Fix handling of empty uncompressed posting list pages in GIN

akorotkov · akorotkov · commit 608793266dbb · 2018-07-19T23:26:15.000+03:00
PostgreSQL 9.4 introduces posting list compression in GIN. This feature supports online upgrade, so that after pg_upgrade uncompressed posting lists are compressed on-the-fly. Underlying code appears to always expect at least one item on uncompressed posting list page. But there could be completely empty pages, because VACUUM never deletes leftmost and rightmost pages from posting trees. This commit fixes that. Reported-by: Sivasubramanian Ramasubramanian Discussion: https://postgr.es/m/1531867212836.63354%40amazon.com Author: Sivasubramanian Ramasubramanian, Alexander Korotkov Backpatch-through: 9.4
diff --git a/src/backend/access/gin/gindatapage.c b/src/backend/access/gin/gindatapage.c
@@ -1394,23 +1394,27 @@ disassembleLeaf(Page page)
 	{
 		/*
 		 * A pre-9.4 format uncompressed page is represented by a single
-		 * segment, with an array of items.
+		 * segment, with an array of items.  The corner case is uncompressed
+		 * page containing no items, which is represented as no segments.
 		 */
 		ItemPointer uncompressed;
 		int			nuncompressed;
 		leafSegmentInfo *seginfo;
 
 		uncompressed = dataLeafPageGetUncompressed(page, &nuncompressed);
 
-		seginfo = palloc(sizeof(leafSegmentInfo));
+		if (nuncompressed > 0)
+		{
+			seginfo = palloc(sizeof(leafSegmentInfo));
 
-		seginfo->action = GIN_SEGMENT_REPLACE;
-		seginfo->seg = NULL;
-		seginfo->items = palloc(nuncompressed * sizeof(ItemPointerData));
-		memcpy(seginfo->items, uncompressed, nuncompressed * sizeof(ItemPointerData));
-		seginfo->nitems = nuncompressed;
+			seginfo->action = GIN_SEGMENT_REPLACE;
+			seginfo->seg = NULL;
+			seginfo->items = palloc(nuncompressed * sizeof(ItemPointerData));
+			memcpy(seginfo->items, uncompressed, nuncompressed * sizeof(ItemPointerData));
+			seginfo->nitems = nuncompressed;
 
-		dlist_push_tail(&leaf->segments, &seginfo->node);
+			dlist_push_tail(&leaf->segments, &seginfo->node);
+		}
 
 		leaf->oldformat = true;
 	}
diff --git a/src/backend/access/gin/ginxlog.c b/src/backend/access/gin/ginxlog.c
@@ -153,15 +153,30 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
 		ItemPointer uncompressed = (ItemPointer) GinDataPageGetData(page);
 		int			nuncompressed = GinPageGetOpaque(page)->maxoff;
 		int			npacked;
-		GinPostingList *plist;
 
-		plist = ginCompressPostingList(uncompressed, nuncompressed,
-									   BLCKSZ, &npacked);
-		Assert(npacked == nuncompressed);
+		/*
+		 * Empty leaf pages are deleted as part of vacuum, but leftmost and
+		 * rightmost pages are never deleted.  So, pg_upgrade'd from pre-9.4
+		 * instances might contain empty leaf pages, and we need to handle
+		 * them correctly.
+		 */
+		if (nuncompressed > 0)
+		{
+			GinPostingList *plist;
+
+			plist = ginCompressPostingList(uncompressed, nuncompressed,
+										   BLCKSZ, &npacked);
+			totalsize = SizeOfGinPostingList(plist);
+
+			Assert(npacked == nuncompressed);
 
-		totalsize = SizeOfGinPostingList(plist);
+			memcpy(GinDataLeafPageGetPostingList(page), plist, totalsize);
+		}
+		else
+		{
+			totalsize = 0;
+		}
 
-		memcpy(GinDataLeafPageGetPostingList(page), plist, totalsize);
 		GinDataPageSetDataSize(page, totalsize);
 		GinPageSetCompressed(page);
 		GinPageGetOpaque(page)->maxoff = InvalidOffsetNumber;
