[Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>]

As mentioned in the near thread, I think there is another oversight in
the cost estimation for aggregate pushdown paths in postgres_fdw, IIUC.
 When costing an aggregate pushdown path using local statistics, we
re-use the estimated costs of implementing the underlying scan/join
relation, cached in the relation's PgFdwRelationInfo (ie,
rel_startup_cost and rel_total_cost).  Since these costs wouldn't yet
contain the costs of evaluating the final scan/join target, as tlist
replacement by apply_scanjoin_target_to_paths() is performed afterwards.
 So I think we need to adjust these costs so that the tlist eval costs
are included, but ISTM that estimate_path_cost_size() forgot to do so.
Attached is a patch for fixing this issue.

Best regards,
Etsuro Fujita

Attachments:

  [text/x-patch] another-fix-postgres-fdw-grouping-path-cost.patch (3.1K, ../5C66A056.60007@lab.ntt.co.jp/2-another-fix-postgres-fdw-grouping-path-cost.patch)
  download | inline diff:
*** a/contrib/postgres_fdw/postgres_fdw.c
--- b/contrib/postgres_fdw/postgres_fdw.c
***************
*** 2705,2716 **** estimate_path_cost_size(PlannerInfo *root,
--- 2705,2719 ----
  		}
  		else if (IS_UPPER_REL(foreignrel))
  		{
+ 			RelOptInfo *outerrel = fpinfo->outerrel;
  			PgFdwRelationInfo *ofpinfo;
  			AggClauseCosts aggcosts;
  			double		input_rows;
  			int			numGroupCols;
  			double		numGroups = 1;
  
+ 			Assert(outerrel);
+ 
  			/*
  			 * This cost model is mixture of costing done for sorted and
  			 * hashed aggregates in cost_agg().  We are not sure which
***************
*** 2719,2725 **** estimate_path_cost_size(PlannerInfo *root,
  			 * and all finalization and run cost are added in total_cost.
  			 */
  
! 			ofpinfo = (PgFdwRelationInfo *) fpinfo->outerrel->fdw_private;
  
  			/* Get rows and width from input rel */
  			input_rows = ofpinfo->rows;
--- 2722,2728 ----
  			 * and all finalization and run cost are added in total_cost.
  			 */
  
! 			ofpinfo = (PgFdwRelationInfo *) outerrel->fdw_private;
  
  			/* Get rows and width from input rel */
  			input_rows = ofpinfo->rows;
***************
*** 2772,2782 **** estimate_path_cost_size(PlannerInfo *root,
  
  			/*-----
  			 * Startup cost includes:
! 			 *	  1. Startup cost for underneath input relation
  			 *	  2. Cost of performing aggregation, per cost_agg()
  			 *-----
  			 */
  			startup_cost = ofpinfo->rel_startup_cost;
  			startup_cost += aggcosts.transCost.startup;
  			startup_cost += aggcosts.transCost.per_tuple * input_rows;
  			startup_cost += aggcosts.finalCost.startup;
--- 2775,2787 ----
  
  			/*-----
  			 * Startup cost includes:
! 			 *	  1. Startup cost for underneath input relation, adjusted for
! 			 *	     tlist replacement by apply_scanjoin_target_to_paths()
  			 *	  2. Cost of performing aggregation, per cost_agg()
  			 *-----
  			 */
  			startup_cost = ofpinfo->rel_startup_cost;
+ 			startup_cost += outerrel->reltarget->cost.startup;
  			startup_cost += aggcosts.transCost.startup;
  			startup_cost += aggcosts.transCost.per_tuple * input_rows;
  			startup_cost += aggcosts.finalCost.startup;
***************
*** 2784,2794 **** estimate_path_cost_size(PlannerInfo *root,
  
  			/*-----
  			 * Run time cost includes:
! 			 *	  1. Run time cost of underneath input relation
  			 *	  2. Run time cost of performing aggregation, per cost_agg()
  			 *-----
  			 */
  			run_cost = ofpinfo->rel_total_cost - ofpinfo->rel_startup_cost;
  			run_cost += aggcosts.finalCost.per_tuple * numGroups;
  			run_cost += cpu_tuple_cost * numGroups;
  
--- 2789,2801 ----
  
  			/*-----
  			 * Run time cost includes:
! 			 *	  1. Run time cost of underneath input relation, adjusted for
! 			 *	     tlist replacement by apply_scanjoin_target_to_paths()
  			 *	  2. Run time cost of performing aggregation, per cost_agg()
  			 *-----
  			 */
  			run_cost = ofpinfo->rel_total_cost - ofpinfo->rel_startup_cost;
+ 			run_cost += outerrel->reltarget->cost.per_tuple * input_rows;
  			run_cost += aggcosts.finalCost.per_tuple * numGroups;
  			run_cost += cpu_tuple_cost * numGroups;