Dig down into sub-selects to look for column statistics.

If a sub-select's output column is a simple Var, recursively look for
statistics applying to that Var, and use them if available.  The need for
this was foreseen ages ago, but we didn't have enough infrastructure to do
it with reasonable speed until just now.

We punt and stick with default estimates if the subquery uses set
operations, GROUP BY, or DISTINCT, since those operations would change the
underlying column statistics (particularly, the relative frequencies of
different values) beyond recognition.  This means that the types of
sub-selects for which this improvement applies are fairly limited, since
most subqueries satisfying those restrictions would have gotten flattened
into the parent query anyway.  But it does help for some cases, such as
subqueries with ORDER BY or LIMIT.

src/backend/utils/adt/selfuncs.c

@@ -168,6 +168,8 @@ static double convert_one_bytea_to_scalar(unsigned char *value, int valuelen,
 							int rangelo, int rangehi);
 static char *convert_string_datum(Datum value, Oid typid);
 static double convert_timevalue_to_scalar(Datum value, Oid typid);
+static void examine_simple_variable(PlannerInfo *root, Var *var,
+						VariableStatData *vardata);
 static bool get_variable_range(PlannerInfo *root, VariableStatData *vardata,
 				   Oid sortop, Datum *min, Datum *max);
 static bool get_actual_variable_range(PlannerInfo *root,
@@ -4153,46 +4155,16 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 		(varRelid == 0 || varRelid == ((Var *) basenode)->varno))
 	{
 		Var		   *var = (Var *) basenode;
-		RangeTblEntry *rte;
 
+		/* Set up result fields other than the stats tuple */
 		vardata->var = basenode;	/* return Var without relabeling */
 		vardata->rel = find_base_rel(root, var->varno);
 		vardata->atttype = var->vartype;
 		vardata->atttypmod = var->vartypmod;
 		vardata->isunique = has_unique_index(vardata->rel, var->varattno);
 
-		rte = root->simple_rte_array[var->varno];
-
-		if (get_relation_stats_hook &&
-			(*get_relation_stats_hook) (root, rte, var->varattno, vardata))
-		{
-			/*
-			 * The hook took control of acquiring a stats tuple.  If it did
-			 * supply a tuple, it'd better have supplied a freefunc.
-			 */
-			if (HeapTupleIsValid(vardata->statsTuple) &&
-				!vardata->freefunc)
-				elog(ERROR, "no function provided to release variable stats with");
-		}
-		else if (rte->rtekind == RTE_RELATION)
-		{
-			vardata->statsTuple = SearchSysCache3(STATRELATTINH,
-												ObjectIdGetDatum(rte->relid),
-												Int16GetDatum(var->varattno),
-												  BoolGetDatum(rte->inh));
-			vardata->freefunc = ReleaseSysCache;
-		}
-		else
-		{
-			/*
-			 * XXX This means the Var comes from a JOIN or sub-SELECT. Later
-			 * add code to dig down into the join etc and see if we can trace
-			 * the variable to something with stats.  (But beware of
-			 * sub-SELECTs with DISTINCT/GROUP BY/etc.	Perhaps there are no
-			 * cases where this would really be useful, because we'd have
-			 * flattened the subselect if it is??)
-			 */
-		}
+		/* Try to locate some stats */
+		examine_simple_variable(root, var, vardata);
 
 		return;
 	}
@@ -4334,6 +4306,109 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 	}
 }
 
+/*
+ * examine_simple_variable
+ *		Handle a simple Var for examine_variable
+ *
+ * This is split out as a subroutine so that we can recurse to deal with
+ * Vars referencing subqueries.
+ *
+ * We already filled in all the fields of *vardata except for the stats tuple.
+ */
+static void
+examine_simple_variable(PlannerInfo *root, Var *var,
+						VariableStatData *vardata)
+{
+	RangeTblEntry *rte = root->simple_rte_array[var->varno];
+
+	Assert(IsA(rte, RangeTblEntry));
+
+	if (get_relation_stats_hook &&
+		(*get_relation_stats_hook) (root, rte, var->varattno, vardata))
+	{
+		/*
+		 * The hook took control of acquiring a stats tuple.  If it did supply
+		 * a tuple, it'd better have supplied a freefunc.
+		 */
+		if (HeapTupleIsValid(vardata->statsTuple) &&
+			!vardata->freefunc)
+			elog(ERROR, "no function provided to release variable stats with");
+	}
+	else if (rte->rtekind == RTE_RELATION)
+	{
+		/*
+		 * Plain table or parent of an inheritance appendrel, so look up the
+		 * column in pg_statistic
+		 */
+		vardata->statsTuple = SearchSysCache3(STATRELATTINH,
+											  ObjectIdGetDatum(rte->relid),
+											  Int16GetDatum(var->varattno),
+											  BoolGetDatum(rte->inh));
+		vardata->freefunc = ReleaseSysCache;
+	}
+	else if (rte->rtekind == RTE_SUBQUERY && !rte->inh)
+	{
+		/*
+		 * Plain subquery (not one that was converted to an appendrel).
+		 *
+		 * Punt if subquery uses set operations, GROUP BY, or DISTINCT --- any
+		 * of these will mash underlying columns' stats beyond recognition.
+		 * (Set ops are particularly nasty; if we forged ahead, we would
+		 * return stats relevant to only the leftmost subselect...)
+		 */
+		Query	   *subquery = rte->subquery;
+		RelOptInfo *rel;
+		TargetEntry *ste;
+
+		if (subquery->setOperations ||
+			subquery->groupClause ||
+			subquery->distinctClause)
+			return;
+
+		/*
+		 * OK, fetch RelOptInfo for subquery.  Note that we don't change the
+		 * rel returned in vardata, since caller expects it to be a rel of the
+		 * caller's query level.  Because we might already be recursing, we
+		 * can't use that rel pointer either, but have to look up the Var's
+		 * rel afresh.
+		 */
+		rel = find_base_rel(root, var->varno);
+
+		/* Subquery should have been planned already */
+		Assert(rel->subroot && IsA(rel->subroot, PlannerInfo));
+
+		/* Get the subquery output expression referenced by the upper Var */
+		ste = get_tle_by_resno(subquery->targetList, var->varattno);
+		if (ste == NULL || ste->resjunk)
+			elog(ERROR, "subquery %s does not have attribute %d",
+				 rte->eref->aliasname, var->varattno);
+		var = (Var *) ste->expr;
+
+		/* Can only handle a simple Var of subquery's query level */
+		if (var && IsA(var, Var) &&
+			var->varlevelsup == 0)
+		{
+			/*
+			 * OK, recurse into the subquery.  Note that the original setting
+			 * of vardata->isunique (which will surely be false) is left
+			 * unchanged in this situation.  That's what we want, since even
+			 * if the underlying column is unique, the subquery may have
+			 * joined to other tables in a way that creates duplicates.
+			 */
+			examine_simple_variable(rel->subroot, var, vardata);
+		}
+	}
+	else
+	{
+		/*
+		 * Otherwise, the Var comes from a FUNCTION, VALUES, or CTE RTE.  (We
+		 * won't see RTE_JOIN here because join alias Vars have already been
+		 * flattened.)  There's not much we can do with function outputs, but
+		 * maybe someday try to be smarter about VALUES and/or CTEs.
+		 */
+	}
+}
+
 /*
  * get_variable_numdistinct
  *	  Estimate the number of distinct values of a variable.