Refactor bitmap heap scan estimation of heap pages fetched.

Currently, we only need this logic in order to cost a Bitmap Heap
Scan.  But a pending patch for Parallel Bitmap Heap Scan also uses
it to help figure out how many workers to use for the scan, which
has to be determined prior to costing.  So, move the logic to
a separate function to make that easier.

Dilip Kumar.  The patch series of which this is a part has been
reviewed by Andres Freund, Amit Khendekar, Tushar Ahuja, Rafia
Sabih, Haribabu Kommi, and me; it is not clear from the email
discussion which of those people have looked specifically at this
part.

Discussion: http://postgr.es/m/CAFiTN-v3QYNJEZnnmKCeATuLbN-h9tMVfeEF0+BrouYDqjXgwg@mail.gmail.com

src/backend/optimizer/path/costsize.c

@@ -813,7 +813,6 @@ cost_bitmap_heap_scan(Path *path, PlannerInfo *root, RelOptInfo *baserel,
 	Cost		startup_cost = 0;
 	Cost		run_cost = 0;
 	Cost		indexTotalCost;
-	Selectivity indexSelectivity;
 	QualCost	qpqual_cost;
 	Cost		cpu_per_tuple;
 	Cost		cost_per_page;
@@ -837,54 +836,18 @@ cost_bitmap_heap_scan(Path *path, PlannerInfo *root, RelOptInfo *baserel,
 	if (!enable_bitmapscan)
 		startup_cost += disable_cost;
 
-	/*
-	 * Fetch total cost of obtaining the bitmap, as well as its total
-	 * selectivity.
-	 */
-	cost_bitmap_tree_node(bitmapqual, &indexTotalCost, &indexSelectivity);
+	pages_fetched = compute_bitmap_pages(root, baserel, bitmapqual,
+										 loop_count, &indexTotalCost,
+										 &tuples_fetched);
 
 	startup_cost += indexTotalCost;
+	T = (baserel->pages > 1) ? (double) baserel->pages : 1.0;
 
 	/* Fetch estimated page costs for tablespace containing table. */
 	get_tablespace_page_costs(baserel->reltablespace,
 							  &spc_random_page_cost,
 							  &spc_seq_page_cost);
 
-	/*
-	 * Estimate number of main-table pages fetched.
-	 */
-	tuples_fetched = clamp_row_est(indexSelectivity * baserel->tuples);
-
-	T = (baserel->pages > 1) ? (double) baserel->pages : 1.0;
-
-	if (loop_count > 1)
-	{
-		/*
-		 * For repeated bitmap scans, scale up the number of tuples fetched in
-		 * the Mackert and Lohman formula by the number of scans, so that we
-		 * estimate the number of pages fetched by all the scans. Then
-		 * pro-rate for one scan.
-		 */
-		pages_fetched = index_pages_fetched(tuples_fetched * loop_count,
-											baserel->pages,
-											get_indexpath_pages(bitmapqual),
-											root);
-		pages_fetched /= loop_count;
-	}
-	else
-	{
-		/*
-		 * For a single scan, the number of heap pages that need to be fetched
-		 * is the same as the Mackert and Lohman formula for the case T <= b
-		 * (ie, no re-reads needed).
-		 */
-		pages_fetched = (2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);
-	}
-	if (pages_fetched >= T)
-		pages_fetched = T;
-	else
-		pages_fetched = ceil(pages_fetched);
-
 	/*
 	 * For small numbers of pages we should charge spc_random_page_cost
 	 * apiece, while if nearly all the table's pages are being read, it's more
@@ -4820,3 +4783,69 @@ get_parallel_divisor(Path *path)
 
 	return parallel_divisor;
 }
+
+/*
+ * compute_bitmap_pages
+ *
+ * compute number of pages fetched from heap in bitmap heap scan.
+ */
+double
+compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel, Path *bitmapqual,
+					 int loop_count, Cost *cost, double *tuple)
+{
+	Cost		indexTotalCost;
+	Selectivity indexSelectivity;
+	double		T;
+	double		pages_fetched;
+	double		tuples_fetched;
+
+	/*
+	 * Fetch total cost of obtaining the bitmap, as well as its total
+	 * selectivity.
+	 */
+	cost_bitmap_tree_node(bitmapqual, &indexTotalCost, &indexSelectivity);
+
+	/*
+	 * Estimate number of main-table pages fetched.
+	 */
+	tuples_fetched = clamp_row_est(indexSelectivity * baserel->tuples);
+
+	T = (baserel->pages > 1) ? (double) baserel->pages : 1.0;
+
+	if (loop_count > 1)
+	{
+		/*
+		 * For repeated bitmap scans, scale up the number of tuples fetched in
+		 * the Mackert and Lohman formula by the number of scans, so that we
+		 * estimate the number of pages fetched by all the scans. Then
+		 * pro-rate for one scan.
+		 */
+		pages_fetched = index_pages_fetched(tuples_fetched * loop_count,
+											baserel->pages,
+											get_indexpath_pages(bitmapqual),
+											root);
+		pages_fetched /= loop_count;
+	}
+	else
+	{
+		/*
+		 * For a single scan, the number of heap pages that need to be fetched
+		 * is the same as the Mackert and Lohman formula for the case T <= b
+		 * (ie, no re-reads needed).
+		 */
+		pages_fetched =
+			(2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);
+	}
+
+	if (pages_fetched >= T)
+		pages_fetched = T;
+	else
+		pages_fetched = ceil(pages_fetched);
+
+	if (cost)
+		*cost = indexTotalCost;
+	if (tuple)
+		*tuple = tuples_fetched;
+
+	return pages_fetched;
+}

src/include/optimizer/cost.h

@@ -183,6 +183,8 @@ extern void set_cte_size_estimates(PlannerInfo *root, RelOptInfo *rel,
 					   double cte_rows);
 extern void set_foreign_size_estimates(PlannerInfo *root, RelOptInfo *rel);
 extern PathTarget *set_pathtarget_cost_width(PlannerInfo *root, PathTarget *target);
+extern double compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel,
+				  Path *bitmapqual, int loop_count, Cost *cost, double *tuple);
 
 /*
  * prototypes for clausesel.c