From 0e9c4cda2bf0f25718b8bfb90e2f318323ae8b29 Mon Sep 17 00:00:00 2001
From: xianyu-w <707512433@qq.com>
Date: Thu, 11 Jul 2024 13:17:10 +0000
Subject: [PATCH] [CP] Fix limit group by cardinality estimation bug

---
 src/sql/optimizer/ob_log_distinct.cpp | 42 ++++++++++++++++++++++++++-
 src/sql/optimizer/ob_log_distinct.h   |  5 +++-
 src/sql/optimizer/ob_log_group_by.cpp |  2 +-
 3 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/src/sql/optimizer/ob_log_distinct.cpp b/src/sql/optimizer/ob_log_distinct.cpp
index ad3eb51bb..9910816d1 100644
--- a/src/sql/optimizer/ob_log_distinct.cpp
+++ b/src/sql/optimizer/ob_log_distinct.cpp
@@ -169,7 +169,15 @@ int ObLogDistinct::do_re_est_cost(EstimateCostInfo &param, double &card, double
         param.need_row_count_ < child_card &&
         param.need_row_count_ < total_ndv_) {
       child_ndv = param.need_row_count_;
-      param.need_row_count_ = child_card * (1 - std::pow((1 - child_ndv / total_ndv_), total_ndv_ / child_card));
+      if (input_sorted_) {
+        if (param.need_row_count_ > 1.0) {
+          param.need_row_count_ = child_card * (param.need_row_count_ - 1.0) / total_ndv_ + 1.0;
+        } else {
+          // do nothing
+        }
+      } else {
+        param.need_row_count_ = child_card * (1 - std::pow((1 - child_ndv / total_ndv_), total_ndv_ / child_card));
+      }
     } else {
       param.need_row_count_ = -1;
       need_scale_ndv = true;
@@ -395,5 +403,37 @@ int ObLogDistinct::check_use_child_ordering(bool &used, int64_t &inherit_child_o
   return ret;
 }
 
+int ObLogDistinct::compute_property()
+{
+  int ret = OB_SUCCESS;
+  ObLogicalOperator *top = get_child(first_child);
+  bool need_sort = false;
+  int64_t prefix_pos = 0;
+  if (OB_ISNULL(top) || OB_ISNULL(get_plan())) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("unexpected null", K(ret));
+  } else if (OB_FAIL(ObLogicalOperator::compute_property())) {
+    LOG_WARN("failed to compute op property", K(ret));
+  } else if (MERGE_AGGREGATE == algo_) {
+    input_sorted_ = true;
+  } else if (OB_FAIL(ObOptimizerUtil::check_need_sort(distinct_exprs_,
+                                                      NULL/*directions*/,
+                                                      top->get_op_ordering(),
+                                                      top->get_fd_item_set(),
+                                                      top->get_output_equal_sets(),
+                                                      top->get_output_const_exprs(),
+                                                      get_plan()->get_onetime_query_refs(),
+                                                      top->get_is_at_most_one_row(),
+                                                      need_sort,
+                                                      prefix_pos))) {
+    LOG_WARN("failed to check need sort", K(ret));
+  } else if (!need_sort) {
+    input_sorted_ = true;
+  } else {
+    input_sorted_ = false;
+  }
+  return ret;
+}
+
 }
 }
diff --git a/src/sql/optimizer/ob_log_distinct.h b/src/sql/optimizer/ob_log_distinct.h
index 28ddd3f8e..1760e77ee 100644
--- a/src/sql/optimizer/ob_log_distinct.h
+++ b/src/sql/optimizer/ob_log_distinct.h
@@ -28,7 +28,8 @@ public:
         is_block_mode_(false),
         is_push_down_(false),
         total_ndv_(-1.0),
-        force_push_down_(false)
+        force_push_down_(false),
+        input_sorted_(false)
   { }
   virtual ~ObLogDistinct()
   { }
@@ -76,6 +77,7 @@ public:
   inline void set_is_partition_gi(bool v) { is_partition_gi_ = v; }
   virtual int get_card_without_filter(double &card) override;
   virtual int check_use_child_ordering(bool &used, int64_t &inherit_child_ordering_index)override;
+  virtual int compute_property() override;
 
 private:
   common::ObSEArray<ObRawExpr*, 16, common::ModulePageAllocator, true> distinct_exprs_;
@@ -85,6 +87,7 @@ private:
   double total_ndv_;
   bool force_push_down_; // control by _aggregation_optimization_settings
   bool is_partition_gi_;
+  bool input_sorted_;
 private:
   DISALLOW_COPY_AND_ASSIGN(ObLogDistinct);
 };
diff --git a/src/sql/optimizer/ob_log_group_by.cpp b/src/sql/optimizer/ob_log_group_by.cpp
index f5e59d5b2..83bece9f0 100644
--- a/src/sql/optimizer/ob_log_group_by.cpp
+++ b/src/sql/optimizer/ob_log_group_by.cpp
@@ -285,7 +285,7 @@ int ObLogGroupBy::do_re_est_cost(EstimateCostInfo &param, double &card, double &
         need_ndv /= selectivity;
       }
       if (child_card > 0) {
-        param.need_row_count_ = child_card * (1 - std::pow((1 - need_ndv / child_ndv), child_ndv / child_card));
+        param.need_row_count_ = child_card * need_ndv / child_ndv;
         param.need_row_count_ /= number_of_copies;
       } else {
         param.need_row_count_ = 0;