54 lines
2.5 KiB
C++
54 lines
2.5 KiB
C++
/**
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* Copyright (c) 2021 OceanBase
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* OceanBase CE is licensed under Mulan PubL v2.
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* You can use this software according to the terms and conditions of the Mulan PubL v2.
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* You may obtain a copy of Mulan PubL v2 at:
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* http://license.coscl.org.cn/MulanPubL-2.0
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* THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
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* EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT,
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* MERCHANTABILITY OR FIT FOR A PARTICULAR PURPOSE.
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* See the Mulan PubL v2 for more details.
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*/
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#ifndef OCEANBASE_SQL_ENGINE_EXPR_ESTIMATE_NDV_H_
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#define OCEANBASE_SQL_ENGINE_EXPR_ESTIMATE_NDV_H_
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#include "sql/engine/expr/ob_expr_operator.h"
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namespace oceanbase {
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namespace sql {
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class ObExprEstimateNdv : public ObFuncExprOperator {
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public:
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explicit ObExprEstimateNdv(common::ObIAllocator &alloc);
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virtual ~ObExprEstimateNdv();
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virtual int calc_result_type1(ObExprResType &type,
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ObExprResType &type1,
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common::ObExprTypeCtx &type_ctx) const;
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static void llc_estimate_ndv(int64_t &result, const common::ObString &bitmap_str);
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static int llc_estimate_ndv(double &estimate_ndv, const common::ObString &bitmap_buf);
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// 计算value的leading zeros。在HyperLogLogCount中，一个hash值的前面若干位要用来做分桶，
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// 这里的传入参数value是通过左移移除掉分桶部分后的部分，它的实际有效位数是高bit_width位。
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static uint64_t llc_leading_zeros(uint64_t value, uint64_t bit_width);
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static bool llc_is_num_buckets_valid(int64_t num_buckets);
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// for engine 3.0
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virtual int cg_expr(ObExprCGCtx &expr_cg_ctx, const ObRawExpr &raw_expr,
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ObExpr &rt_expr) const override;
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static int calc_estimate_ndv_expr(const ObExpr &expr, ObEvalCtx &ctx,
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ObDatum &res_datum);
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private:
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// 计算HyperLogLogCount中的 alpha * m^2 的函数。计算涉及变量m(uint64_t)
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// 转double的步骤，调用者需要考虑可能的精度损失（目前m通常不超过4096，无损失）。
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static inline double llc_alpha_times_m_square(const uint64_t m);
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// 根据Google的HLLC论文桶数至少取2^4(16)个，至多取2^16(65536)个。
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static const int LLC_NUM_BUCKETS_MIN = (1 << 4);
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static const int LLC_NUM_BUCKETS_MAX = (1 << 16);
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private:
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DISALLOW_COPY_AND_ASSIGN(ObExprEstimateNdv);
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};
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} /* namespace sql */
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} /* namespace oceanbase */
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#endif /* OCEANBASE_SQL_ENGINE_EXPR_ESTIMATE_NDV_H_ */
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