Teach CLUSTER to use seqscan-and-sort when it's faster than indexscan.

... or at least, when the planner's cost estimates say it will be faster.

Leonardo Francalanci, reviewed by Itagaki Takahiro and Tom Lane

src/include/optimizer/cost.h

@@ -84,6 +84,7 @@ extern void cost_ctescan(Path *path, PlannerInfo *root, RelOptInfo *baserel);
 extern void cost_recursive_union(Plan *runion, Plan *nrterm, Plan *rterm);
 extern void cost_sort(Path *path, PlannerInfo *root,
 		  List *pathkeys, Cost input_cost, double tuples, int width,
+		  Cost comparison_cost, int sort_mem,
 		  double limit_tuples);
 extern void cost_material(Path *path,
 			  Cost input_startup_cost, Cost input_total_cost,

src/include/optimizer/plancat.h

@@ -31,6 +31,8 @@ extern void get_relation_info(PlannerInfo *root, Oid relationObjectId,
 extern void estimate_rel_size(Relation rel, int32 *attr_widths,
 				  BlockNumber *pages, double *tuples);
 
+extern int32 get_relation_data_width(Oid relid);
+
 extern bool relation_excluded_by_constraints(PlannerInfo *root,
 								 RelOptInfo *rel, RangeTblEntry *rte);
 

src/include/optimizer/planner.h

@@ -37,4 +37,6 @@ extern Plan *subquery_planner(PlannerGlobal *glob, Query *parse,
 
 extern Expr *expression_planner(Expr *expr);
 
+extern bool plan_cluster_use_sort(Oid tableOid, Oid indexOid);
+
 #endif   /* PLANNER_H */