BRIN de-summarization

When the BRIN summary tuple for a page range becomes too "wide" for the
values actually stored in the table (because the tuples that were
present originally are no longer present due to updates or deletes), it
can be useful to remove the outdated summary tuple, so that a future
summarization can install a tighter summary.

This commit introduces a SQL-callable interface to do so.

Author: Álvaro Herrera
Reviewed-by: Eiji Seki
Discussion: https://postgr.es/m/20170228045643.n2ri74ara4fhhfxf@alvherre.pgsql

src/include/access/brin_xlog.h

@@ -33,7 +33,7 @@
 #define XLOG_BRIN_UPDATE			0x20
 #define XLOG_BRIN_SAMEPAGE_UPDATE	0x30
 #define XLOG_BRIN_REVMAP_EXTEND		0x40
-#define XLOG_BRIN_REVMAP_VACUUM		0x50
+#define XLOG_BRIN_DESUMMARIZE		0x50
 
 #define XLOG_BRIN_OPMASK			0x70
 /*
@@ -124,6 +124,24 @@ typedef struct xl_brin_revmap_extend
 #define SizeOfBrinRevmapExtend	(offsetof(xl_brin_revmap_extend, targetBlk) + \
 								 sizeof(BlockNumber))
 
+/*
+ * This is what we need to know about a range de-summarization
+ *
+ * Backup block 0: revmap page
+ * Backup block 1: regular page
+ */
+typedef struct xl_brin_desummarize
+{
+	BlockNumber	pagesPerRange;
+	/* page number location to set to invalid */
+	OffsetNumber heapBlk;
+	/* offset of item to delete in regular index page */
+	OffsetNumber regOffset;
+} xl_brin_desummarize;
+
+#define SizeOfBrinDesummarize	(offsetof(xl_brin_desummarize, regOffset) + \
+								 sizeof(OffsetNumber))
+
 
 extern void brin_redo(XLogReaderState *record);
 extern void brin_desc(StringInfo buf, XLogReaderState *record);