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src/sql/engine/expr/ob_expr_estimate_ndv.cpp

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+/**
+ * Copyright (c) 2021 OceanBase
+ * OceanBase CE is licensed under Mulan PubL v2.
+ * You can use this software according to the terms and conditions of the Mulan PubL v2.
+ * You may obtain a copy of Mulan PubL v2 at:
+ *          http://license.coscl.org.cn/MulanPubL-2.0
+ * THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+ * EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT,
+ * MERCHANTABILITY OR FIT FOR A PARTICULAR PURPOSE.
+ * See the Mulan PubL v2 for more details.
+ */
+
+#define USING_LOG_PREFIX SQL_ENG
+#include "sql/engine/expr/ob_expr_estimate_ndv.h"
+
+constexpr double DOUBLE_OVERFLOW_UINT64 = static_cast<double>(UINT64_MAX);
+
+using namespace oceanbase::common;
+
+namespace oceanbase {
+namespace sql {
+ObExprEstimateNdv::ObExprEstimateNdv(ObIAllocator& alloc)
+    : ObFuncExprOperator(alloc, T_FUN_SYS_ESTIMATE_NDV, "estimate_ndv", 1, NOT_ROW_DIMENSION)
+{}
+
+ObExprEstimateNdv::~ObExprEstimateNdv()
+{}
+
+int ObExprEstimateNdv::calc_result_type1(ObExprResType& type, ObExprResType& type1, ObExprTypeCtx& type_ctx) const
+{
+  UNUSED(type_ctx);
+  int ret = OB_SUCCESS;
+  type1.set_calc_type(ObVarcharType);
+  type1.set_calc_collation_type(ObCharset::get_system_collation());
+  if (OB_LIKELY(NOT_ROW_DIMENSION == row_dimension_)) {
+    type.set_uint64();
+    type.set_precision(ObAccuracy::DDL_DEFAULT_ACCURACY[ObUInt64Type].precision_);
+    type.set_scale(ObAccuracy::DDL_DEFAULT_ACCURACY[ObUInt64Type].scale_);
+  } else {
+    ret = OB_ERR_INVALID_TYPE_FOR_OP;
+  }
+  return ret;
+}
+
+int ObExprEstimateNdv::calc_result1(ObObj& result, const ObObj& obj, ObExprCtx& expr_ctx) const
+{
+  return llc_estimate_ndv(result, obj, expr_ctx);
+}
+
+int ObExprEstimateNdv::llc_estimate_ndv(ObObj& result, const ObObj& obj, ObExprCtx& expr_ctx)
+{
+  int ret = OB_SUCCESS;
+  UNUSED(expr_ctx);
+  TYPE_CHECK(obj, ObVarcharType);
+  const ObString& bitmap_str = obj.get_string();
+  double res_double = 0.0;
+  if (OB_FAIL(llc_estimate_ndv(res_double, bitmap_str))) {
+    LOG_WARN("calculate estimate ndv failed.");
+  } else if (OB_UNLIKELY(res_double >= DOUBLE_OVERFLOW_UINT64)) {
+    LOG_WARN("estimate ndv value overflows", K(res_double));
+    result.set_null();
+  } else {
+    result.set_uint64(static_cast<uint64_t>(res_double));
+  }
+  return ret;
+}
+
+void ObExprEstimateNdv::llc_estimate_ndv(int64_t& result, const ObString& bitmap_str)
+{
+  int ret = OB_SUCCESS;
+  double res_double = 0.0;
+  result = OB_INVALID_COUNT;
+  if (OB_FAIL(llc_estimate_ndv(res_double, bitmap_str))) {
+    LOG_WARN("calculate estimate ndv failed.");
+  } else if (OB_UNLIKELY(res_double >= DOUBLE_OVERFLOW_UINT64)) {
+    LOG_WARN("estimate ndv value overflows", K(res_double));
+  } else {
+    result = static_cast<int64_t>(res_double);
+  }
+}
+uint64_t ObExprEstimateNdv::llc_leading_zeros(uint64_t value, uint64_t bit_width)
+{
+  return std::min(bit_width, static_cast<uint64_t>(__builtin_clzll(value)));
+}
+
+inline double ObExprEstimateNdv::llc_alpha_times_m_square(const uint64_t m)
+{
+  double alpha = 0.0;
+  switch (m) {
+    case 16:
+      alpha = 0.673;
+      break;
+    case 32:
+      alpha = 0.697;
+      break;
+    case 64:
+      alpha = 0.709;
+      break;
+    default:
+      alpha = 0.7213 / (1 + 1.079 / static_cast<double>(m));
+      break;
+  }
+  return alpha * static_cast<double>(m) * static_cast<double>(m);
+}
+
+int ObExprEstimateNdv::llc_estimate_ndv(double& estimate_ndv, const ObString& bitmap_buf)
+{
+  int ret = OB_SUCCESS;
+  double sum_of_pmax = 0.0;
+  uint64_t num_empty_buckets = 0;
+  ObString::obstr_size_t llc_num_buckets = bitmap_buf.length();
+  if (OB_UNLIKELY(!llc_is_num_buckets_valid(llc_num_buckets))) {
+    ret = OB_INVALID_ARGUMENT;
+    LOG_WARN("number of buckets is invalid", K(llc_num_buckets));
+  } else {
+    for (int64_t i = 0; i < llc_num_buckets; ++i) {
+      sum_of_pmax += 1.0 / static_cast<double>((1ULL << (bitmap_buf[i])));
+      if (bitmap_buf[i] == 0) {
+        ++num_empty_buckets;
+      }
+    }
+    estimate_ndv = llc_alpha_times_m_square(llc_num_buckets) / sum_of_pmax;
+    if (OB_UNLIKELY(estimate_ndv >= DOUBLE_OVERFLOW_UINT64)) {
+      LOG_WARN("estimate ndv value overflows", K(estimate_ndv));
+    } else {
+      if (estimate_ndv <= 2.5 * static_cast<double>(llc_num_buckets)) {
+        if (0 != num_empty_buckets) {
+          // use linear count
+          estimate_ndv = static_cast<double>(llc_num_buckets) *
+                         log(static_cast<double>(llc_num_buckets) / static_cast<double>(num_empty_buckets));
+        }
+      }
+    }
+  }
+  return ret;
+}
+
+bool ObExprEstimateNdv::llc_is_num_buckets_valid(int64_t num_buckets)
+{
+  return (num_buckets >= LLC_NUM_BUCKETS_MIN) && (num_buckets <= LLC_NUM_BUCKETS_MAX) &&
+         !(num_buckets & (num_buckets - 1));
+}
+
+int ObExprEstimateNdv::calc_estimate_ndv_expr(const ObExpr& expr, ObEvalCtx& ctx, ObDatum& res_datum)
+{
+  int ret = OB_SUCCESS;
+  ObDatum* arg = NULL;
+  if (OB_FAIL(expr.eval_param_value(ctx, arg))) {
+    LOG_WARN("eval arg failed", K(ret));
+  } else if (arg->is_null()) {
+    ret = OB_INVALID_ARGUMENT;
+    LOG_WARN("arg cannot be null", K(ret));
+  } else {
+    const ObString& bitmap_str = arg->get_string();
+    double res_double = 0.0;
+    if (OB_FAIL(ObExprEstimateNdv::llc_estimate_ndv(res_double, bitmap_str))) {
+      LOG_WARN("calculate estimate ndv failed.");
+    } else if (OB_UNLIKELY(res_double >= DOUBLE_OVERFLOW_UINT64)) {
+      LOG_WARN("estimate ndv value overflows", K(res_double));
+      res_datum.set_null();
+    } else {
+      res_datum.set_uint(static_cast<uint64_t>(res_double));
+    }
+  }
+  return ret;
+}
+
+int ObExprEstimateNdv::cg_expr(ObExprCGCtx& expr_cg_ctx, const ObRawExpr& raw_expr, ObExpr& rt_expr) const
+{
+  int ret = OB_SUCCESS;
+  UNUSED(expr_cg_ctx);
+  UNUSED(raw_expr);
+  rt_expr.eval_func_ = calc_estimate_ndv_expr;
+  return ret;
+}
+
+} /* namespace sql */
+} /* namespace oceanbase */