Fix BRIN cost estimation

The original code was overly optimistic about the cost of scanning a
BRIN index, leading to BRIN indexes being selected when they'd be a
worse choice than some other index.  This complete rewrite should be
more accurate.

Author: David Rowley, based on an earlier patch by Emre Hasegeli
Reviewed-by: Emre Hasegeli
Discussion: https://postgr.es/m/CAKJS1f9n-Wapop5Xz1dtGdpdqmzeGqQK4sV2MK-zZugfC14Xtw@mail.gmail.com

src/include/access/brin.h

@@ -25,6 +25,17 @@ typedef struct BrinOptions
 	bool		autosummarize;
 } BrinOptions;
 
+
+/*
+ * BrinStatsData represents stats data for planner use
+ */
+typedef struct BrinStatsData
+{
+	BlockNumber	pagesPerRange;
+	BlockNumber	revmapNumPages;
+} BrinStatsData;
+
+
 #define BRIN_DEFAULT_PAGES_PER_RANGE	128
 #define BrinGetPagesPerRange(relation) \
 	((relation)->rd_options ? \
@@ -35,4 +46,7 @@ typedef struct BrinOptions
 	 ((BrinOptions *) (relation)->rd_options)->autosummarize : \
 	  false)
 
+
+extern void brinGetStats(Relation index, BrinStatsData *stats);
+
 #endif   /* BRIN_H */