improve_bitmap_cost_v2.patch

diff --git a/src/backend/nodes/tidbitmap.c b/src/backend/nodes/tidbitmap.c
index c4e53ad..6689c63 100644
--- a/src/backend/nodes/tidbitmap.c
+++ b/src/backend/nodes/tidbitmap.c
@@ -295,10 +295,7 @@ tbm_create(long maxbytes, dsa_area *dsa)
 	 * for our purpose.  Also count an extra Pointer per entry for the arrays
 	 * created during iteration readout.
 	 */
-	nbuckets = maxbytes /
-		(sizeof(PagetableEntry) + sizeof(Pointer) + sizeof(Pointer));
-	nbuckets = Min(nbuckets, INT_MAX - 1);	/* safety limit */
-	nbuckets = Max(nbuckets, 16);	/* sanity limit */
+	nbuckets = tbm_calculate_entires(maxbytes);
 	tbm->maxentries = (int) nbuckets;
 	tbm->lossify_start = 0;
 	tbm->dsa = dsa;
@@ -1562,3 +1559,26 @@ pagetable_free(pagetable_hash *pagetable, void *pointer)
 		tbm->dsapagetableold = InvalidDsaPointer;
 	}
 }
+
+/*
+ * tbm_calculate_entires
+ *
+ * Calculate the number of estimated entries in the bitmap for given memory
+ * in bytes.
+ */
+long
+tbm_calculate_entires(double maxbytes)
+{
+	long	nbuckets;
+
+	/*
+	 * Estimate number of hashtable entries we can have within maxbytes. This
+	 * estimates the hash cost as sizeof(PagetableEntry).
+	 */
+	nbuckets = maxbytes /
+		(sizeof(PagetableEntry) + sizeof(Pointer) + sizeof(Pointer));
+	nbuckets = Min(nbuckets, INT_MAX - 1);		/* safety limit */
+	nbuckets = Max(nbuckets, 16);		/* sanity limit */
+
+	return nbuckets;
+}
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 051a854..5b15f54 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -5132,6 +5132,10 @@ compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel, Path *bitmapqual,
 	double		T;
 	double		pages_fetched;
 	double		tuples_fetched;
+	double		heap_pages = 0;
+	double		exact_pages = 0;
+	double		lossy_pages = 0;
+	long		maxentries;
 
 	/*
 	 * Fetch total cost of obtaining the bitmap, as well as its total
@@ -5176,8 +5180,42 @@ compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel, Path *bitmapqual,
 	else
 		pages_fetched = ceil(pages_fetched);
 
+	/*
+	 * Calculate the number of pages fetched from the heap.  Then based on
+	 * current work_mem estimate get the estimated maxentries in the bitmap.
+	 */
+	heap_pages = Min(pages_fetched, baserel->pages);
+	maxentries = tbm_calculate_entires(work_mem * 1024L);
+
+	/*
+	 * After the initial lossification that is maxentries / 2, the number of
+	 * lossy pages grows slower. It is good enough to reflect this initial
+	 * sharp increase in the lossy page number.
+	 */
+	lossy_pages = Max(0, heap_pages - maxentries / 2);
+	exact_pages = heap_pages - lossy_pages;
+
+	/*
+	 * If there are lossy pages then recompute the  number of tuples processed
+	 * by the bitmap heap node.  For the exact_pages it's baserel->tuples *
+	 * indexSelectivity and for lossy_pages we have to process all the tuples.
+	 */
+	if (exact_pages > 0 && lossy_pages)
+		tuples_fetched = clamp_row_est (indexSelectivity *
+					(exact_pages / heap_pages) * baserel->tuples +
+					(lossy_pages / heap_pages) * baserel->tuples);
+
 	if (cost)
+	{
 		*cost = indexTotalCost;
+
+		/*
+		 * 	If exact_pages are -ve then even after lossifying we will not be
+		 * 	able to fit them into work_mem. Hence, disable this path.
+		 */
+		if (exact_pages < 0)
+			*cost += disable_cost;
+	}
 	if (tuple)
 		*tuple = tuples_fetched;
 
diff --git a/src/include/nodes/tidbitmap.h b/src/include/nodes/tidbitmap.h
index f9a1902..1a8d9fc 100644
--- a/src/include/nodes/tidbitmap.h
+++ b/src/include/nodes/tidbitmap.h
@@ -70,5 +70,6 @@ extern void tbm_end_iterate(TBMIterator *iterator);
 extern void tbm_end_shared_iterate(TBMSharedIterator *iterator);
 extern TBMSharedIterator *tbm_attach_shared_iterate(dsa_area *dsa,
 						  dsa_pointer dp);
+extern long tbm_calculate_entires(double maxbytes);
 
 #endif							/* TIDBITMAP_H */