From bf291247e8f49d4558ab70fa335bd5a7bdda88b4 Mon Sep 17 00:00:00 2001
From: amit <amitlangote09@gmail.com>
Date: Fri, 23 Jun 2017 11:20:41 +0900
Subject: [PATCH 1/3] Set pd_lower correctly in the GIN metapage.

Also tell xlog.c to treat the metapage like a standard page, so any
hole in it is compressed.
---
 src/backend/access/gin/gininsert.c |  4 ++--
 src/backend/access/gin/ginutil.c   |  9 +++++++++
 src/backend/access/gin/ginxlog.c   | 24 +++++++++---------------
 3 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/src/backend/access/gin/gininsert.c b/src/backend/access/gin/gininsert.c
index 5378011f50..c9aa4ee147 100644
--- a/src/backend/access/gin/gininsert.c
+++ b/src/backend/access/gin/gininsert.c
@@ -348,7 +348,7 @@ ginbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 		Page		page;
 
 		XLogBeginInsert();
-		XLogRegisterBuffer(0, MetaBuffer, REGBUF_WILL_INIT);
+		XLogRegisterBuffer(0, MetaBuffer, REGBUF_WILL_INIT | REGBUF_STANDARD);
 		XLogRegisterBuffer(1, RootBuffer, REGBUF_WILL_INIT);
 
 		recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_CREATE_INDEX);
@@ -447,7 +447,7 @@ ginbuildempty(Relation index)
 	START_CRIT_SECTION();
 	GinInitMetabuffer(MetaBuffer);
 	MarkBufferDirty(MetaBuffer);
-	log_newpage_buffer(MetaBuffer, false);
+	log_newpage_buffer(MetaBuffer, true);
 	GinInitBuffer(RootBuffer, GIN_LEAF);
 	MarkBufferDirty(RootBuffer);
 	log_newpage_buffer(RootBuffer, false);
diff --git a/src/backend/access/gin/ginutil.c b/src/backend/access/gin/ginutil.c
index 136ea27718..e926649dd2 100644
--- a/src/backend/access/gin/ginutil.c
+++ b/src/backend/access/gin/ginutil.c
@@ -374,6 +374,15 @@ GinInitMetabuffer(Buffer b)
 	metadata->nDataPages = 0;
 	metadata->nEntries = 0;
 	metadata->ginVersion = GIN_CURRENT_VERSION;
+
+	/*
+	 * Set pd_lower just past the end of the metadata.  This is not essential
+	 * but it makes the page look compressible to xlog.c, as long as the
+	 * buffer containing the page is passed to XLogRegisterBuffer() as a
+	 * REGBUF_STANDARD page.
+	 */
+	((PageHeader) page)->pd_lower =
+			((char *) metadata + sizeof(GinMetaPageData)) - (char *) page;
 }
 
 /*
diff --git a/src/backend/access/gin/ginxlog.c b/src/backend/access/gin/ginxlog.c
index 7ba04e324f..f5c11b2d9a 100644
--- a/src/backend/access/gin/ginxlog.c
+++ b/src/backend/access/gin/ginxlog.c
@@ -514,7 +514,7 @@ ginRedoUpdateMetapage(XLogReaderState *record)
 	Assert(BufferGetBlockNumber(metabuffer) == GIN_METAPAGE_BLKNO);
 	metapage = BufferGetPage(metabuffer);
 
-	GinInitPage(metapage, GIN_META, BufferGetPageSize(metabuffer));
+	GinInitMetabuffer(metabuffer);
 	memcpy(GinPageGetMeta(metapage), &data->metadata, sizeof(GinMetaPageData));
 	PageSetLSN(metapage, lsn);
 	MarkBufferDirty(metabuffer);
@@ -656,7 +656,7 @@ ginRedoDeleteListPages(XLogReaderState *record)
 	Assert(BufferGetBlockNumber(metabuffer) == GIN_METAPAGE_BLKNO);
 	metapage = BufferGetPage(metabuffer);
 
-	GinInitPage(metapage, GIN_META, BufferGetPageSize(metabuffer));
+	GinInitMetabuffer(metabuffer);
 
 	memcpy(GinPageGetMeta(metapage), &data->metadata, sizeof(GinMetaPageData));
 	PageSetLSN(metapage, lsn);
@@ -768,6 +768,7 @@ void
 gin_mask(char *pagedata, BlockNumber blkno)
 {
 	Page		page = (Page) pagedata;
+	PageHeader	pagehdr = (PageHeader) page;
 	GinPageOpaque opaque;
 
 	mask_page_lsn(page);
@@ -776,18 +777,11 @@ gin_mask(char *pagedata, BlockNumber blkno)
 	mask_page_hint_bits(page);
 
 	/*
-	 * GIN metapage doesn't use pd_lower/pd_upper. Other page types do. Hence,
-	 * we need to apply masking for those pages.
+	 * For GIN_DELETED page, the page is initialized to empty. Hence, mask
+	 * the page content.
 	 */
-	if (opaque->flags != GIN_META)
-	{
-		/*
-		 * For GIN_DELETED page, the page is initialized to empty. Hence, mask
-		 * the page content.
-		 */
-		if (opaque->flags & GIN_DELETED)
-			mask_page_content(page);
-		else
-			mask_unused_space(page);
-	}
+	if (opaque->flags & GIN_DELETED)
+		mask_page_content(page);
+	else if (pagehdr->pd_lower != 0)
+		mask_unused_space(page);
 }
-- 
2.11.0