BUG#38380978: Regression for TPC-DS Q1 with statistics cache

Problem: Stats were being exchanged between table scan and eq_ref paths, exchanging stats here lead to issues, because table scan and index lookups have different output rows, and should not be exchanged. Change-Id: Ifa672a761442c579ba576158c118914eb09000b6

Author: shaginhekvs

sql/join_optimizer/join_optimizer.cc

@@ -6494,6 +6494,19 @@ void CostingReceiver::CommitBitsetsToHeap(AccessPath *path) const {
  return all_ok;
 }
 
+bool IsAccessPathRefType(AccessPath *path) {
+ return path->type == AccessPath::REF || path->type == AccessPath::EQ_REF;
+}
+/*
+ * This function returns true when 2 equivalent paths representing a Subgraph
+ * pair should not exchange their statistics from statistics cache, because due
+ * to actual execution characteristics the statistics could indeed be quite
+ * different.
+ */
+bool ArePathsStatsExchangeIneligible(AccessPath *path, AccessPath *other_path) {
+ return IsAccessPathRefType(path) || IsAccessPathRefType(other_path);
+}
+
 /**
  Propose the given access path as an alternative to the existing access paths
  for the same task (assuming any exist at all), and hold a “tournament” to find
@@ -6612,16 +6625,18 @@ AccessPath *CostingReceiver::ProposeAccessPath(
  secondary_engine_nrows_params.access_path = path;
  bool cur_use_sc =
  ApplySecondaryEngineNrowsHook(secondary_engine_nrows_params);
- /* we equalise Nrows for other similar paths, but not for REF's cause
- * lookup and scan can have different Nrows. */
- if (was_other_used_sc && path->type != AccessPath::REF) {
+ /* we equalise Nrows for other similar paths, but not for access paths
+ * where it might not make sense due to different execution
+ * characteristics. */
+ if (was_other_used_sc &&
+ !ArePathsStatsExchangeIneligible(path, other_path)) {
  path->set_num_output_rows(other_path->num_output_rows());
  if (path->num_output_rows_before_filter < path->num_output_rows()) {
  path->num_output_rows_before_filter = path->num_output_rows();
  }
 
  } else if (!was_other_used_sc && cur_use_sc &&
- other_path->type != AccessPath::REF) {
+ !ArePathsStatsExchangeIneligible(path, other_path)) {
  other_path->set_num_output_rows(path->num_output_rows());
  if (other_path->num_output_rows_before_filter <
  other_path->num_output_rows()) {