Teach estimate_array_length() to use statistics where available.

If we have DECHIST statistics about the argument expression, use
the average number of distinct elements as the array length estimate.
(It'd be better to use the average total number of elements, but
that is not currently calculated by compute_array_stats(), and
it's unclear that it'd be worth extra effort to get.)

To do this, we have to change the signature of estimate_array_length
to pass the "root" pointer.  While at it, also change its result
type to "double".  That's probably not really necessary, but it
avoids any risk of overflow of the value extracted from DECHIST.
All existing callers are going to use the result in a "double"
calculation anyway.

Paul Jungwirth, reviewed by Jian He and myself

Discussion: https://postgr.es/m/CA+renyUnM2d+SmrxKpDuAdpiq6FOM=FByvi6aS6yi__qyf6j9A@mail.gmail.com

src/backend/optimizer/path/costsize.c

@@ -1256,7 +1256,7 @@ cost_tidscan(Path *path, PlannerInfo *root,
 	QualCost	qpqual_cost;
 	Cost		cpu_per_tuple;
 	QualCost	tid_qual_cost;
-	int			ntuples;
+	double		ntuples;
 	ListCell   *l;
 	double		spc_random_page_cost;
 
@@ -1283,7 +1283,7 @@ cost_tidscan(Path *path, PlannerInfo *root,
 			ScalarArrayOpExpr *saop = (ScalarArrayOpExpr *) qual;
 			Node	   *arraynode = (Node *) lsecond(saop->args);
 
-			ntuples += estimate_array_length(arraynode);
+			ntuples += estimate_array_length(root, arraynode);
 		}
 		else if (IsA(qual, CurrentOfExpr))
 		{
@@ -4770,7 +4770,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 		Node	   *arraynode = (Node *) lsecond(saop->args);
 		QualCost	sacosts;
 		QualCost	hcosts;
-		int			estarraylen = estimate_array_length(arraynode);
+		double		estarraylen = estimate_array_length(context->root, arraynode);
 
 		set_sa_opfuncid(saop);
 		sacosts.startup = sacosts.per_tuple = 0;
@@ -4808,7 +4808,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 			 */
 			context->total.startup += sacosts.startup;
 			context->total.per_tuple += sacosts.per_tuple *
-				estimate_array_length(arraynode) * 0.5;
+				estimate_array_length(context->root, arraynode) * 0.5;
 		}
 	}
 	else if (IsA(node, Aggref) ||
@@ -4859,7 +4859,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 		context->total.startup += perelemcost.startup;
 		if (perelemcost.per_tuple > 0)
 			context->total.per_tuple += perelemcost.per_tuple *
-				estimate_array_length((Node *) acoerce->arg);
+				estimate_array_length(context->root, (Node *) acoerce->arg);
 	}
 	else if (IsA(node, RowCompareExpr))
 	{

src/backend/utils/adt/arrayfuncs.c

@@ -6340,7 +6340,7 @@ array_unnest_support(PG_FUNCTION_ARGS)
 			/* We can use estimated argument values here */
 			arg1 = estimate_expression_value(req->root, linitial(args));
 
-			req->rows = estimate_array_length(arg1);
+			req->rows = estimate_array_length(req->root, arg1);
 			ret = (Node *) req;
 		}
 	}

src/backend/utils/adt/selfuncs.c

@@ -2128,10 +2128,11 @@ scalararraysel(PlannerInfo *root,
 /*
  * Estimate number of elements in the array yielded by an expression.
  *
- * It's important that this agree with scalararraysel.
+ * Note: the result is integral, but we use "double" to avoid overflow
+ * concerns.  Most callers will use it in double-type expressions anyway.
  */
-int
-estimate_array_length(Node *arrayexpr)
+double
+estimate_array_length(PlannerInfo *root, Node *arrayexpr)
 {
 	/* look through any binary-compatible relabeling of arrayexpr */
 	arrayexpr = strip_array_coercion(arrayexpr);
@@ -2152,11 +2153,39 @@ estimate_array_length(Node *arrayexpr)
 	{
 		return list_length(((ArrayExpr *) arrayexpr)->elements);
 	}
-	else
+	else if (arrayexpr)
 	{
-		/* default guess --- see also scalararraysel */
-		return 10;
+		/* See if we can find any statistics about it */
+		VariableStatData vardata;
+		AttStatsSlot sslot;
+		double		nelem = 0;
+
+		examine_variable(root, arrayexpr, 0, &vardata);
+		if (HeapTupleIsValid(vardata.statsTuple))
+		{
+			/*
+			 * Found stats, so use the average element count, which is stored
+			 * in the last stanumbers element of the DECHIST statistics.
+			 * Actually that is the average count of *distinct* elements;
+			 * perhaps we should scale it up somewhat?
+			 */
+			if (get_attstatsslot(&sslot, vardata.statsTuple,
+								 STATISTIC_KIND_DECHIST, InvalidOid,
+								 ATTSTATSSLOT_NUMBERS))
+			{
+				if (sslot.nnumbers > 0)
+					nelem = clamp_row_est(sslot.numbers[sslot.nnumbers - 1]);
+				free_attstatsslot(&sslot);
+			}
+		}
+		ReleaseVariableStats(vardata);
+
+		if (nelem > 0)
+			return nelem;
 	}
+
+	/* Else use a default guess --- this should match scalararraysel */
+	return 10;
 }
 
 /*
@@ -6540,7 +6569,7 @@ genericcostestimate(PlannerInfo *root,
 		if (IsA(rinfo->clause, ScalarArrayOpExpr))
 		{
 			ScalarArrayOpExpr *saop = (ScalarArrayOpExpr *) rinfo->clause;
-			int			alength = estimate_array_length(lsecond(saop->args));
+			double		alength = estimate_array_length(root, lsecond(saop->args));
 
 			if (alength > 1)
 				num_sa_scans *= alength;
@@ -6820,7 +6849,7 @@ btcostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 			{
 				ScalarArrayOpExpr *saop = (ScalarArrayOpExpr *) clause;
 				Node	   *other_operand = (Node *) lsecond(saop->args);
-				int			alength = estimate_array_length(other_operand);
+				double		alength = estimate_array_length(root, other_operand);
 
 				clause_op = saop->opno;
 				found_saop = true;
@@ -7414,7 +7443,7 @@ gincost_scalararrayopexpr(PlannerInfo *root,
 	{
 		counts->exactEntries++;
 		counts->searchEntries++;
-		counts->arrayScans *= estimate_array_length(rightop);
+		counts->arrayScans *= estimate_array_length(root, rightop);
 		return true;
 	}
 

src/include/utils/selfuncs.h

@@ -200,7 +200,7 @@ extern Selectivity scalararraysel(PlannerInfo *root,
 								  ScalarArrayOpExpr *clause,
 								  bool is_join_clause,
 								  int varRelid, JoinType jointype, SpecialJoinInfo *sjinfo);
-extern int	estimate_array_length(Node *arrayexpr);
+extern double estimate_array_length(PlannerInfo *root, Node *arrayexpr);
 extern Selectivity rowcomparesel(PlannerInfo *root,
 								 RowCompareExpr *clause,
 								 int varRelid, JoinType jointype, SpecialJoinInfo *sjinfo);