1 JOIN t2 ON a = b OR (a IS NULL AND b IS NULL)
 * SELECT * FROM t1 JOIN t2 ON a <=> b OR (a IS NULL AND b IS NULL)
 *
 * will be transformed into
 *
 * SELECT * FROM t1 JOIN t2 ON a <=> b
 * -------------------------------
 *
 * 7. Remove redundant equality checks on boolean functions.
 *  - these requndant checks cause the primary index to not be used when if the query involves any primary key columns
 * -------------------------------
 * SELECT * FROM t1 WHERE a IN (n) = 1
 * SELECT * FROM t1 WHERE a IN (n) = 0
 *
 * will be transformed into
 *
 * SELECT * FROM t1 WHERE a IN (n)
 * SELECT * FROM t1 WHERE NOT a IN (n)
 * -------------------------------
 */

class LogicalExpressionOptimizerPass final : public IQueryTreePass
{
public:
    String getName() override { return "LogicalExpressionOptimizer"; }

    String getDescription() override
    {
        return "Transforms chains of logical expressions if possible, i.e. "
            "replace chains of equality functions inside an OR with a single IN operator";
    }

    void run(QueryTreeNodePtr & query_tree_node, ContextPtr context) override;
};

}