Fix GIN data page split ratio calculation.

The code that tried to split a page at 75/25 ratio, when appending to the
end of an index, was buggy in two ways. First, there was a silly typo that
caused it to just fill the left page as full as possible. But the logic as
it was intended wasn't correct either, and would actually have given a ratio
closer to 60/40 than 75/25.

Gaetano Mendola spotted the typo. Backpatch to 9.4, where this code was added.

diff --git a/src/backend/access/gin/gindatapage.c b/src/backend/access/gin/gindatapage.c
index 76e0cb3e19f7bdea54edb9e468efea1d902b8940..e2d15a85fe41f91224b4c595181318f8a7d435e6 100644 (file)
--- a/src/backend/access/gin/gindatapage.c
+++ b/src/backend/access/gin/gindatapage.c
@@ -621,9 +621,9 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
        /*
         * Had to split.
         *
-        * We already divided the segments between the left and the right
-        * page. The left page was filled as full as possible, and the rest
-        * overflowed to the right page. When building a new index, that's
+        * leafRepackItems already divided the segments between the left and
+        * the right page. It filled the left page as full as possible, and
+        * put the rest to the right page. When building a new index, that's
         * good, because the table is scanned from beginning to end and there
         * won't be any more insertions to the left page during the build.
         * This packs the index as tight as possible. But otherwise, split
@@ -631,9 +631,10 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
         * until they're balanced.
         *
         * As a further heuristic, when appending items to the end of the
-        * page, split 75/25, one the assumption that subsequent insertions
-        * will probably also go to the end. This packs the index somewhat
-        * tighter when appending to a table, which is very common.
+        * page, try make the left page 75% full, one the assumption that
+        * subsequent insertions will probably also go to the end. This packs
+        * the index somewhat tighter when appending to a table, which is very
+        * common.
         */
        if (!btree->isBuild)
        {
@@ -645,14 +646,18 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
                if (lastleftinfo->action != GIN_SEGMENT_DELETE)
                {
                    segsize = SizeOfGinPostingList(lastleftinfo->seg);
+
+                   /*
+                    * Note that we check that the right page doesn't become
+                    * more full than the left page even when appending. It's
+                    * possible that we added enough items to make both pages
+                    * more than 75% full.
+                    */
+                   if ((leaf->lsize - segsize) - (leaf->rsize + segsize) < 0)
+                       break;
                    if (append)
                    {
-                       if ((leaf->lsize - segsize) - (leaf->lsize - segsize) < BLCKSZ / 4)
-                           break;
-                   }
-                   else
-                   {
-                       if ((leaf->lsize - segsize) - (leaf->rsize + segsize) < 0)
+                       if ((leaf->lsize - segsize) < (BLCKSZ * 3) / 4)
                            break;
                    }