v15-0002-Allow-estimate_num_groups-to-pass-back-further-d.patch

From 5e9d5a0efc45a252c8201c29ca6858bd25137d82 Mon Sep 17 00:00:00 2001
From: "dgrowley@gmail.com" <dgrowley@gmail.com>
Date: Thu, 2 Jul 2020 16:06:36 +1200
Subject: [PATCH v15 2/5] Allow estimate_num_groups() to pass back further
 details about the estimation

Here we add a new output parameter to estimate_num_groups() to allow it to
set a flags variable with some bits to allow it to pass back additional
details to the caller which may be useful for decision making.

For now, the only new flag is one which indicates if the estimation
fell back on using the hard-coded constants in any part of the estimation.
Callers may like to change their behavior if this is set, and this gives
them the ability to do so. Callers may pass the flag pointer as NULL if
they have no interest in any of the flags.

We're not adding any actual usages of these flags here.  Some follow-up
commits will make use of this feature.
---
 contrib/postgres_fdw/postgres_fdw.c    |  2 +-
 src/backend/optimizer/path/costsize.c  |  3 ++-
 src/backend/optimizer/path/indxpath.c  |  1 +
 src/backend/optimizer/plan/planner.c   | 10 ++++++----
 src/backend/optimizer/prep/prepunion.c |  1 +
 src/backend/optimizer/util/pathnode.c  |  1 +
 src/backend/utils/adt/selfuncs.c       | 22 +++++++++++++++++++++-
 src/include/utils/selfuncs.h           | 17 ++++++++++++++++-
 8 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 35b48575c5..ed33d819e7 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3086,7 +3086,7 @@ estimate_path_cost_size(PlannerInfo *root,
 			numGroups = estimate_num_groups(root,
 											get_sortgrouplist_exprs(root->parse->groupClause,
 																	fpinfo->grouped_tlist),
-											input_rows, NULL);
+											input_rows, NULL, NULL);
 
 			/*
 			 * Get the retrieved_rows and rows estimates.  If there are HAVING
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index a25b674a19..b92c948588 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1969,7 +1969,8 @@ cost_incremental_sort(Path *path,
 
 	/* Estimate number of groups with equal presorted keys. */
 	if (!unknown_varno)
-		input_groups = estimate_num_groups(root, presortedExprs, input_tuples, NULL);
+		input_groups = estimate_num_groups(root, presortedExprs, input_tuples,
+										   NULL, NULL);
 
 	group_tuples = input_tuples / input_groups;
 	group_input_run_cost = input_run_cost / input_groups;
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 8c447cf0a2..8de302ddd3 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -1990,6 +1990,7 @@ adjust_rowcount_for_semijoins(PlannerInfo *root,
 			nunique = estimate_num_groups(root,
 										  sjinfo->semi_rhs_exprs,
 										  nraw,
+										  NULL,
 										  NULL);
 			if (rowcount > nunique)
 				rowcount = nunique;
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 20adb77ccc..3f9344b026 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -3717,7 +3717,8 @@ get_number_of_groups(PlannerInfo *root,
 					double		numGroups = estimate_num_groups(root,
 																groupExprs,
 																path_rows,
-																&gset);
+																&gset,
+																NULL);
 
 					gs->numGroups = numGroups;
 					rollup->numGroups += numGroups;
@@ -3742,7 +3743,8 @@ get_number_of_groups(PlannerInfo *root,
 					double		numGroups = estimate_num_groups(root,
 																groupExprs,
 																path_rows,
-																&gset);
+																&gset,
+																NULL);
 
 					gs->numGroups = numGroups;
 					gd->dNumHashGroups += numGroups;
@@ -3758,7 +3760,7 @@ get_number_of_groups(PlannerInfo *root,
 												 target_list);
 
 			dNumGroups = estimate_num_groups(root, groupExprs, path_rows,
-											 NULL);
+											 NULL, NULL);
 		}
 	}
 	else if (parse->groupingSets)
@@ -4807,7 +4809,7 @@ create_distinct_paths(PlannerInfo *root,
 												parse->targetList);
 		numDistinctRows = estimate_num_groups(root, distinctExprs,
 											  cheapest_input_path->rows,
-											  NULL);
+											  NULL, NULL);
 	}
 
 	/*
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index becdcbb872..037dfaacfd 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -338,6 +338,7 @@ recurse_set_operations(Node *setOp, PlannerInfo *root,
 				*pNumGroups = estimate_num_groups(subroot,
 												  get_tlist_exprs(subquery->targetList, false),
 												  subpath->rows,
+												  NULL,
 												  NULL);
 		}
 	}
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 69b83071cf..d5c66780ac 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1713,6 +1713,7 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.rows = estimate_num_groups(root,
 											  sjinfo->semi_rhs_exprs,
 											  rel->rows,
+											  NULL,
 											  NULL);
 	numCols = list_length(sjinfo->semi_rhs_exprs);
 
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 52314d3aa1..2306602a51 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3241,6 +3241,7 @@ typedef struct
 	Node	   *var;			/* might be an expression, not just a Var */
 	RelOptInfo *rel;			/* relation it belongs to */
 	double		ndistinct;		/* # distinct values */
+	bool		isdefault;		/* true if DEFAULT_NUM_DISTINCT was used */
 } GroupVarInfo;
 
 static List *
@@ -3287,6 +3288,7 @@ add_unique_group_var(PlannerInfo *root, List *varinfos,
 	varinfo->var = var;
 	varinfo->rel = vardata->rel;
 	varinfo->ndistinct = ndistinct;
+	varinfo->isdefault = isdefault;
 	varinfos = lappend(varinfos, varinfo);
 	return varinfos;
 }
@@ -3311,6 +3313,12 @@ add_unique_group_var(PlannerInfo *root, List *varinfos,
  *	pgset - NULL, or a List** pointing to a grouping set to filter the
  *		groupExprs against
  *
+ * Outputs:
+ *	estinfo - When passed as non-NULL, the function will set bits in the
+ *		"flags" field in order to provide callers with additional information
+ *		about the estimation.  Currently, we only set the SELFLAG_USED_DEFAULT
+ *		bit if we used any default values in the estimation.
+ *
  * Given the lack of any cross-correlation statistics in the system, it's
  * impossible to do anything really trustworthy with GROUP BY conditions
  * involving multiple Vars.  We should however avoid assuming the worst
@@ -3358,7 +3366,7 @@ add_unique_group_var(PlannerInfo *root, List *varinfos,
  */
 double
 estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
-					List **pgset)
+					List **pgset, EstimationInfo *estinfo)
 {
 	List	   *varinfos = NIL;
 	double		srf_multiplier = 1.0;
@@ -3366,6 +3374,10 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 	ListCell   *l;
 	int			i;
 
+	/* Zero the estinfo output parameter, if non-NULL */
+	if (estinfo != NULL)
+		memset(estinfo, 0, sizeof(EstimationInfo));
+
 	/*
 	 * We don't ever want to return an estimate of zero groups, as that tends
 	 * to lead to division-by-zero and other unpleasantness.  The input_rows
@@ -3569,6 +3581,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 					if (relmaxndistinct < varinfo2->ndistinct)
 						relmaxndistinct = varinfo2->ndistinct;
 					relvarcount++;
+
+					/*
+					 * When varinfo2's isdefault is set then we'd better set
+					 * the SELFLAG_USED_DEFAULT bit in the EstimationInfo.
+					 */
+					if (estinfo != NULL && varinfo2->isdefault)
+						estinfo->flags |= SELFLAG_USED_DEFAULT;
+
 				}
 
 				/* we're done with this relation */
diff --git a/src/include/utils/selfuncs.h b/src/include/utils/selfuncs.h
index f9be539602..78cde58acc 100644
--- a/src/include/utils/selfuncs.h
+++ b/src/include/utils/selfuncs.h
@@ -68,6 +68,20 @@
 			p = 1.0; \
 	} while (0)
 
+/*
+ * A set of flags which some selectivity estimation functions can pass back to
+ * callers to provide further details about some assumptions which were made
+ * during the estimation.
+ */
+#define SELFLAG_USED_DEFAULT		(1 << 0)	/* Estimation fell back on one
+												 * of the DEFAULTs as defined
+												 * above. */
+
+typedef struct EstimationInfo
+{
+	uint32			flags;		/* Flags, as defined above to mark special
+								 * properties of the estimation. */
+} EstimationInfo;
 
 /* Return data from examine_variable and friends */
 typedef struct VariableStatData
@@ -197,7 +211,8 @@ extern void mergejoinscansel(PlannerInfo *root, Node *clause,
 							 Selectivity *rightstart, Selectivity *rightend);
 
 extern double estimate_num_groups(PlannerInfo *root, List *groupExprs,
-								  double input_rows, List **pgset);
+								  double input_rows, List **pgset,
+								  EstimationInfo *estinfo);
 
 extern void estimate_hash_bucket_stats(PlannerInfo *root,
 									   Node *hashkey, double nbuckets,
-- 
2.27.0