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 77725acf9c40209c86f86ce49fd05f7491d4261b..25c36a2985af2e2f99fcd34d3b51d7006d3459cb 100644 (file)
--- a/src/backend/access/gin/gindatapage.c
+++ b/src/backend/access/gin/gindatapage.c
@@ -1470,7 +1470,8 @@ 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;
@@ -1478,15 +1479,18 @@ 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 7f93ce6526fecea4c01833b86b93bfc6d0226453..130ee7d5b2c7756611aded15c6909fd12b9e623c 100644 (file)
--- a/src/backend/access/gin/ginxlog.c
+++ b/src/backend/access/gin/ginxlog.c
@@ -160,15 +160,30 @@ 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;