Fix handling of empty uncompressed posting list pages in GIN

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
index 93f2d8f350dba96699d7664d5fe87412ff526e25..d25684949f5796f88b689eb7d88aa5e27ace9ef9 100644 (file)
--- a/src/backend/access/gin/gindatapage.c
+++ b/src/backend/access/gin/gindatapage.c
@@ -1392,7 +1392,8 @@ 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;
@@ -1400,15 +1401,18 @@ disassembleLeaf(Page page)
 
        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
index d9b0ce02ac4dd39475a627f1afb404caad125560..9b57fd4978af0ee76ad3e0f37f189ef04fe7a60a 100644 (file)
--- a/src/backend/access/gin/ginxlog.c
+++ b/src/backend/access/gin/ginxlog.c
@@ -151,15 +151,30 @@ 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;