Comment fixes for extended statistics

Clean up some code comments in new extended statistics code, from
7b504eb2.

src/backend/optimizer/util/plancat.c

@@ -79,6 +79,7 @@ static List *get_relation_statistics(RelOptInfo *rel, Relation relation);
  *	min_attr	lowest valid AttrNumber
  *	max_attr	highest valid AttrNumber
  *	indexlist	list of IndexOptInfos for relation's indexes
+ *	statlist	list of StatisticExtInfo for relation's statistic objects
  *	serverid	if it's a foreign table, the server OID
  *	fdwroutine	if it's a foreign table, the FDW function pointers
  *	pages		number of pages

src/backend/statistics/dependencies.c

@@ -900,7 +900,13 @@ find_strongest_dependency(StatisticExtInfo * stats, MVDependencies * dependencie
 
 /*
  * dependencies_clauselist_selectivity
- *		Attempt to estimate selectivity using functional dependency statistics
+ *		Return the estimated selectivity of the given clauses using
+ *		functional dependency statistics, or 1.0 if no useful functional
+ *		dependency statistic exists.
+ *
+ * 'estimatedclauses' is an output argument that gets a bit set corresponding
+ * to the (zero-based) list index of clauses that are included in the
+ * estimated selectivity.
  *
  * Given equality clauses on attributes (a,b) we find the strongest dependency
  * between them, i.e. either (a=>b) or (b=>a). Assuming (a=>b) is the selected
@@ -935,7 +941,6 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 	AttrNumber *list_attnums;
 	int			listidx;
 
-
 	/* check if there's any stats that might be useful for us. */
 	if (!has_stats_of_kind(rel->statlist, STATS_EXT_DEPENDENCIES))
 		return 1.0;

src/backend/statistics/extended_stats.c

@@ -428,8 +428,9 @@ has_stats_of_kind(List *stats, char requiredkind)
 
 /*
  * choose_best_statistics
- *		Look for statistics with the specified 'requiredkind' which have keys
- *		that match at least two attnums.
+ *		Look for and return statistics with the specified 'requiredkind' which
+ *		have keys that match at least two of the given attnums.  Return NULL if
+ *		there's no match.
  *
  * The current selection criteria is very simple - we choose the statistics
  * referencing the most attributes with the least keys.

src/backend/utils/cache/relcache.c

@@ -4508,7 +4508,6 @@ RelationGetStatExtList(Relation relation)
 								 NULL, 1, &skey);
 
 	while (HeapTupleIsValid(htup = systable_getnext(indscan)))
-		/* TODO maybe include only already built statistics? */
 		result = insert_ordered_oid(result, HeapTupleGetOid(htup));
 
 	systable_endscan(indscan);

src/include/nodes/relation.h

@@ -693,17 +693,15 @@ typedef struct ForeignKeyOptInfo
  * StatisticExtInfo
  *		Information about extended statistics for planning/optimization
  *
- * This contains information about which columns are covered by the
- * statistics (stakeys), which options were requested while adding the
- * statistics (*_enabled), and which kinds of statistics were actually
- * built and are available for the optimizer (*_built).
+ * Each pg_statistic_ext row is represented by one or more nodes of this
+ * type, or even zero if ANALYZE has not computed them.
  */
 typedef struct StatisticExtInfo
 {
 	NodeTag		type;
 
 	Oid			statOid;		/* OID of the statistics row */
-	RelOptInfo *rel;			/* back-link to index's table */
+	RelOptInfo *rel;			/* back-link to statistic's table */
 	char		kind;			/* statistic kind of this entry */
 	Bitmapset  *keys;			/* attnums of the columns covered */
 } StatisticExtInfo;