Provide statistics for hypothetical BRIN indexes

Trying to use hypothetical indexes with BRIN currently fails when trying
to access a relation that does not exist when looking for the
statistics.  With the current API, it is not possible to easily pass
a value for pages_per_range down to the hypothetical index, so this
makes use of the default value of BRIN_DEFAULT_PAGES_PER_RANGE, which
should be fine enough in most cases.

Being able to refine or enforce the hypothetical costs in more
optimistic ways would require more refactoring by filling in the
statistics when building IndexOptInfo in plancat.c.  This would involve
ABI breakages around the costing routines, something not fit for stable
branches.

This is broken since 7e534adc, so backpatch down to v10.

Author: Julien Rouhaud, Heikki Linnakangas
Reviewed-by: Álvaro Herrera, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/CAOBaU_ZH0LKEA8VFCocr6Lpte1ab0b6FpvgS0y4way+RPSXfYg@mail.gmail.com
Backpatch-through: 10

src/backend/utils/adt/selfuncs.c

@@ -102,6 +102,7 @@
 #include <math.h>
 
 #include "access/brin.h"
+#include "access/brin_page.h"
 #include "access/gin.h"
 #include "access/table.h"
 #include "access/tableam.h"
@@ -6865,12 +6866,34 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 							  &spc_seq_page_cost);
 
 	/*
-	 * Obtain some data from the index itself.  A lock should have already
-	 * been obtained on the index in plancat.c.
+	 * Obtain some data from the index itself, if possible.  Otherwise invent
+	 * some plausible internal statistics based on the relation page count.
 	 */
-	indexRel = index_open(index->indexoid, NoLock);
-	brinGetStats(indexRel, &statsData);
-	index_close(indexRel, NoLock);
+	if (!index->hypothetical)
+	{
+		/*
+		 * A lock should have already been obtained on the index in plancat.c.
+		 */
+		indexRel = index_open(index->indexoid, NoLock);
+		brinGetStats(indexRel, &statsData);
+		index_close(indexRel, NoLock);
+
+		/* work out the actual number of ranges in the index */
+		indexRanges = Max(ceil((double) baserel->pages /
+							   statsData.pagesPerRange), 1.0);
+	}
+	else
+	{
+		/*
+		 * Assume default number of pages per range, and estimate the number
+		 * of ranges based on that.
+		 */
+		indexRanges = Max(ceil((double) baserel->pages /
+							   BRIN_DEFAULT_PAGES_PER_RANGE), 1.0);
+
+		statsData.pagesPerRange = BRIN_DEFAULT_PAGES_PER_RANGE;
+		statsData.revmapNumPages = (indexRanges / REVMAP_PAGE_MAXITEMS) + 1;
+	}
 
 	/*
 	 * Compute index correlation
@@ -6970,10 +6993,6 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 											 baserel->relid,
 											 JOIN_INNER, NULL);
 
-	/* work out the actual number of ranges in the index */
-	indexRanges = Max(ceil((double) baserel->pages / statsData.pagesPerRange),
-					  1.0);
-
 	/*
 	 * Now calculate the minimum possible ranges we could match with if all of
 	 * the rows were in the perfect order in the table's heap.